cie 157 2004 (control of damage to museum objects by optical radiation)

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CONTROL OF DAMAGE TO MUSEUM OBJECTS BY OPTICAL RADIATION

CIE 157:2004 UDC: 535.683 Descriptor: Influence of radiation 535.683.1 Fastness to light

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THE INTERNATIONAL COMMISSION ON ILLUMINATION

The International Commission on Illumination (CIE) is an organisation devoted to international co-operation and exchange of information among its member countries on all matters relating to the art and science of lighting. Its membership consists of the National Committees in 38 countries and one geographical area and of 4 associate members. The objectives of the CIE are: 1. To provide an international forum for the discussion of all matters relating to the science, technology and art in the fields

of light and lighting and for the interchange of information in these fields between countries. 2. To develop basic standards and procedures of metrology in the fields of light and lighting. 3. To provide guidance in the application of principles and procedures in the development of international and national

standards in the fields of light and lighting. 4. To prepare and publish standards, reports and other publications concerned with all matters relating to the science,

technology and art in the fields of light and lighting. 5. To maintain liaison and technical interaction with other international organisations concerned with matters related to the

science, technology, standardisation and art in the fields of light and lighting. The work of the CIE is carried on by seven Divisions each with about 20 Technical Committees. This work covers subjects ranging from fundamental matters to all types of lighting applications. The standards and technical reports developed by these international Divisions of the CIE are accepted throughout the world. A plenary session is held every four years, at which the work of the Divisions and Technical Committees is reviewed, reported and plans are made for the future. The CIE is recognised as the authority on all aspects of light and lighting. As such it occupies an important position among international organisations.

LA COMMISSION INTERNATIONALE DE L'ECLAIRAGE

La Commission Internationale de l'Eclairage (CIE) est une organisation qui se donne pour but la coopération internationale et l'échange d'informations entre les Pays membres sur toutes les questions relatives à l'art et à la science de l'éclairage. Elle est composée de Comités Nationaux représentant 38 pays plus un territoire géographique, et de 4 membres associés. Les objectifs de la CIE sont : 1. De constituer un centre d'étude international pour toute matière relevant de la science, de la technologie et de l'art de la

lumière et de l'éclairage et pour l'échange entre pays d'informations dans ces domaines. 2. D'élaborer des normes et des méthodes de base pour la métrologie dans les domaines de la lumière et de l'éclairage. 3. De donner des directives pour l'application des principes et des méthodes d'élaboration de normes internationales et

nationales dans les domaines de la lumière et de l'éclairage. 4. De préparer et publier des normes, rapports et autres textes, concernant toutes matières relatives à la science, la

technologie et l'art dans les domaines de la lumière et de l'éclairage. 5. De maintenir une liaison et une collaboration technique avec les autres organisations internationales concernées par

des sujets relatifs à la science, la technologie, la normalisation et l'art dans les domaines de la lumière et de l'éclairage. Les travaux de la CIE sont effectués par 7 Divisions, ayant chacune environ 20 Comités Techniques. Les sujets d'études s'étendent des questions fondamentales, à tous les types d'applications de l'éclairage. Les normes et les rapports techniques élaborés par ces Divisions Internationales de la CIE sont reconnus dans le monde entier. Tous les quatre ans, une Session plénière passe en revue le travail des Divisions et des Comités Techniques, en fait rapport et établit les projets de travaux pour l'avenir. La CIE est reconnue comme la plus haute autorité en ce qui concerne tous les aspects de la lumière et de l'éclairage. Elle occupe comme telle une position importante parmi les organisations internationales.

DIE INTERNATIONALE BELEUCHTUNGSKOMMISSION

Die Internationale Beleuchtungskommission (CIE) ist eine Organisation, die sich der internationalen Zusammenarbeit und dem Austausch von Informationen zwischen ihren Mitgliedsländern bezüglich der Kunst und Wissenschaft der Lichttechnik widmet. Die Mitgliedschaft besteht aus den Nationalen Komitees in 38 Ländern und einem geographischen Gebiet und aus 4 assoziierten Mitgliedern. Die Ziele der CIE sind: 1. Ein internationaler Mittelpunkt für Diskussionen aller Fragen auf dem Gebiet der Wissenschaft, Technik und Kunst der

Lichttechnik und für den Informationsaustausch auf diesen Gebieten zwischen den einzelnen Ländern zu sein. 2. Grundnormen und Verfahren der Meßtechnik auf dem Gebiet der Lichttechnik zu entwickeln. 3. Richtlinien für die Anwendung von Prinzipien und Vorgängen in der Entwicklung internationaler und nationaler Normen

auf dem Gebiet der Lichttechnik zu erstellen. 4. Normen, Berichte und andere Publikationen zu erstellen und zu veröffentlichen, die alle Fragen auf dem Gebiet der

Wissenschaft, Technik und Kunst der Lichttechnik betreffen. 5. Liaison und technische Zusammenarbeit mit anderen internationalen Organisationen zu unterhalten, die mit Fragen der

Wissenschaft, Technik, Normung und Kunst auf dem Gebiet der Lichttechnik zu tun haben. Die Arbeit der CIE wird in 7 Divisionen, jede mit etwa 20 Technischen Komitees, geleistet. Diese Arbeit betrifft Gebiete mit grundlegendem Inhalt bis zu allen Arten der Lichtanwendung. Die Normen und Technischen Berichte, die von diesen international zusammengesetzten Divisionen ausgearbeitet werden, sind von der ganzen Welt anerkannt. Tagungen werden alle vier Jahre abgehalten, in der die Arbeiten der Divisionen überprüft und berichtet und neue Pläne für die Zukunft ausgearbeitet werden. Die CIE wird als höchste Autorität für alle Aspekte des Lichtes und der Beleuchtung angesehen. Auf diese Weise unterhält sie eine bedeutende Stellung unter den internationalen Organisationen.

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ISBN 3 901 906 27 4


CIE 157:2004 UDC: 535.683 Descriptor: Influence of radiation 535.683.1 Fastness to light



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CIE 157:2004

This Technical Report has been prepared by CIE Technical Committee 3-22 of Division 3 “Interior Environment and Lighting Design” and has been approved by the Board of Administration of the Commission Internationale de l'Eclairage for study and application. The document reports on current knowledge and experience within the specific field of light and lighting described, and is intended to be used by the CIE membership and other interested parties. It should be noted, however, that the status of this document is advisory and not mandatory. The latest CIE proceedings or CIE NEWS should be consulted regarding possible subsequent amendments.

Ce rapport technique a été élaboré par le Comité Technique CIE 3-22 de la Division 3 “Environnement intérieur et étude de l'éclairage” et a été approuvé par le Bureau de la Commission Internationale de l'Eclairage, pour étude et emploi. Le document expose les connaissances et l'expérience courantes dans le domaine particulier de la lumière et de l'éclairage décrit ici. Il est destiné à être utilisé par les membres de la CIE et par tout les intéressés. Il faut cependant noter que ce document est indicatif et non obligatoire. Il faut consulter les plus récents comptes rendus de la CIE, ou le CIE NEWS, en ce qui concerne des amendements nouveaux éventuels.

Dieser Technische Bericht ist vom CIE Technischen Komitee 3-22 der Division 3 “Innenraum und Beleuchtungsentwurf” ausgearbeitet und vom Vorstand der Commission Internationale de l'Eclairage gebilligt worden. Das Dokument berichtet über den derzeitigen Stand des Wissens und Erfahrung in dem behandelten Gebiet von Licht und Beleuchtung; es ist zur Verwendung durch CIE-Mitglieder und durch andere Interessierte bestimmt. Es sollte jedoch beachtet werden, daß das Dokument eine Empfehlung und keine Vorschrift ist. Die neuesten CIE-Tagungsberichte oder das CIE NEWS sollten im Hinblick auf mögliche spätere Änderungen zu Rate gezogen werden.

Any mention of organisations or products does not imply endorsement by the CIE. Whilst every care has been taken in the compilation of any lists, up to the time of going to press, these may not be comprehensive.

Toute mention d'organisme ou de produit n'implique pas une préférence de la CIE. Malgré le soin apporté à la compilation de tous les documents jusqu'à la mise sous presse, ce travail ne saurait être exhaustif.

Die Erwähnung von Organisationen oder Erzeugnissen bedeutet keine Billigung durch die CIE. Obgleich große Sorgfalt bei der Erstellung von Verzeichnissen bis zum Zeitpunkt der Drucklegung angewendet wurde, ist es möglich, daß diese nicht vollständig sind.

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CIE 157:2004

FOREWORD

The following members of TC 3-22 "Museum lighting" took part in the preparation of this Technical Report. The TC comes under CIE Division 3 "Interior Environment and Lighting Design". This present report replaces CIE 89/3-1991 "On the deterioration of exhibited museum objects by optical radiation".

