visibility of approach and runway lights - … bound...visibility of approach and runway lights ......

R 168 Philips Res. Rep. 6, 224·239, 1951

VISIBILITY OF APPROACH AND RUNWAY LIGHTS

by J. B. de BOER 628.971.8

SummaryIn this paper the physiological data governing the visibility of steadylights are surveyed. For circular sourcesof light extensive informationcan be obtained from the results of observations published by severalinvestigators. The influence of the shape of the sources and t.hat ofneighbouring light sources has been empirically studied for rectangu-lar sources and for sources arranged in rows. Further a brief accountis given on what is known regarding the influence of the colour of thelight. All these influences can be allowed for by applying correctionfactors. To this end "size", "shape", "row" and "colour" factors havebeen introduced. The threshold values found in the laborator y have tobe multiplied by a safety factor in order to get data suitabl.e for cal-culating the luminous intensities required in order that a pilot inactual flight may be able to see the lights.

RésuméDans eet article, on passe en revue les données physiobgiques quirégissent la visibilité des feux permanents. Pour les sources de lu-mière circulaires, on peut tirer de nombreux renseignements desrésultats d'observations publiés par plusieurs cherchcurs. L'bfluencede la forme des sources et celle des sources lumineuses voisine s a étéétudiée cmpiriquement pour des sources rectangulaires et pour dessources disposées en files. En outre, on donne un bref aperçu desconnaissances actuelles concernant l'influence de la couleur sur la'lumière. On peut tenir compte de toutes ces influences en appliquantdes facteurs de correction. Dans ce but, on a intro duit les facteurs"dimension", "forme", "rangée" et "couleur". Les valeurs de seuiltrouvëes au laboratoire doivent être multipliées par un facteur desëcuritë afin d'obtenir des données convenables pour calculer lesintensités lumineuses requises, afin qu'un pilote puisse voir les feuxpendant Ie vol.

ZusammenfassungIn dieser Arbeit wird eine Übersicht der physiologischen Daten he-züglich der Sichtbarkeit ruhender Lichtquellen gegeben. "Oberkreis-förmige Lichtquellen können die von mehreren Untersuchern ver-öffentlichen Beobaehtungsergebuisse genaue Aufklärung verschaffen.Es wurde der EinfluB der Form der Liehtquellen und der benach-barter Liehtquellen empirisch untersueht für rechteckige und inReihen angeordnete Lichtquellen. Ferner wird kurz berichtet, wasüber den EinfluB der Farbe des Lichtes bekannt ist. Alle diese Ein-flüsse können dureh Anwendung von Korrektionsfaktoren für"GröBe", "Form", "Reihe" und "Farbe" berücksichtigt werden.Die im Laboratorium gefundenen Schwellenwerte sind dazu noch miteinem Sicherheitsfaktor zu multiplizieren, um geeignete Daten zuerhalten zur Berechnung jener Leuchtdiehten, welche erforderlichsind, damit ein Pilot während des Fluges die Scheinwerferlichte zusehen vermag.

Introduetion

Duriug the last five years or so much work has beeu done on the subjectof approach and runway lighting. Very ingenious configurations of the

VISIBILITY OF APPROACH AND RUNWAY LIGHTS 225

lights have been worked out in order to arrive at a system of lights thatunder all possible weather conditions gives the pilot of an approachingairplane complete indications enabling him to determine his positionrelative to the runway at every moment of the approach. Up to now mostattention has been paid to the problem of the most efficient configuration.Only a very carefully planned configuration of approach and runwaylights can give a pilot all the indications just mentioned. It is not intendedto deal with this problem here. To our mind, Calvert's last publicationon this subject 1) gives an excellent idea of what this problem involvesand of the direction in which a solution can be found which satisfies allthe requirements in this particular field of lighting.A problem subordinated to that of the configuration of approach and

runway lights but undoubtedly of very great importance in this lightingproblem as a whole is that of the luminous intensity of the lights usedand its proper distribution in space. Valuable information regarding theselight-technical questions can be derived from many publications, but thefundamental data required by a designer of airfield lights have to be collectedfrom a number of sources and even so he \vill still not find everythinghe needs. For instance, such problems as the influence of the size and theshape of the lights under consideration and that of neighbouring lightson the visibility of the former have, practically speaking, so far beenneglected. In this respect mention is to be made of Kevern's article 2)which is intended to give an indication of the direction in which thesolution of this part of the problem is to be sought and which, as such,does not give a final solution.In this paper a number of physiological data of importance in relation

to the visibility of lights have been collected from various publications.Regarding the influence of size, shape and surrounding light sourceson the visibility we have carried out some experiments in which the mostimportant conditions to be encountered in airfield lighting have beenconsidered (lights having a circular and a rectangular apparent shapeand lights placed at different spacings in rows). In this way a number ofdata have been obtained which a lighting engineer should have at hisdisposal to be able to calculate the spatial distribution of the luminousintensity of approach and runway lights.