Dr. William (Bill) Allen United Kingdom Dr. Sirri Aydinli Germany Pierrette Chauvel France Christopher (Kit) Cuttle (Editor) New Zealand M.J.F. Dempster South Africa Jean-Jacques Ezrati (Secretary) France Barbara Fischer Germany Frank Florentine USA Philip Gabriel Canada Dr. Tamara Gatalskaya-Sidorova Russia John Goodall Australia Günter Hilbert Germany Kohtaro Kohmoto Japan Dr. Heinrich Kramer Germany Maria Lashkowska Poland David Loe United Kingdom Prof. Kunio Matsuura Japan Stefan Michalski Canada John Moore United Kingdom Prof. Eliyahu Ne’eman (Chairperson) Israel Julle Oksanen Finland Zeynep Özver-Krochmann Germany Edwin Robinson USA Prof. Masako Saito Japan

Dr. David Saunders United Kingdom

Prof. Gong-Xia Yang China

Committee meetings have been conducted in various centres to suit the changing focus of the committee’s work:

New York, USA July 1994 Paris, France September 1994 Berlin, Germany February 1995 New Delhi, India November 1995 London, United Kingdom February 1996 Berlin, Germany October 1996 Seattle, USA August 1997 Ottawa, Canada September 1997 London, United Kingdom February 1998 Ottawa, Canada May 1998 Warsaw, Poland June 1999

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CIE 157:2004

TABLE OF CONTENTS

SUMMARY V RESUME V ZUSAMMENFASSUNG V INTRODUCTION 1 1. THE SCIENTIFIC PRINCIPLES 3

1.1 The processes of damage to museum objects 3 1.2 Photochemical action 3

1.2.1 The process of photochemical action 3 1.2.2 Effects of photochemical action 3 1.2.3 Causes of photochemical action 3 1.2.4 Irradiance and duration of exposure 4 1.2.5 Spectral power distribution of incident irradiation 4 1.2.6 Action spectrum of receiving material 5

1.3 Radiant heating effect 7 1.3.1 The process of radiant heating 7 1.3.2 Effects of radiant heating 8

2. CURRENT KNOWLEDGE AND RECENT RESEARCH 9 2.1 Measurement of damage 9

2.1.1 The "Blue Wool" scale 9 2.1.2 The CIELAB system 9 2.1.3 Threshold effective radiant exposure 10 2.1.4 Responses of colorants to exposure 13

2.2 Tuning the spectrum 15 2.2.1 Correlated colour temperature of lighting 15 2.2.2 Spectral power distribution of lighting 16

3. RECOMMENDATIONS FOR LIGHTING IN MUSEUMS 18 3.1 Materials to be protected 18

3.1.1 Four categories of responsivity 18 3.1.2 Classifying pigments for responsivity 19

3.2 Procedure to control damage to museum objects 22 3.2.1 To minimise exposure of museum objects 22 3.2.2 Use of electronic flash 24 3.2.3 Exposure rate 25 3.2.4 Outline of procedure 26

REFERENCES 28

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SUMMARY

The report comprises three parts. The first part reviews the scientific principles that govern the processes of radiation-induced damage to museum objects with the aim of providing fundamental information for museum conservators and research workers. The second part reviews current knowledge and recent research to provide a commentary on the efforts of researchers to better understand how these processes may be retarded or eliminated in the museum environment. The final part gives the committee’s recommendations for lighting in museums in the form of a practical procedure that covers setting up a new display and monitoring the lighting during the life of the display. This procedure takes account of the research findings that have been reviewed as well as recommendations published by other organisations, and is modelled on current practice in several of the world’s leading museum institutions.

MAITRISE DES DEGRADATIONS OCCASIONEES PAR LES RADIATIONS OPTIQUES AUX COLLECTIONS MUSEALES

RESUME

Le présent rapport est constitué de trois parties. La première partie est un rappel des principes scientifiques qui régissent les dégradations induites par les radiations optiques aux objets de musée avec comme objectif de donner au personnel des musées une information de base. La seconde partie fait le point sur les connaissances actuelles et les derniers travaux des chercheurs réalisés dans le but de mieux envisager les moyens à mettre en œuvre pour retarder, voire éliminer les causes de ces dégradations. La dernière partie donne les recommandations du comité pour un éclairage muséographique, sous une forme pratique, par la mise en place de nouvelles présentations et d’un contrôle de la lumière durant celles-ci. Ces recommandations tiennent compte aussi bien des études citées dans le rapport que des recommandations éditées par d’autres organismes, le tout à la lumière de la pratique en place dans les principaux musées du monde.

BEGRENZUNG DES SCHADENS AN MUSEUMSOBJEKTEN DURCH OPTISCHE STRAHLUNG

ZUSAMMENFASSUNG

Der Bericht besteht aus drei Teilen. Der erste Teil gibt einen Überblick über die wissenschaftlichen Grundlagen der durch Strahlung verursachten Schädigungsprozesse an musealen Objekten. Das Ziel dabei ist, grundlegende Information für Museumskonservatoren und wissenschaftliche Mitarbeiter bereitzustellen. Der zweite Teil gibt einen Überblick über aktuelle Forschungsarbeiten und den Kenntnisstand zur Erläuterung der Bemühungen von Forschern, um besser zu verstehen, wie diese Prozesse im Bereich von Museen verzögert oder eliminiert werden können. Der letzte Teil enthält Empfehlungen des Komitees hinsichtlich der Beleuchtung in Museen in Form einer praktischen Vorgehensweise für die Darbietung eines neuen Ausstellungsgegenstandes und für die Überwachung der Beleuchtung während dessen Ausstellungszeit. Das Verfahren berücksichtigt die hier behandelten wissenschaftlichen Ergebnisse, aber auch Empfehlungen, die von anderen Organisationen veröffentlicht wurden, und folgt der gängigen Praxis von einigen der führenden Museumsinstitutionen der Welt.

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INTRODUCTION

This report comprises three parts.

• Part 1 reviews the scientific principles of radiation exposure and conservation.

• Part 2 reviews current knowledge and recent research.

• Part 3 gives recommendations for lighting in museums.

Parts 1 and 2 comprise important information for professional conservators and research workers, but it is expected that some museum staff and exhibitions designers will choose to turn directly to Part 3.

The starting point for the committee’s work was CIE 89/3-1991 (CIE, 1991), which had established a basis for relating the optical radiation exposure of an object to the photochemical response of the object. Initially the committee’s intention was to develop this approach so that it could be applied to the practical task of controlling the degradation of museum exhibits.

The CIE 89/3-1991 (CIE, 1991) approach depends upon the action spectrum of the receiving object being defined, and in practical situations this incurs several problems. Materials vary in both absolute and relative spectral responsivity, and many museum objects comprise a combination of materials. However, recent research has made important contributions to knowledge of how museum materials react to exposure, and the work initiated by Prof. Jürgen Krochmann and continued by his colleagues in Berlin deserves particular mention. These researchers have selectively exposed a range of typical museum material samples, and from data of measured colour shifts have derived typical responsivity functions for several material groups. Each of these groups is defined by a logarithmic responsivity function, and this suggested a practical means of enabling museum staff to predict visible effects of exposure. For a given lamp and filter combination, the duration of exposure that would cause a given material to undergo a just noticeable change of colour is shown to be inversely proportional to the illuminance.

Attempts to apply this approach to practical situations revealed fundamental problems. The formulae defining the responsivity functions do not distinguish between radiant energy that is visually useful and that which is not. Large gains in permitted exposure are obtained by reducing short-wavelength radiation, so that long-wavelength rich light sources are always favoured regardless of the visual effect intended by the lighting designer. Furthermore, as only photochemical degradation is considered, the detrimental effects of infra-red exposure are ignored.

For these reasons, the committee has recommended a procedure that seeks to give lighting designers opportunities to achieve their display objectives while avoiding unnecessary exposure of the exhibits, and where necessary, achieving control by restricting the duration of exposure.

For Part 1, the committee’s aim has been to outline the known facts that govern the processes of radiation-induced damage to museum objects. Part 2 provides a commentary on the efforts of researchers to better understand how these processes may be retarded or eliminated in the museum environment. The recommendations given in Part 3 are based upon these research findings, but clearly, this is a developing field and it must be expected that notions of good practice will change as research progresses and the technology of control develops. References are given throughout the document for more detailed information.

The committee’s aim has been to give recommendations for good lighting practice wherever the conservation of museum objects is a cause for concern. These recommendations are modelled on the lighting policies in leading national institutions, and are considered to represent practical standards to which all museums should aspire. They include procedures for setting up lighting for a new display and for periodic monitoring during the life of a display, and in several respects these differ from conventional current practice. After much discussion, the committee has proposed a four-category material classification for limiting light exposure that includes a high responsivity category, for which permanent display is precluded. Also, the committee arrived at the decision that there is no rational basis for

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recommending that museum objects may be subjected to any level of ultraviolet exposure, and elimination of UV is recommended.

It is recognised that some museums will decide that not all of its displays need to be fully compliant with the committee’s recommendations. Some objects may be classified as inresponsive to radiation exposure, while others may be considered to have a limited lifespan, or to be of insufficient value to justify the expense of instituting the recommended procedure. In any such cases, the museum should consider all aspects of the committee’s recommendations to ensure that its own policy is comprehensive. Also, staff should be alerted to the risks of putting light-responsive objects on long-term display in non-compliant conditions.



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1. THE SCIENTIFIC PRINCIPLES

1.1 The processes of damage to museum objects

It is natural for many substances to change with time. Nature has provided processes to dismantle the substances produced by organic growth in order to provide for recycling of the molecules that are the building blocks of life on this planet. Even inorganic substances are not necessarily permanent, although change may occur much more slowly.

The principal aim of conservation policies in museums is to retard these processes, and these policies must address all causes of change that might affect objects in the museum's collection. It is only in the case of exposure to light that the needs for conservation and display are directly in conflict. For the conservation of many materials, the ideal environment would be completely dark.

There are two processes by which exposure to light may cause damage:

Photochemical action; •

• Radiant heating effect.

1.2 Photochemical action

1.2.1 The process of photochemical action

Photochemical action is the process by which a molecule undergoes a chemical change, with the activation energy for the change being derived from the absorption of a photon. While the initial event of the photon absorption is independent of the surrounding environment, subsequent chemical actions may be strongly affected by environmental factors such as temperature and humidity. The photon absorption may be the initial stage in a complex series of chemical changes, but whether or not the process is complex, the change is irreversible. All forms of chemical action affecting museum objects constitute damage, and where the aim is to minimise the effects of photochemical action on museum objects, the museum must have a comprehensive policy for environmental control.

Materials differ substantially in their responsivity to light exposure. The process by which a photographic film forms an image is also due to photochemical action, and obviously, even the most susceptible museum objects, including photographic prints, have low sensitivities in comparison with unexposed photographic emulsion materials. Even so, unless an object is totally irresponsive to light exposure, for every incident photon there is a finite probability of permanent damage. There is no safe level of exposure for a light-responsive object.

1.2.2 Effects of photochemical action

Colour change is usually the most obvious indication of light-induced damage to museum objects. The appearance of fading is well known, but the visible effects of photochemical action may be quite different. It can cause some colorants to darken, and some to undergo changes of hue that are quite unlike the yellowing and lightening associated with fading.

The other main effect of light-induced damage is loss of strength, which may be evident as fraying of fibres on fabrics, or embrittlement and surface cracking of artefacts. These effects may be difficult to distinguish from effects of radiant heating discussed in Section 1.3.

1.2.3 Causes of photochemical action

Four factors determine the level of photochemical action:

• Irradiance;

• Duration of exposure;

• Spectral power distribution of incident radiation;

• Action spectrum of receiving material.



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1.2.4 Irradiance and duration of exposure

The principle that, for a given source of irradiation, exposure (H) is the integration of irradiance and time may be referred to as the Reciprocity Law, or the Bunsen-Roscoe Law, or, as in this document, the Reciprocity Principle. Whichever title is used, the principle may be expressed as:

H = ∫ Ee dt Wh/m2 (1.1) t

where

Ee is irradiance incident on the surface (W/m2) t is time in hours (h)

When Ee is constant, as with electric lighting, the equation takes the simple form: H = Ee⋅ t

According to this principle, a total exposure of 10 W/m2 for 10 hours is equivalent to 20 W/m2 for 5 hours or 5 W/m2 for 20 hours if the spectral power distribution is the same. It should be noted that the principle is defined in terms of irradiance, which is a measure of the density of incident radiant power, or as it is referred to hereafter, incident radiant flux. It needs to be recognised that illuminance is not a reliable alternative measure, as it represents the density of luminous flux, being radiant flux evaluated according to a typical human visual response, defined by the photopic spectral luminous efficiency function V(λ). Not only does illuminance take no account of irradiance outside the visible spectrum, but also radiant flux within the visible spectrum is weighted according to its relative visual effect, which is not related to its damage effect. Illuminance meters are reasonably affordable and easy to operate, whereas meters that measure radiant flux irrespective of wavelength are more specialised scientific instruments. It is common practice for museum staff to use an illuminance meter to monitor light exposure, but they should be aware that an illuminance reading can not alert them to the presence of non-visible radiation, either UV or IR, nor is it an entirely reliable indicator of the damage effect due to visible radiant flux. Some hand-held UV meters are available, but it is important to check that the spectral range covered is appropriate for conservation assessments. These instruments are useful only for checking the presence of UV. They can not indicate the extent of possible damage, because for this both the UV spectrum and the action spectrum of the receiving object must be known. See Section 3.2 for more discussion of this topic.

The aim of preventive conservation is to retard the photochemical process and extend it over many years, and this makes it more likely that extraneous factors will complicate the process. For example, the radiant heating effect of lighting may raise the surface temperature of the object to the extent that chemical reactions which may follow photochemical action are accelerated. Also, as discussed in the following subsection, the spectral power distribution of the incident radiant flux affects the rate of photochemical action.

1.2.5 Spectral power distribution of incident irradiation

Radiant flux, which includes ultraviolet (UV), visible and infrared (IR) wavebands may be envisaged as a stream of photons in which each photon is a discrete energy package. Photons differ widely in energy level, although they all travel at the same velocity in vacuum. Photon energy level E (Joules) is directly proportional to frequency, and is given by the expression:

E = hν J (1.2)

where h is Planck's constant, a number that relates the units of frequency to energy (h = 6,626 x 10-34 J s) ν is frequency (Hz)

The photons are the "bullets" that trigger photochemical reactions, and if absorbed, their energy levels indicate their potential to cause damage (Feller, 1964). Different molecules have different photon energy thresholds. A molecule that is highly responsive to light exposure will have a low photon energy threshold, so that a low level of photon energy is sufficient to trigger a chemical change.



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Photon energy defines the energy level of an individual photon. It is not to be confused with irradiance, except that under conditions of very high irradiance a molecule may receive photons in such rapid succession that a biphotonic process is initiated. This occurs when the photon energy threshold is exceeded by the sum of two or more photons. However, this effect may be ignored in this context as it is unlikely to occur at the low irradiances encountered in properly managed museums (Thomson, 1986).

As shown in Equation (1.2), photon energy is proportional to frequency. However, frequency is inversely proportional to wavelength λ, so that Equation (1.2) can be rewritten

E= hc/λ J (1.2a)

where c is the velocity of light in vacuum.

This shows that photon energy is proportional to the reciprocal of wavelength 1/λ, so that short wavelength luminous flux (i.e. blue light) has higher photon energies than long wavelength luminous flux (i.e. red light), and UV flux has higher photon energies still. It may be noted that the CIE defines UV as radiant energy of wavelength less than 400 nm, even though the visible spectrum is defined as extending down to 380 nm. However, the visual response to radiation in the 380 to 400 nm waveband is very low, and for museum conservation purposes, all wavelengths less than 400 nm are considered to be UV and unwanted.

The amount of UV produced by different light sources is often indicated in terms of microwatts of UV per lumen, µW/lm, and Table 1.1 lists typical proportions for various light sources. These data do not take account of the wavelength distribution of the UV radiation, and so do not provide a reliable indication of relative damage potential. It can be seen that no practical light source is entirely devoid of UV emission, so that except for materials that are totally irresponsive to UV, there is always scope to reduce damage by using filters that block UV. The use of UV blocking filters is discussed in Section 3.2, and is generally recommended.

Table 1.1 Typical ultraviolet proportions for various light sources.

Light source UV content (µW/lm)

Daylight Tungsten incandescent Tungsten halogen* Fluorescent lamps Metal halide Light emitting diode (LED)**

400 –1500 70 – 80 40 – 170 30 – 100

160 – 700 <5

* Includes "UV-STOP" lamps. ** These lamps are not of suitably high colour quality for museum use at present, but have

future potential as very low UV power sources. Data provided by Dr David Saunders, Scientific Department, The National Gallery, London, UK.

1.2.6 Action spectrum of receiving material

In 1953, Harrison (1953) proposed a procedure for evaluating the relative damage potential of different types of light sources and source/filter combinations. He introduced a Damage function D(λ), which is an action spectrum that defines the relative spectral responsivity of a receiving material (Figure 1.1), and he proposed that this be used to determine a Damage Index DI for incident radiation. The intention was that this index would be used to compare the damage potential of alternative light sources, or source and filter combinations.



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0

1

2

3

4

5

6

7

8

300 400 500 600

Wavelength (nm)

Rel

ativ

e D

amag

e

700

Figure 1.1 Harrison’s Damage function D(λ).

The relative damage flux is given by:

Fdm,rel = ∫ Φ(λ) ⋅ T(λ) ⋅ D(λ) ⋅ dλ (1.3) λ

where:

Φ (λ) is spectral radiant power W/nm T(λ) is spectral transmittance of filter D(λ) is damage function λ is wavelength nm

And the relative luminous flux:

Fv,rel = ∫ Φ (λ) ⋅ T(λ) ⋅ V(λ) ⋅ dλ (1.4) λ

where V(λ) is the spectral luminous efficiency for photopic vision

Then the damage index for the incident radiation:

DI = Fdm,rel / Fv,rel (1.5)

Although Harrison’s proposal aroused a lot of interest at the time, the conservators were sceptical that a single damage function could be representative of all the diverse materials found in museums. Thomson (1978, p. 178) stated "for more fugitive materials … the figure for visible radiation would be higher. On the other hand…the fastest dyes are probably affected only by UV. Thus it can be seen that no single figure can be given for damage versus wavelength". Confidence was further eroded when it became evident that the precisely defined D(λ) function had been extrapolated from a very limited number of measurements based on exposing samples of "low-grade paper", such as newsprint.

Because of the scientists’ mistrust of D(λ), Harrison’s proposal failed to gain acceptance as the procedure for comparing the damage potential of different types of light sources. It was, nonetheless, influential in the wider museum community. As can be seen in Figure 1.1, the D(λ) function gives the impression that damage is due almost entirely to UV. Furthermore, the relatively small amount of damage shown to be attributable to visible radiation is due to the shorter wavelengths, with damage becoming negligible at wavelengths



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longer than 500 nm. This cemented in the minds of many museum directors that the incandescent lamp was safe and the museum light source of choice (the halogen lamp had not been invented), and that daylight must be eliminated from galleries.