Some physiological data governing the visibility of approach and runwaylights

Under conditions of bad visibility only a small part of the whole systemof approach and runway lights will be visible to the pilot at each moment.In the designing of the configuration of the lighting system this restrietionshould be taken into account. On the other hand the lights should be

226 J. B. de BOER

so designed that the visibility of tros particular part' of the system isassured under the conditions for which-the approach lighting system isintended. This is a matter of intensity and distribution of the individuallights.

A light is visible to an observer when the illumination produced bythe light on the observer's eyes exceeds a certain minimum value Em:n'

This minimum eye-illumination depends on:

(a) the background brightness B;(b) the size of the source;(c) the apparent shape of its light-emitting surface;(d) the influence of light sources in the neighbourhood;(e) the colour of the light;(f) the character of the light;(g) the time available for observing the light;(h) the general conditions of observation (in how far the observer's

attention is concentrated on his observing task, the relative movementof the lights in the visual field, whether or not the place of the lightsin the visual field is known to the observer, etc.).

The above factors influencing Emin are all of a physiological nature.The question whether or not a certain.light produces the required mini-mum eye-illumination Emin (in lux) depends on its luminous intensity I(in cd), its distance r from the observer (in m) and the transmission tv perm by the atmosphere, according to the formula:

I rEmin = _·tv•r2

From this it is clear that to be able to calculate the light distributionof approach lights we need quantitative information on a number ofphysiological factors as well as on some physical factors. The latter willbe treated in a further article on this subject.

Re (a) and (b). The visibility of light sources of circular shape

Many investigators have experimentally determined the lowest bright-ness of a light source of a circular shape at which the light source canjust be perceived. This has been done for different brightnesses of thevisual field and for different sizes of the light sources. Other investigatorshave measured the lowest brightness difference that can be just dis-cerned between a stimulus of circular form and its surrounding field,this also for different sizes of the stimuli and for different brightnessesof the surrounding field.


From the results of these investigations Emin can be calculated as afunction of the apparent size of the light source and of the brightnessof the visual field when Emin is' the eye-illumination required for justperceiving a circular light source.

Bouma 3) collected and presented in a surveyable form a large numberof data published up to 1941 concerning the visibility of circular light

/ö· _'I-I-I--+I+1+Il-Ill__ I--II-+--lIf--l-rlH-++-_+---+-!-+++++II~-I111II I 11 ~ ./I---l--I-~I+1+ll-Ill----~-l--I-~T+T++I----+--l---l-++++~I--I--+-~~w_---L~~-L~~----+--l--I-~,~~·~I--~1--1--1--1------ B/ackwe//

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5 2 5 100 66336- Cl(; in minutes

Fig. 1.Minimum illumination on the observer's eye Emin (in lux), due to the light sourceonly, required for perceiving a light of circular shape through an atmosphere with abrightness B (in cd/m2) and the source at a distance greater than the metereologic visibility.Emin is plotted for different values of B, as a function of the diameter of the sourceexpressed in the angle a (in minutes) subtended by that diameter at the observer's eye.

10

227

228 J. B. de BOER

sources. Since his publication new observations have heen carried outby Schönwald 4), Knoll, Tousey and Hulburt 5), Blackwell ") and Hill 7).Here in figs 1 and 2 the data have been collected which can be calculatedfrom the results given in the publications just mentioned in connectionwith the relation between Emill' the background brightness B and theangular size of the light source a (angle subtended at the observer's eyeby the diameter of the source). Fig. 1 also includes the results obtainedby Weigel and Knoll 8) already mentioned in Bouma's 3) survey. Schön-wald's data were taken from his fig. 2 (page 17).

____ extrapolated from Blackwell's resulls6) /1(i3 I--+---+-l--- Bouma 31 I-l---I--l-~-l

___ Hill71 /_~ _ •._.. Schönwald 'J 5J ~ "*'.f:

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-- Background brightness in cdfm2

66337

Fig. 2. Minimum illumination on the observer's eye Emin (in lux) required for perceivinga point source of light, as a function of the background brightness (in cd/m2) againstwhich this source should be seen.