Despite the shortcomings of D(λ), scientists realised that Harrison’s procedure was basically sound. If a reliable way of expressing the relative damage potential of radiation as a function of wavelength could be found, it would be practical to compare the effects of exposing objects to different spectra of incident radiation. Modern computers would make the calculations very simple. It has been shown that, despite its appearance in Figure 1.1, D(λ) is actually a simple logarithmic function (Cuttle, 1988) with a slope of –1,25 logD(λ) units per 100 nm of wavelength. (This way of characterising relative damage functions is employed in Section 2.1, where this function would be shown as –1,25 logD(λ)/100 nm.)

Because the range of photochemical reactions that can occur in a museum is vast and the actions themselves are complex, a simple model of spectral responsivity can not be expected to be precise or entirely reliable. Nonetheless, such a model could take the form:

s(λ)dm,rel = α(λ) ⋅ 1/λ ⋅ f(λ) (1.6)

where s(λ)dm,rel is relative spectral responsivity α(λ) is spectral absorptance f(λ) is a function of wavelength determined by the receiving material

The rationale for this model is that first, energy has to be absorbed to cause damage; second, the chance of a photochemical response is related to the photon energy level which, as shown in the previous subsection, is proportional to the reciprocal of wavelength; and third, there will be some function of wavelength that is determined by the inherent properties of the material. It may be noted that at a given wavelength, α(λ) = 1-[ρ(λ)+τ(λ)], where ρ(λ) and τ(λ) are spectral reflectance and transmittance, respectively.

The growth of scientific interest in relating light exposure to damage led museum professionals to question what actually constitutes damage. Before researchers could make progress with defining action spectra of museum materials, they needed a reliable measure of damage. This is discussed in Section 2.1.

1.3 Radiant heating effect

1.3.1 The process of radiant heating

Radiant heating effect is the raising of surface temperature above ambient temperature due to absorption of incident radiant flux. It has been shown (Feller, 1968) that the maximum attainable temperature of an irradiated object is given by:

camax

khAETT e+= (1.7)

where

Ta is ambient (air) temperature (C) k is a proportionality constant A is absorptance of the object* Ee is irradiance (W/m2) hc is coefficient of convection heat loss

*In general, unlike UV energy, IR energy is not evaluated on a wavelength basis but rather in terms of all such energy incident on a surface. In this expression, A is assumed to be averaged over the spectrum, but some situations may require spectral variations to be taken into account.

Equation (1.7) shows that the elevation of the object's surface temperature above ambient temperature is proportional to irradiance, and is independent of the object's thermal capacity, density or thickness. When radiant flux is directed onto an object, some proportion is absorbed, depending on the spectral power distribution of the incident flux and the spectral absorptance of the object. Some small proportion of the absorbed radiation may promote



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photochemical action as discussed in the previous section, but regardless of whether the object is light-responsive, its surface temperature will rise towards the maximum attainable temperature, Tmax.

1.3.2 Effects of radiant heating

Radiant heating is a less serious source of concern than photochemical action, and it is not uncommon for museums to disregard it completely. However, it does cause damage, and as museums become more rigorous in the control of photochemical action, so radiant heating effects are more likely to become the predominant cause of damage. Museum staff may not recognise this, as the visible effects are not readily distinguished from photochemical damage.

Radiant heating has the effect of raising the temperature of an illuminated surface, and this encourages chemical activity. Also, in a changing thermal environment, materials undergo corresponding dimensional changes and deformations. Stresses occur where materials having different coefficients of thermal expansion are in contact, and particularly where materials having high coefficients are involved. Partial shading of the object may cause differential heating effects. Variation of relative humidity causes migrations of moisture between hygroscopic objects and the surrounding atmosphere. Preventive conservation seeks to minimise these effects by maintaining air temperature and humidity within prescribed limits.

Lighting adds a variable effect, as some proportion of the incident radiant flux is absorbed by the objects causing a radiant heating effect. This results in local elevation of surface temperature and dehydration. Daily on/off switching of lighting causes cyclic surface expansions and contractions, and migrations of moisture. The visible effects of these processes are surface hardening, discolouration and cracking, which may be difficult to distinguish from the effects of photochemical action. Damage is particularly likely in materials that are hygroscopic (which includes virtually all organic materials) or where the surface comprises layers of dissimilar materials, such as varnish over pigment, or pigment over a substrate.

IR radiant flux is associated with incandescent lamps, and where the light from this type of source is focussed onto an object to provide strong visual impact, the radiant heating effect can become a significant source of damage. Museum staff tend to think of incandescent lighting as being relatively safe, as its UV content is lower than for most other types of lighting. Procedures for addressing the issue of radiant heating effect are described in Section 3.2.



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2. CURRENT KNOWLEDGE AND RECENT RESEARCH

2.1 Measurement of damage

Several research studies have exposed samples of typical museum materials to controlled levels of radiation, and have sought to define relationships between light exposure and damage due to photochemical action. Generally these studies have ignored radiant heating effects. While it is relatively straightforward to measure exposure to the causes of photochemical action (Section 1.2), damage is less easily quantified.

2.1.1 The "Blue Wool" scale

For many materials, the most obvious effect of exposure is "on-line" fading, which is characterised by loss of colour saturation and, particularly for darker colours, lightening of appearance. This has led some researchers to adopt the ISO rating system (ISO, 1995) as a general classification system for museum materials. This system is based on the "blue wool" scale of light-fastness, which comprises eight categories. ISO 1 is the most responsive to light; ISO 2 is approximately half as responsive as ISO 1; and so on to ISO 8 which is the least responsive. A material is categorised by exposing it to a broad-spectrum light source in a controlled environment cabinet alongside a standard card that has eight dyed-wool samples. Both the material and the sample card are partly covered, and visual comparisons are made at intervals to match the rate of fading of the material to one of the wool samples. In this way, a material rated as ISO 3 is approximately half as responsive as a material rated ISO 2, and twice as responsive as a material rated ISO 4.

2.1.2 The CIELAB system

Not all materials demonstrate on-line fading under exposure. Some materials show yellowing, some darken, and some change hue. Researchers need to be able to record changes of surface colour over time, and for this they need a precise system of colour measurement. Currently, the most widely used system for exposure research is CIELAB (CIE, 1986). This system defines a three-dimensional colour space within which the colour characteristics of a sample material are specified in terms of a lightness dimension L*, and two chromatic dimensions, a* and b* (Figure 2.1). The L* dimension ranges from black to white. Positive values of a* indicate redness, and negative values greenness. Positive values of b* indicate yellowness, and negative values blueness. The value of this system as a research tool is that it enables extents of colour difference to be measured and compared.

Suppose, for example, that a medium red coloured material has been exposed, and the before and after exposure measurements are being compared. The "before" measurement for this material would have a moderate L* value; a relatively high positive a* value; and a low b* value that might be positive or negative. If the "after" measurement shows a reduction of L*, this indicates a loss of lightness, or a darkening of the material. A reduction of a* indicates a loss of redness, and if b* is proportionately reduced, there has been a loss of chroma without a change of hue. The magnitude of any colour difference can be represented by a vector, which is indicated by the symbol ∆E*ab, and takes account of the changes on all three dimensions. The scales of the three dimensions have been so chosen that when the colour difference is just discernable in a side-by-side comparison, ∆E*ab has a value of one. In this condition the human eye is a very responsive discriminator, and it requires quite elaborate equipment to reliably measure colour differences as small as one unit of ∆E*ab.



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*L∆ baL 111 baL 111 L*1a*1b*1

*L∆

L*0a*0b*0 *a∆

*b∆

Figure 2.1 Distance between two objects in CIELAB colour space.

The three rectangular coordinates defining an object in CIELAB colour space are:

L*, CIE 1976 Lightness units; a*, CIE 1976 Red-green units; b*, CIE 1976 Yellow-blue units.

Using the subscript 0 for the standard and 1 for the sample, the distance ∆E*ab between L*0 a*0 b*0 and L*1 a*1 b*1 in rectangular coordinates:

( ) ( ) ( )222 **** baLE ab ∆+∆+∆=∆

where: ∆L* = L*1 – L*0 ∆a* = a*1 – a*0 ∆b* = b*1 – b*0

The CIELAB system has enabled researchers to gather precise data on the progressive nature of colour change due to light exposure for many materials. The reciprocity principle (Equation 1.1) has been confirmed for museum materials (Saunders and Kirby, 1996; Ezrati, 1996). Furthermore, a scientific model for the visible effects of exposure has been proposed by a team of researchers working in Berlin, Germany (Krochmann, 1988; CIE, 1991; Hilbert et al., 1991).

2.1.3 Threshold effective radiant exposure

In the Berlin model, the damage suffered by an exposed object DM is a function of the effective radiant exposure Hdm:

DM = f(Hdm) (2.1)

The Effective Irradiance that causes the damage takes account of the spectrum of incident radiation and the relative spectral response of the receiving material:

Edm = ∫ Ee,λ ⋅ s(λ)dm,rel ⋅ dλ W/m2 (2.2) λ

where

Ee,λ is spectral irradiance W/m2 s(λ)dm,rel is relative spectral responsivity normalised at 300 nm, so that s(λ)dm,rel = 1,0

for λ = 300 nm λ is wavelength nm



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Note: The SI unit for irradiance is W/m2, irrespective of spectral power distribution. The suffix dm indicates that effective irradiance is evaluated according to the spectral responsivity of the receiving material.

The Effective Radiant Exposure is the effective irradiance over time:

Hdm = ∫ Edm ⋅ dt W h/m2 (2.3) t

where t is time h

The Threshold Effective Radiant Exposure Hs,dm is the value of Hdm that will cause a just noticeable colour change, that is to say, for which ∆E*ab = 1.

Hs,dm = Edm ⋅ ts W h/m2 (2.4)

where ts is the critical duration of exposure in h

The basis of the Berlin model is illustrated in Figure 2.2. The cause of damage is effective radiant exposure, shown on the horizontal scale, and the effect is change of colour, shown on the vertical scale. When the material is first exposed the curve is steep and the effect rapid, so that it requires only a relatively small level of Hs,dm to cause one unit of ∆E*ab to occur, but as damage continues the density of susceptible molecules reduces, so that greater exposure is required to produce the same visible effect. Eventually the material stabilises, and no more colour change occurs because all of the colorant has faded.

Colour difference ∆E*ab

Effective radiant exposure Hdm

Hs-dm

∆E*ab=1

Hs-dm

∆E*ab=1

y Ypre-exposure

(no more colour change)

x

Xpre-exposure

Figure 2.2 The cause of damage (effective radiant exposure, Hdm) and the effect (colour change, ∆E*ab) according to the Berlin model. The threshold effective radiant exposure Hs,dm is the exposure that causes one unit of ∆E*ab for the material concerned, and this increases as damage progresses.



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The responsivity of an object is defined by its threshold effective radiant exposure Hs,dm and its relative spectral responsivity s(λ)dm,rel. The general form of the s(λ)dm,rel function given in Equation (1.6):

s(λ)dm,rel = α(λ) ⋅ 1/λ ⋅ f(λ)

which can be simplified to:

s(λ)dm,rel ≅ α(λ) ⋅ f '(λ)

and for many non-pigmented materials α(λ) is nearly constant, so that for these it may be assumed that s(λ)dm,rel = f '(λ). The Berlin researchers have exposed samples representing various categories of museum materials to a xenon source, with portions of each sample being shielded from selected spectral bands by a series of sharp cut-off filters. Data from periodic colorimetric measurements have indicated that s(λ)dm,rel may be represented by an exponential function of the form:

s(λ)dm,rel = exp [-b(λ-300)] (2.5)

The s(λ)dm,rel function defines the action spectrum for each category of materials, and is normalised at 300 nm so that equation (2.5) returns a value of one for λ = 300 nm.

Table 2.1 Threshold effective radiant exposure Hs,dm and b values for the relative spectral responsivity function [Equation (2.5)] for five categories of museum materials.

Group Samples Hs,dm

(W h/m2) b

a Low-grade paper 5 0,038

b Rag paper 1200 0,0125

c Oil paints on canvas 850 0,0115

d Textiles 290 0,0100

e Water colours on rag paper 175 0,0115

The sample materials have been classified into five categories, and values of Hs,dm and b are given in Table 2.1. For incident monochromatic radiation of 300 nm wavelength, the values of Hs,dm in Table 2.1 indicate the exposures required to cause the samples to undergo a just discernable colour change. It should be noted, therefore, that reducing values of Hs,dm indicate increasing responsivity. For other wavelengths, the required exposures correspond to the value of Hs,dm/s(λ)dm,rel, and this is shown on a logarithmic scale in Figure 2.3. The combined effects of Hs,dm and s(λ)dm,rel are clearly evident. It is apparent that the slopes of the curves are similar for categories b through e, while category a is distinctly different. The slopes are determined by the value of b, and may be more conveniently expressed in terms of logarithmic units of Hs,dm per 100 nm (logHs,dm/100 nm).

For categories b through e, which covers a reasonably representative range of museum materials, the typical value of b is 0,0115 for which the slope of Hs,dm conveniently turns out to be 0,50 logHs,dm/100 nm. In everyday language, this means that for every 200 nm of wavelength, responsivity changes by a factor of ten. It may be noted also that at all wavelengths, the range of threshold effective exposure for these materials lie within one logarithmic unit, indicating that the range from the least to the most responsive of these materials is not more than a factor of ten. For these four categories of museum materials, this research leads to a reasonably simple model of material type, exposure and damage.



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300 400 500 600 700Wavelength (nm)

0

1

2

3

4

5

6

7

8

log

thre

sho l

d ef

fect

ive

expo

sure

, lgH

s,dm a

bc

de

Figure 2.3 The spectral distribution of the logarithm of threshold effective radiant exposure logHs,dm for five categories of museum materials:

a Low-grade paper b Rag paper c Oil paints on canvas d Textiles e Watercolours on rag paper (Cuttle, 1996, after CIE 89/3-1991)

Category a, which comprised newsprint, is quite different from the other materials. The slope is 1,65 logHs,dm/100 nm, and compared with the other categories, this material is much more responsive to UV and much less responsive to visible radiation. The slope is fairly close to that of Harrison’s D(λ), which was discussed in Section 1.2, and this finding vindicates the refusal of the museum community to accept Harrison’s D(λ) as being representative of typical museum materials.

The aim of this continuing research is that where museum staff know the values of Hs,dm and b for a material that is to be put on display, they will be able to make decisions on display lighting with knowledge of the consequences of exposure. The appropriate critical duration of exposure will become a matter of policy, for which the question posed is: what is an acceptable number of hours of display before the material will undergo a just perceptible change of colour? This decision determines the permissible effective irradiance Edm of the display lighting, which takes account of the illuminance, the spectral power distribution of incident radiation, and the action spectrum, or spectral responsivity distribution, of the material.

There are some difficult issues still to be resolved. It has been explained that Hs,dm is not constant, but increases as exposure causes the material to become less responsive. Also, fading and other induced colour changes are visible evidence of changing spectral absorptance of the material, and this effect may become critical in the case of pigments.

2.1.4 Responses of colorants to exposure

Laboratory studies of colorant fading have produced data that show close correlations with physical laws. Johnston-Feller (1968) has exposed carefully prepared samples of a colorant, alizarin lake, combined with titanium dioxide and dispersed in a PVC vehicle, and her data demonstrate that the rate of loss of colorant was proportional to the logarithm of colorant concentration. This is in accordance with the kinetic first order rate equation, which may be expressed as:



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ln Ct = ln C0 + k1t (2.6)

where:

C0 is initial colorant concentration Ct is colorant concentration after time t k1 is a constant ln refers to logarithm to the base e

This expression suggests a simple model for duration of exposure and loss of colorant. However, researchers concerned with museum conservation issues generally have directed their efforts towards exposing samples of museum materials and assessing damage by measuring the resulting colour changes.

Saunders and Kirby (1994) have examined the spectral sensitivities of various artists’ pigments. They exposed samples of the pigments to radiant flux in seven 70 nm bandwidths at 50 nm intervals in the range 400 nm to 700 nm, and measured colour shifts in terms of ∆E*ab to give detailed data for wavelength effects in the visible spectrum. Figure 2.4 is a representation of their data, which shows how the differing spectral sensitivities of the pigments relate to their spectral absorptance values. Also shown is the Berlin spectral responsivity function (Equation 2.5) for b=0,0115, in this case normalised for λ = 400 nm, and it can be seen that the individual spectral responsivity curves have a general tendency to follow this function while their different spectral absorptance curves impart variations upon the trend. This pattern can be seen to be in accord with Equation (1.6). While it is clear that "typical" spectral responsivity functions, such as the Berlin function, cannot be expected to accurately represent the spectral sensitivities of individual pigments, it might be supposed that they could represent the overall responsivity of a palette of pigments. However, this supposition assumes that all the pigments have similar overall sensitivities. For example, if there is one highly responsive pigment in a low responsivity multi-coloured art-work, then to assess the effective irradiance on the basis of a typical spectral responsivity function could be seriously misleading.

Figure 2.4 Spectral absorptance (solid line) and relative spectral responsivity (broken line) for four artist’s pigments (Cuttle and Ne’eman, 1999, after Saunders and Kirby, 1994). The dotted lines show relative spectral sensitivities normalised at 400 nm based on the Berlin relative spectral responsivity function, Equation (2.5).



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It can be seen in Figure 2.4 that spectral responsivity and spectral absorptance curves do not show exact correspondence, and this has been the subject of experiments in several countries. In Korea, Kim and Kim (2000) have irradiated samples of white and coloured paper, and have reported finding "a close relationship between the spectral damage factor and the spectral absorptance in the visible wavelength range". However, researchers in Japan (Katano et al., 1999) have exposed samples of dyed fabrics to different types of fluorescent lamps, and found that in some cases the spectral absorptance of the sample did not provide a reliable indication of its relative spectral responsivity. A workable system for characterising action spectra for colorants, including pigments and dyes, remains an unattained goal.

While these activities indicate progress, to rely only on CIELAB measurements for measures of the effects of radiation exposure assumes that the effects are entirely visible. At least as important as fading is the loss of strength caused by light exposure, as evidenced by the embrittlement of paper and the fraying of textiles. Although it will often happen that noticeable fading will precede significant weakening of the material, it is not safe to assume that there is correspondence between the two effects. For example, a material that incorporates a light-fast colorant may show little visible effect of light exposure while its physical structure is undergoing serious damage. Although progress is being made towards limiting rates of exposure based on the visible effect, more work is needed before this approach can provide reliable guidance for practice in museums.