To avoid possible misunderstanding about the value of Emin, plotted infig. 1, the following explanation is to be added. The general laboratorypractice is to ask an observer to perceive a light source in a visual field witha homogeneous brightness B. He can see this light source if its brightnessis at least equal to Bs. One measures the minimum illumination Bs w,at the observer's eye required for seeing the source, w being the solidangle of the source subtended at the observer's eye. The results of thistest have to be applied for the conditions under which light sources areperceived in fog. The apparent brightness of the light source required forseeing it in fog is again Bs, where B is the brightness of tb e fog. The brightness


Bs is now equal to the sum of the brightness B of the fog and the brightnessB L of the source, the latter multiplied by some transmission factor t.So the light source itself gives at the observer's eye an illumination equalto tBLw. To he able to see the source this illumination must be greaterthan the value of Emill plotted in fig. 1 for different values of the brightnessB of the visual field.

For large light sources the eye-illumination as required for our purposecould not be very accurately calculated from Schönwald's curves becauseSchönwald's method ofpresenting the results does not allow ofthe differencebetween the brightness of the source and the brightness of the surroundingfields being determined with sufficient accuracy. The results of Knollc.s. (only valid for point sources, see fig. 2) have been given by theirequation (expressing the results within a factor 3 over the entire range):

Emill = 1.076.10-9 (1+ 3.14.105 Br"in which Emin is expressed in lux and B in caJm2•

As Blackwell's results have been given not only in curves but alsonumerically, the functions Emin = f(B) could be calculated for the angularsize of the stimuli applied by Blackwell in his experiments.

From these functions the curves given in fig. 1 (Emin as a functionof a) could be determined. The smallest value of a used by Blackwellis 3.6'. The curves representing Blackwell's results have been extrapolatedin fig. 1 downwards to 0·2'. It seems that this could be done with a quitesatisfactory degree of accuracy since the points of these curves apper-taining to the values of a applied in the original experiments could veryeasily he connected by smooth curves. From these extrapolations wedetermined the curve in fig. 2, representing the function Emin = f(B) forpoint sources of light, as derived from Blackwell's investigations.

Fig. 2 shows a very good agreement between Bouma's curve and thecurve derived from Blackwell's results. Schönwald's results agree verywell with those of Bouma above B = 10-1 cdJcm2• The original observa-tions of Knoll c.s. come closer to Bouma's and Blackwell's curves thantheir approximate equation does. Only' Hill's results deviate somewhatmore from Bouma's and Blackwell's curves due to the fact that Hill con-sidered 'only foveal observations.

Summarizing the results given in fig. 2 it may be said that Bouma's curve.still holds very well for the relationship between Emin and B. If greataccuracy should he required the curve derived. from Blackwell's workmight better he taken for values of B lower than 10-2 cdJcm2• The mostimportant observations of a pilot during the approach procedure aredone foveally. Therefore since the curves repreaenting the results ofBlackwell, Bouma and Knoll c.s. are valid for parafoveal observation at

229

230 J. B. de BOER

the lower background brightnesses we consider Hill's curve, which con-firms fairly well Schönwald's results, to be the most reliable'for the problemof visibility of approach _lights.

Regarding the functions Emin = f(a) for different values of B given infig. 1, the curves interpolated from Blackwell's results seem to be themost reliable, not only on account of what has already been mentionedbut also in view of the very large number of observations carried outby this investigator. Especially for the lower values of background bright-ness, however, it should not be forgotten that Blackwell's results are basedupon parafoveal observations. Bouma already stated that for large valuesof a his curve will give too high values for Emin'

From the curves given in fig. 1 a "size factor" can be calculated, beingthe factor by which Emin, found for a point source of light at a certainsurrounding brightness, has to be .multiplied in order to find the eye-illumination required for perceiving a finite source at the same surroundingbrightness. Fig. 3 gives this size factor as a function of a for differentvalues of B. These functions have been calculated from' the curves ofBlackwell in fig. 1, whilst the results of some observations of our ownhave been added (each point of the latter curves represents the averageof 15 observations of 5 observers with normal eyes; for further detailsabout the measuring procedure, see below).The agreement with Blackwell's results is rather good.The curves of fig. 3 confirm in the first place Ricco's law: For very