2.2 Tuning the spectrum

2.2.1 Correlated colour temperature of lighting

While the variations of spectral responsivity for individual materials, particularly pigments, remains problematic, the overall tendency for responsivity to increase at shorter wavelengths is reasonably well defined. It has been shown that there is a general effect for the relative damage potential to increase as colour temperature increases (Cuttle, 1988), and Table 2.2 and Figure 2.5 show this relationship for two types of broad-spectrum, UV-free light sources. It can be seen that UV filtered illumination from a black-body source at a colour temperature of 6000 K has twice the damage potential of the light from a regular incandescent lamp (CIE Standard Illuminant A). The D series sources, which are standard daylight spectral distributions, show a similar effect as would any broad-spectrum light source, whether natural or artificial. The practical implications are discussed in Section 3.2.

Table 2.2 Damage potential relative to CIE Standard Illuminant A (2856 K) according to Equation (2.5) where b=0,0115, for a Planckian (i.e. black-body) source, and three D series sources. In all cases, wavelengths shorter than 400 nm are excluded. Colour temperature of

a Planckian source Relative damage

potential D series source Relative damage potential

2500 K 3000 K 3500 K 4000 K 4500 K 5000 K 5500 K 6000 K 6500 K 7000 K 7500 K

0,92 1,04 1,20 1,37 1,54 1,71 1,87 2,01 2,15 2,28 2,40

D55 D65 D75

1,63 1,87 2,07



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Figure 2.5 Relative damage potential and colour temperature for a Planckian (i.e. black-body) source and three D series sources, based on Equation (2.5) and excluding wavelengths shorter than 400 nm.

2.2.2 Spectral power distribution of lighting

Museum staff tend to favour light sources having continuous spectral power distributions as the correlated colour temperature together with the colour rendering index provide a fairly reliable indication of the colour characteristics of the lighting. However, some novel approaches have been proposed for modifying the spectrum to reduce the effects of light exposure.

Miller (1993) has proposed a reflected energy matching (REM) procedure by which filters are inserted into a fibre optic lighting system so that the spectrum of incident light closely matches that of the reflected light. This reduces the amount of energy being absorbed by the object and significant reductions of fading are claimed. It should also be expected that the procedure would have the effect of increasing the apparent colour saturation of the displayed materials. While this may be found an attractive feature in some situations, it raises questions concerning the ethics of modifying the apparent colours of displayed objects.

Thornton (1975) has proposed an approach that purports to maintain, or even enhance, the colour rendering properties of the lighting while reducing the irradiance. His technique is to use a "prime colour" light source, which provides a spectrum that comprises three narrow bands of radiant flux that relate to the response peaks of the three types of retinal cones. This approach is employed widely in tri-phosphor fluorescent lamps and colour television screens, but so far its application in museums has aroused little interest. Of the available fluorescent lamps, Thomson (1986) has stated "there will be certain museum situations not demanding the best colour rendering where they will be the choice".

Application of Thornton's proposal in an art gallery situation has been the focus of a recent research study (Cuttle, 2000) in which subjects made comparative settings of lighting for a broad-band source (tungsten halogen lamp) and a special tri-band light source in simulated art gallery settings. The subjects matched luminances for equal preference, and did not show a preference for one source over the other. However, the irradiance on the art works for the tri-band source was less by 30 to 40%, depending on the correlated colour temperature of the illumination, and this suggests that there may be potential to significantly reduce the damage exposure of the displayed objects without reducing illuminance.



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While researchers are able to demonstrate some conservation advantages by departing from the continuous spectrum exemplified by the incandescent lamp, museum curators need to be cautious about employing the proposed spectra in situations where colour rendering is of prime importance. It would seem likely that new light sources will become available with claims for reduced damage effect, and it will be up to curators to assess their visual effect and decide in what situations they might be acceptable. There clearly is potential for reduced damage rates, but small differences of CIE colour rendering index Ra should not be relied upon to compare "good" colour rendering sources. The acceptability of "tuned spectrum" light sources must ultimately be determined by critical viewing, and this could lead to beneficial interaction between the museum community and scientific researchers.

This review of recent research does not lead to a clear conclusion, but rather it shows the diversity of investigations being pursued by researchers around the world. Some of this work is aimed towards developing understanding of the conservation issues, and some to developing a more scientific approach to museum lighting. Meanwhile, it appears that the techniques that are currently employed and the equipment that is available rely more on progressive refinement than upon scientific breakthroughs.



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3. RECOMMENDATIONS FOR LIGHTING IN MUSEUMS

3.1 Materials to be protected

The objects in a museum’s collection can be classified into two main conservation categories: materials of mineral or inorganic origin (stone, metals and glass), and organic materials, which include materials of vegetable origin (paper, papyrus, wood, natural textiles, many pigments and dyes, etc.) and materials of animal origin (bone, ivory, skins, etc., as well as some pigments and dyes). In general, inorganic materials show little or no responsivity to light, while organic materials are moderately or highly responsive.

To classify materials as inorganic or organic is fairly straightforward, but the items of a museum collection must also be classed according to their responsivities to exposure. The recommended classification uses four categories.

3.1.1 Four categories of responsivity

The recommended categories of responsivity to exposure are shown in Table 3.1. Apart from the irresponsive category, the materials described are mostly natural materials and traditional pigments. This table is necessarily a simplification. As is explained in the following subsection, pigment fading is a complicated topic and it takes skill to reliably identify a pigment. Synthetic materials are generally more difficult to identify than natural materials.

Synthetic materials add another level of complication. Polymeric substances form the basis of modern plastics, textiles, rubber, paints, varnishes, adhesives, pigments and dyes. In their pure form the polymers are generally colourless and quite stable at room temperatures, but invariably they are combined with other substances to give them particular properties. Some plastics and synthetic rubbers appear to "sweat" when placed on display, and this is because added plasticiser is leaching out of the material. Added pigments may fade while the base material is relatively unaffected. Some synthetic materials become "chalky" when exposed, and this is separation of filler that has been added to the material. It might actually be chalk.

Conservators have made substantial advances in identifying materials through use of non-destructive infrared spectroscopy, and where there is doubt and the consequences of error are serious, the advice of a professional conservator should be sought. Even so, many lighting decisions will be made without this advanced technology being applied, and both lighting designers and conservators will have to look elsewhere for guidance. Hebblethwaite (1986) gives extensive information on artists’ materials and their relative responsivity to exposure.

Table 3.1 Four category classification of materials according to responsivity to visible light.

Category Description

1. Irresponsive The object is composed entirely of materials that are permanent, in that they have no light responsivity. Examples: most metals, stone, most glass, genuine ceramic, enamel, most minerals.

2. Low responsivity The object includes durable materials that are slightly light responsive. Examples: oil and tempera painting, fresco, undyed leather and wood, horn, bone, ivory, lacquer, some plastics.

3. Medium responsivity

The object includes fugitive materials that are moderately light responsive. Examples: costumes, watercolours, pastels, tapestries, prints and drawings, manuscripts, miniatures, paintings in distemper media, wallpaper, gouache, dyed leather and most natural history objects, including botanical specimens, fur and feathers.

4. High responsivity The object includes highly light responsive materials. Examples: silk, colorants known to be highly fugitive, newspaper.



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3.1.2 Classifying pigments for responsivity

Pigments are a special concern for conservators because it often happens that the first visible sign of damage due to exposure is the deterioration of pigments, and pigments vary widely in responsivity to exposure. Identification of pigments requires the skills of a professional conservator or museum scientist.

The ISO rating based on the "blue wool" scale has been described in Section 2.1. Although this scale was devised for categorising materials such as clothing and furnishing textiles, several researchers have used it for classifying the sensitivities of artists’ pigments, and based on these studies, the responsivity classifications in Table 3.1 may be related to ISO ratings for pigments as shown in Table 3.2.

Table 3.2 Relationship of responsivity categories and blue wool categories.

Responsivity category ISO Rating

1. Irresponsive

2. Low responsivity

3. Medium responsivity

4. High responsivity

----

7 & 8

4, 5 & 6

1, 2 & 3

The skills of a professional conservator are needed to make the identification, and if

there is doubt, it is necessary to assume that the most responsive pigment likely to have been used is present. Table 3.3 has been developed by Michalski (1987, 1997) and is recommended for relating pigments to ISO ratings and estimates of probable fading.

Modern pigments offer artists a full palette of colours that has been developed to have high resistance to the effects of light exposure, and pigments that are rated ASTM D4303 Category 1, or Winsor and Newton AA, are all in the irresponsive or low responsivity categories of light-fastness. However, in the past, artists have used many pigments that are much more responsive to light exposure, and Table 3.3 shows examples of these.

Furthermore, the light-fastness of a pigment may be substantially affected by how it is applied by the artist. Indigo on wool is a low responsivity material (ISO 7), and there are many examples in museums of woollen tapestries where indigo is the only pigment that is not seriously faded. However, on paper, cotton or silk, indigo becomes a high responsivity material (ISO 3) and must be treated with great care if its rich blue hue has not already been faded.

The importance of relating illuminance to the light-fastness of the most susceptible pigment present is indicated by the "Mlx h for noticeable fade" data given in Table 3.3. Consider the case of a medium responsivity material with an ISO rating of 5 on permanent display (3000 hours per year), where the display illuminance is 50 lux and UV is eliminated. The annual exposure is 3000 hours x 50 lux = 150 kilolux hours/year. The Table indicates that noticeable fading is probable after 30 megalux hours of exposure, and this will occur after 30000/150 = 200 years of display. Suppose now that a highly responsive material with an ISO 2 rating is placed in the same display situation. Probable fading will occur after 1 megalux hour of exposure, and this will occur in 1000/150 = 6,7 years. It is for this reason that the "highly responsive" category has been included in Table 3.1, and it is recommended that materials in this category are not placed on permanent display.



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21

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CIE 157:2004

It should not be supposed that because a material is classified as having low responsivity, it does not need to be protected from UV exposure. Table 3.3 shows that as the ISO rating increases, there is increasing difference between the exposure for noticeable fading for "UV rich" and "no UV" lighting. This is because the more resistant materials tend to be much more resistant to visible radiation but, at best, only slightly more resistant to UV. It should be assumed that all organic materials are responsive to UV exposure, and even where the displayed objects are inorganic and irresponsive to UV, display support materials are likely to be affected by UV exposure. If it is decided that some areas of a museum will not be UV protected, museum staff must be alerted against placing responsive materials on display in these areas. Because this can be restrictive and difficult to administer, it is recommended that UV is eliminated throughout museums.

3.2 Procedure to control damage to museum objects

Museum staff should give careful consideration to developing a policy for conservation that is appropriate for the objects to be displayed. According to the scope of the collection, this may involve establishing more than one procedure to guide museum staff when designing and commissioning new exhibitions and when maintaining displays. The following procedure is an outline that may be adapted to meet circumstances.

The extent to which control can be exercised over environmental conditions, including lighting, is likely to vary from room to room within a museum. The first stage of developing a policy should be to classify zones within the museum according to their suitability for housing objects that require strict environmental control. These classifications should be taken into account when planning new exhibitions, with the aim of ensuring that the more responsive objects are located where appropriate control can be exercised. In the case of lighting, it is recommended that highly responsive objects should be grouped in locations where low ambient light levels can be maintained. Direct visual contact with areas that have significantly higher illuminances adversely affects visual adaptation, and should be avoided. While it is to be expected that exhibition designers will want to group objects according to their context in the sequence of the exhibition, conservation concerns should not be disregarded.

3.2.1 To minimise exposure of museum objects

The aim is to achieve the designer's objectives for effective display with minimum exposure of the objects. The first aspect to examine is the duration of exposure, and switching controls should be arranged so that the display lighting is in use only when required (Ezrati, 1994). There should be alternative lighting which does not direct light onto the objects available for cleaning, and if needed, for security. In some instances, it may be practical to employ motion detectors (Ginthner & Rummel, 1997), or viewer-operated time switches to restrict use of lighting during the museum's opening hours, or dynamic lighting which is cycled brighter and dimmer, so that the average must be used in calculating the exposure.

Once the necessary duration of light exposure has been determined, the next step is to minimise irradiance. To achieve this, start by ensuring that objects are protected from non-visible radiant flux, both UV and IR. The role of UV in causing damage has been discussed in Sections 1.2 and 3.1. It is practical to virtually eliminate all radiant flux of wavelengths shorter than 400 nm, and generally, this is recommended.

UV blocking filters are available for every type of light source used for museum lighting. Organic filters are available in various grades of clear acrylic sheet; as plastic tubes to be placed over fluorescent lamps; as plastic interlayers for laminated glass; and as varnishes for coating skylight and window glazing. It is generally recommended that the filter is applied to the light sources and the window glazing, so that UV control is provided throughout the space, although in some instances it will be appropriate to use clear sheet UV-blocking materials for picture glazing or display cabinets. Mineral filters, in the form of dichroic coatings on hard glass, can withstand high temperatures and are preferred for use with spotlights, although acrylic plastic may be a more economical alternative despite needing regular replacement in this application.

Recently some lamps with quartz envelopes (halogen and metal halide) have been marketed with "UV STOP" or "UV BLOCK" labels. Examination of the spectral emissions of these lamps has shown that UV output is significantly reduced compared with conventional



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lamps, but that it is not reduced to the level recommended in this document. One difficulty is that the lighting industry considers that the visible spectrum extends to 380 nm, and that "UV" applies only to wavelengths shorter than this value, whereas the museum community insists that the UV spectrum extends up to 400 nm. A lamp that is claimed to "eliminate 99% of UV" may have significant emission in the 380 nm - 400 nm waveband. The performance of these lamps may improve, but at the time of writing they do not provide the recommended level of UV control, and UV blocking filters should be applied. Nonetheless, a general policy of using these lamps may be advantageous. Should a UV-blocking filter deteriorate or be accidentally omitted, less damage will result if this type of lamp in use. Their use in non-gallery areas will lower overall UV levels, and if a light-responsive material is mistakenly located in one of these areas, again, damage will be reduced.

Some fibre optics systems claim to achieve elimination of UV. Conservators should require that manufacturers provide evidence of such claims. Both glass and plastic fibres eliminate short wavelength UV (<315 nm), but do not necessarily eliminate UV in the 315 nm - 400 nm waveband. Some systems using plastic fibres incorporate UV blocking filters to protect the fibres from degradation, but it is up to the conservator to ensure that level of UV control meets the recommended standard. Many fibre optic illuminators use metal halide lamps, and these are powerful sources of UV.

If it is decided to use a halogen lamp without a glass UV filter, it is recommended that a cover glass is used. Some halogen reflector lamps have an integral glass cover, but otherwise a separate cover glass must be installed. This has two useful effects. It blocks short wavelength UV, which although produced in very small quantities by the halogen source, is freely transmitted through the quartz lamp envelope and has high damage potential. Also, it can happen (rarely, fortunately) that halogen lamps shatter. As they operate at high internal pressure, the resulting shower of hot particles is another potential conservation hazard, as well as a safety hazard for staff and visitors. Although some manufacturers claim that their lamps do not require cover glasses, it is a prudent policy to use them for all halogen lamps.

Accurate measurement of UV is a notoriously difficult technical problem. Although portable UV meters are available, there is no single type of detector that responds to the entire range of the UV spectrum, and even within its stated wavelength range, the response of a portable meter is likely to depart significantly from linearity. As has been explained in Section 2.1, the spectral sensitivities of museum materials are distinctly non-linear, so that no correspondence can be expected between a UV detector response and the probable photochemical response of a museum object.

This document recommends UV elimination, and so the need is for reliable detection of UV rather than accurate measurement. UV levels of less than 10 µW/lm are difficult to detect, and so this may be taken as the practical limit for control. Where radiation has passed through glass (not quartz) it is safe to assume that all wavelengths shorter than 315 nm have been blocked, and the need is for a UV responsive instrument that can be relied upon to detect even low levels of radiation throughout the UV-A spectral region, this being the range 315 nm - 400 nm.

It is recommended that wherever conservation is a concern, every source of light should be equipped with an appropriate UV filter. The conservator should check the effectiveness of these filters, which involves not only checking new installations, but also making periodic checks that filters are in place and that their performances have not deteriorated, as loss of performance as a consequence of ageing has been recorded (Ezrati, 1987). Treatments on glazing that is exposed to sunlight are particularly susceptible, and the condition of these and other types of filter can be checked with a UV-A meter and a UV-blocking filter of known performance. Measurements are taken with and without the filter in question, and compared with "with" and "without" measurements for the known filter. It may be noted that it is not uncommon to find that portable UV meters have some response in the visible spectrum (i.e. wavelengths longer than 400 nm).

It is recognised that many museums show little concern for controlling IR exposure, and it is acknowledged that the radiant heating is generally a less serious source of damage than photochemical action. Even so, the cyclic effects of IR exposure are detrimental (Section 1.3) and are not easily distinguished from the effects of photochemical action, so that museum staff should be aware that this is a potential source of damage. Furthermore, the



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main source of concern is incandescent lamps, which are often thought of as being the safe choice for museum lighting. Significant radiant heating may occur where these lamps are mounted close to museum objects, as in display cabinets, or where a spotlight luminaire concentrates both visible light and IR onto an object. Lamps with dichroic reflectors reduce the problem by allowing more than 50% of the IR to pass through the reflector so that it is not reflected into the beam, and "hard" dichroic reflectors should be specified as they retain their performance better than the older type of soft-coated dichroic lamps. If "hard" dichroic reflector lamps of suitable performance are not available, glass IR filters are generally recommended for spotlighting organic materials.

Total elimination of IR is not practical, and direct measurement is difficult. It is possible to obtain a measure of relative radiant heating effect by placing small black and white metal plates at the display location for a short time and measuring their temperatures with an infrared thermometer. The temperature of the white metal plate will be close to the ambient air temperature Ta, and the extent to which the temperature of the black plate exceeds this value gives an indication of the extent to which Tmax could exceed Ta for a material of high absorptance (Equation 1.7). Alternatively, a simple test is to place one's hand between the spotlight and the object and to sense the heating effect on the skin. Section 1.3 discusses materials that are most likely to be affected by cyclic radiant heating, and museum staff should be aware that there are means of controlling IR exposure.

3.2.2 Use of electronic flash

Many museum visitors like to record their visits by taking their own photographs, and most of them use cameras that have built-in flash devices that fire automatically in indoor lighting. This preference of visitors has to be weighed against possible damage to museum objects.

To expose photographic film of ISO 100 rating with a lens aperture of f/8, an electronic flash produces an exposure of approximately 600 lx s (lux seconds) on the object being photographed (Saunders, 1995). This energy arrives at the object within a very short period, typically in the order of 0,001 s, and this has given rise to concern for biphotonic processes, which occur when two photons arrive at the same molecule in such rapid succession that their effect is combined. The possibility of such two-photon processes being caused by photographic flash exposure has been examined by Schaeffer (2001) and found to be very unlikely. Accordingly, it may be assumed that reciprocity applies, so that an exposure of 600 lx s is equivalent to 0,17 lx h.