smalllight sources (a < 0'1 minute) at a certain value of the backgroundbrightness B there is a constant threshold value of the eye-illuminationwhich determines the visibility of the source.If the light source becomes' larger this eye-illumination increases,

whereas with very large light sources it is only the brightness of the sourcewhich determines its visibility. As eye-illumination Emin and sourcebrightness Bs are related according to E = coBs, where co is the solidangle of the light source subtended at the eye, at a constant value of thebrightness of the source the eye-illumination is proportional to co. Itfollows that, as very large light sources are visible when a certain constantvalue of Bs is exceeded, the curves giving the size factor in fig. 3 will tendto show the same slope as a curve giving coas a function of a. A comparisonof the slope of the line co= 2:n: [1- cos (a/2)] in fig. 3 with the maximumslope of the curves giving the size factor for different values of the back-ground brightness confirms very well what has just been said about thebrightness of large light sources determining their visibility. At highervalues of the surrounding brightness B the maximum slope is attainedat smaller values of a. This can be understood from the mutual couplingof the elements of the retina, which is the more extended the lower the


background brightness. Thus the lower the background brightness themore elements of the retina are coupled and acting as one unit. Thisdiminishes visual acuity and implies that a light source must have greaterapparent size before it can be perceived as a source of finite size whichas such can be seen with a certain brightness. It may he concluded thatfor light sources of circular apparent shape the threshold value of theeye-illumination can he found from Hill's curve given in fig. 2 after correc-tion by an appropriate size factor according to the curve in fig. 3 derivedfrom Blackwell's work,

Re (c). The inHuence of the shape of a light source on its visibility

The apparent surface of approach lights is not always circular in shape.

66338

Fig. 3. "Size factor" at different values B of the background brightness (in cdJm2), asa function of the angular sizeof a circular light source expressed in the angle a (in minutes)subtended by the diameter of the source at the observer's eye. The "size factor" is definedas the eye-illumination, due to the light source only, required for perceiving a light sourceof circular shape and finite size when seen through an atmosphere with a brightnessB (in cdJm2) divided by the eye-illumination required for perceiving a point source oflight under the same conditions.The curves giving these functions tend to show the same slope at large values of a as thatof the curve giving the solid angle t» subtended by the source as a function of a. The fullydrawn lines between 3'6' and 121' represent factors derived from Blackwell's results 6).

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5 2 5 ID 2 5 ID 2 5--- oe; in minutes

232 J. B. de BOER

Often rectangular forms are applied, e.g. when gas-discharge tubes areused as light sources, whilst in other cases a number of lights placedclosely together have to act as a single lighting unit, either in order toobtain a high candle power of the unit as a whole or in order to get aparticular form (e.g. a complete slope-light of the slope-line system, beinga pole with a number oflamps on it at equal and rather narrow spacings *).

It is not sure whether the data given in the figures 2 and 3 may beapplied to rectangular light sources where one dimension is many timeslarger than the other or to lighting units composed of several single lights.In order to get information about this we have carried out some ohser-vations with an arrangement given schematically in fig. 4.

66339

Fig. 4. Scheme of testing arrangement.

An observer is placed at a distance of 2·5 m from a sheet of mirror-glass1 m square. Only the side of the glass facing the observer has the normalregular reflection of a glass sheet; the back is painted with white paint andhas therefore lost its regular reflection proporties. Thus, through the regularreflection of the front of the glass at a distance of 5·5 m the observersees the mirror-image of the diaphragm in a special light-box L, thebrightness of which is thereby reduced. to about 4% of its proper value.This image is seen against a background formed by the light from a numberof floodlights F reflected diffusely by the layer of white paint on the backof the glass sheet.

By means of a special arrangement of screens in the light-box L thebrightness of th~ opal glass immediately behind the diaphragm can be variedover a wide range without affecting the colour of the light. Several dia-phragms provided with apertures of different shape and dimensions can

*) For a description of the slope-line system see the publication mentioned under reference-),


be placed in front of this opal glass. For each observation the brightnesswas gradually increased, starting from a value not visible to the observer.The observer had to indicate the moment at which the stimulus becamevisible to him.

, '

As indicated in fig. 4, the mirror-glass could be tilted over a smallangle. Before each observation the setting of the glass was changed slightlyat random.