The limiting annual exposure for a medium responsivity object is 150000 lx h/y (Table 3.4), and one flash represents approximately one millionth of this value. It can be seen that a single electronic flash subjects the receiving object to only a very small degree of exposure, but the effect of multiple exposures is cumulative. For a medium responsivity object that is on display for 3000 h/y, the limiting exposure equals 300 flashes per hour. This means that if the display lighting is already subjecting the object to the limiting exposure, just 30 flashes per hour would cause the total exposure of the object to exceed the limiting level by ten percent.

High responsivity objects should not be exposed to more than 15000 lx h/y. If this is achieved by illuminating to 30 lx for 500 h/y, it would take only 18 flashes per hour, or one flash every three and a half minutes, to cause the limiting exposure to be exceeded by ten percent. It would seem very likely that this flash rate would be exceeded for popular exhibits in public museums.

It is clear that a museum’s efforts to control the light exposure of exhibits risk being jeopardised if there is no restriction on the use of photographic flash. While this particularly concerns popular exhibits in the medium or high responsivity categories, it may not be practical to restrict flash use to certain objects or locations. In these situations it is recommended that flash use not be permitted in museums. It may be noted that other reasons are often cited for enforcing a flash ban. Flash use can be disturbing to other viewers, and in any case, for objects that are displayed behind glass, an image taken with a built-in flash is likely to be unsatisfactory due to the reflected image of the flash.

These comments refer only to use of flash by museum visitors. Conservators often recommend use of flash for professional photography of museum objects, as the object can be set up under normal room lighting and subjected to the photographic lighting for only very short duration (Schaeffer, 2001; Sancho-Arroyo and Rioux, 1996).



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3.2.3 Exposure rate

Now the designer or conservator can turn their attention to the central issue: how to achieve the display objectives with the minimum of radiant flux incident on the objects. By this stage of the procedure, the necessary duration of exposure has been determined, and steps will have been taken to eliminate UV and, if necessary, to control IR. The remaining decisions that affect conservation concern display illuminances and the spectral distribution of the lighting.

It is common practice for museum lighting designers to select continuous spectrum light sources of high colour rendering index, and to choose the correlated colour temperature to suit the overall appearance of the display and its setting. The use of discontinuous spectrum sources is discussed in Section 2.2. While colour rendering is an important design decision, it does not affect conservation considerations. Generally CIE Colour Rendering Group 1A will be specified (CIE, 1995), but if Group 1B is specified for some locations, this does not affect the exposure rate.

The effect of increasing damage potential with increasing correlated colour temperature of lighting is shown in Figure 2.5, and this effect can lead museum staff to discriminate against daylight and to justify general use of low colour temperature light sources as the preferred choice for conservation. Such lighting can be very effective for some types of display, notably rare books and old manuscripts where some yellowness of appearance is not objectionable. However, a higher colour temperature must be provided where a whiter colour appearance is required. It is recommended that decisions on the colour temperature of lighting should be made with concern for the visible characteristics of the objects on display and the setting in which they will be seen. Where the viewing conditions call for moderate or high colour temperature lighting, conservation concerns should not override design objectives for the display. If necessary, the duration of exposure should be restricted rather than the visual qualities of the display be compromised. It should be borne in mind that low colour temperature artificial lighting is likely to be judged unsatisfactory where it is seen in combination with daylight. This occurs because the eye is adapted to the higher colour temperature of daylight. For an observer who is fully adapted to the low colour temperature artificial lighting, the appearance may be quite satisfactory.

The accepted practical measure of exposure rate is illuminance. Museum staff must never overlook that lux readings can grossly understate the exposure rate where UV control is lacking or ineffective. Also it can be seen from Figure 2.5 that even where UV is eliminated, illuminance cannot be an entirely reliable indicator of exposure rate.

The data in Table 3.4 take account of the recommendations of various authorities (AFE, 1997; CIBSE, 1994; IESNA, 1996) and provide initial guidance on exposure rates. Consider the limiting illuminances. There is no reason to restrict the exposure of irresponsive materials on account of conservation concerns, but in practice illuminances have to be considered in the context of exhibitions that include responsive objects. With proper control of the surrounding environment, 200 lx is generally sufficient to provide for adequate visibility and for object appearances that will satisfy exhibition design objectives (Loe et al., 1982), and it is recommended generally that object illuminances should not exceed this value. In cases where illuminances below 200 lx are required, visibility of the exhibit can be enhanced by lighting the background to a lower level. This has the effect of reducing visual adaptation and making the exhibit the brightest part of the field of view. It has been suggested that a ratio of 3:1 for object illuminance to background illuminance be used (Loe et al., 1982).

Medium responsivity materials require more care. Sometimes satisfactory viewing can be achieved with less than 50 lx, particularly if the object is light in colour and does not contain fine detail, and advantage should be taken of these situations. However, for some objects, particularly those that are dark in colour, it may not be possible to achieve a satisfactory appearance at 50 lx. Even so, the limiting illuminance should never be quoted as the justification for unsatisfactory display. It is thoroughly bad policy to place an object on display, where it inevitably will suffer some damage, and to fail to present it adequately. Where an illuminance greater than 50 lx is found to be necessary to provide for a satisfactory appearance of an object that is composed, even in part, of a light-responsive material, the duration of display should be restricted to comply with the limiting exposure value. It is recommended that materials classified as having high responsivity are not placed on permanent display.



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It is easier to achieve control over lighting where daylight is eliminated. However, there are some museum objects, particularly art objects, for which the presence of daylight forms part of the total experience. This consideration has led to some elaborate and technically sophisticated installations that automatically respond to daylight variations. There have been reports of severe operational problems associated with such installations (Cannon-Brookes, 2000), and some designers have recently advocated hybrid strategies that combine passive control elements with active devices (Sedgwick and Shaw, 2000). For damage control procedures to be effective, it is necessary not only that they are based on sound scientific principles, but also that their operation and maintenance is within the technical competence of the museum staff.

Table 3.4 Limiting illuminance (lux) and limiting annual exposure (lux hours per year) for material responsivity classifications.

Material classification Limiting illuminance (lx)

Limiting exposure (lx h/y)

1. Irresponsive 2. Low responsivity 3. Medium responsivity 4. High responsivity

no limit 200 50 50

no limit 600000 150000 15000

3.2.4 Outline of procedure

It is the intention of this document that the conservator and the exhibition designer should use Table 3.4 as a guide for ensuring that all of their concerns are taken into account. Finally, Table 3.5 gives the outline for a practical procedure for control of museum lighting. Museum staff may use this as a guideline for establishing working procedures and ensuring that responsibilities are appropriately allocated. In particular, the procedures to occur during the life of the display will be very much dependant upon the nature of the museum objects, the type of lighting being used, and whether the life of a display is measured in weeks or years. All museums are encouraged to check their procedures against this outline, and to assess whether they are taking adequate steps to avoid unnecessary damage to their collection.



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Table 3.5 A practical procedure for control of museum lighting.

When setting up lighting for a new display:

(a) Classify all exhibits according to the four-category scale given in Table 3.1.

(b) Install UV filters on all light sources, including windows and skylights, and check each source with a UV meter to ensure that UV is below the detection threshold (UV <10 µW/lm).

(c) Focus the lighting, and visually assess the effect of reducing display illumination with the aim of ensuring that illuminances are no greater than is necessary to satisfy display objectives. Check illuminance values. The limiting illuminance is the maximum illuminance at any point on the exhibit's surface.

(d) Check the radiant heating effect for each object, particularly where incandescent filament spotlighting is in use. If radiant heating effect seems to be significant, consider use of dichroic reflector lamps or IR filters.

(e) Check controls and procedures for restricting the duration of display lighting. Estimate annual hours of exposure.

(f) Measure and record illuminances for each object or group of objects. Calculate annual exposures and plan for the duration of display to be restricted as necessary, both for the exhibition and for individual objects at risk.

During the life of the display:

(g) Periodically check the lighting with a UV meter, and replace filters where necessary.

(h) Periodically check radiant heating effect and reduce IR if necessary. (i) Periodically check illuminances, and adjust if necessary. (j) Check that procedures for restricting the duration of display are operating

satisfactorily, both for the exhibition and for individual objects at risk.



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CANNON-BROOKES, S., 2000. Daylighting museum galleries: a review of performance criteria. Lighting Res. & Technol., 32(3), 161-168, 2000.

CIBSE, 1994. CIBSE Lighting Guide LG08 – 1994. Museums and art galleries. Chartered Institution of Building Services Engineers, London, 1994.

CIE, 1986. CIE 15.2-1986. Colorimetry. 1986.

CIE, 1991. CIE 89/3-1991. On the deterioration of exhibited museum objects by optical radiation. 1991. In CIE Technical Collection 1990.

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CUTTLE, C., 1988. Lighting works of art for exhibition and conservation. Lighting Res. & Technol., 20(2), 43-53, 1988.

CUTTLE, C., 1996. Damage to museum objects due to light exposure. Lighting Res. & Technol., 28(1), 1-10, 1996.

CUTTLE C., 2000. A proposal to reduce the exposure to light of museum objects without reducing illuminance or the level of visual satisfaction of museum visitors. J. Am. Inst. for Conservation, 39, 229-244, 2000.

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EZRATI, J-J., 1987. Propriétés anti-UV des films de sécurité. In Preprints ICOM CC - Sydney, The Getty Conservation Institute, 871-874, 1987.

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cie 157 2004 (control of damage to museum objects by optical radiation)

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