Consequently the observer did not know where the mirror-image of thestimulus would appear and he had to scan all over the mirror glass. Bythis procedure of observation it was certain that values for the requiredeye-illumination would be found considerably higher than those foundin tbe case where the observer knows beforehand where the stimuli shouldappear. However, our only object was to obtain comparative data and asour method of observation is more in line with the pilot's activity precedinghis perception of the lights we think the results of our testing procedureare more reliable than in the case where the direction of observation isfixed and known to the observer. Of course we had to accept a largerspread in the results and this necessitated a larger number of observationsin order to arrive at a satisfactory degree of accuracy.

Our results with circular sources of different diameter presented in fig. 3have likewise been obtained 'with the aid of this set-up.The experiments carried out to determine the influence of the shape

of the light source upon its visibility were confined to light sources ofrectangular shape either of uniform brightness or composed of a numberof lights of circular shape each in itself with a uniform brightness. Followingthis line of thought the slope-light of a slope-line system containing anumber of lights on a pole is considered as being a rectangular light sourcewith a length equal to the distance between the outer edges of the firstand the last light and with a width equal to the diameter of one light.We have investigated the influence of rectangles with a width subtending1·25' and 3·75' at the observer's eye and a length that was varied from avalue equal to the width to many times that value. These rectangles werein every case viewed with their longitudinal axis in the horizontal direction.Fig. 5 gives the average results of the'se observations carried out by 5observers with normal eyes. Again the results have been expressed asthe eye-illumination required in order to see the rectangles in relation to,the eye-illumination required to see a point source at the same backgroundbrightness (shape factor). The observations were carried out with a back-ground brightness of 300 cdfm2 only. Each point in fig. 5 represents theaverage of 15 observations of each observer. ,

In view ofthe facts that also long rectangles were included in our observa-tions, and that it is tedious work to calculate for these long light sources

233

234 J. B. de BOER

the required minimum hrightness of the source from the eye-illumination,we have also plotted in fig. 5 the curves showing the hrightness of theohserved rectangles as a function of the length of the source"'). These curvesprove that a similar statement as 'already given with respect to largecircular light sources can also he made hcre, viz. with very long rectangularlight sources and a certain hackground hrightness there is a constant value ofthe source hrightness~which determines whether the light is visible or not.

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~ w=375)1>' "'IQ~....~ ~4-,'

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i:2 5

- Length of the rectan9ular light source in arcmilutes subtended at the observer's eye

6634D

Fig. 5. "Shape factor" and minimum relative brightness required for sceing a rectangularsource of light as functions of the length of a rectangular source expressed in are minutessubtended by this length at the observer's eye. The width of the source expressed in thesame units is w.The "shape factor" is defined as the eye-illumination required for seeing a light source ofrectangular shape divided by the eye-illumination required for seeing a point sourceof light at the same background brightness.Background brightness for these investigations is 300 cd/m2.

Furthermore we carried out a numher of ohservations with rectangularlight units composed of several circular light sources. To explain what ismeant hy such units two specimen are given in fig. 6:(a) representing a slope-light in a slope-line system,(h) a unit of very high intensity at the heginning of an approach lighting

system **)

*)If a is the angle subtended at the observer's eye by the length of the rectangularlight source and fJ is the same angle related to the width of the source (expressed inradians) for not too large values of fJ the relation between eye-illumination E and.source brightness B, is E = 2 BsfJ sin (a/2).

**)One might be inclined to consider a light unit with large dimensions as shown schema-tically in fig. 6b as very inefficient. A point source may have a much smaller luminousintensity then a large one before it becomes invisible under the same conditions.But it is for the very reason that the dimensions of the light sources are perceivable andrecognizable to the pilot that he is able to derive therefrom important indicationsabout height and so on. That is why we are paying so much attention to extendedlight units.


We have considered a number of combinations of the kind given infig. 6 with different values of l up to 40' and some values of w up to 4'.Rectangles up to these dimensions were composed of circular sources insuch a way that, the distance between two adjacent sources was equal toor less than the diameter of the sources. With background brightnesses of10 and 300 cd/m2 the eye-illumination required for discerning such a corn- .posite light unit was found, to a good approximation, to be equal to thatrequired in the case of a rectangle with uniform brightness and the samedimensions land w. In other words, the total candle power of compositelight sources such as those considered here determines their visibility.

Thus the shape factors given in fig. 5 can be applied also to compositelight units of rectangular overall shape of the type given schematicallyin fig. 6, provided the dimensions are not too large.

~-o"ö-cnJ-aw I Q_.Q_p-n_o~

66341

Fig. 6. Examples of composite light units.

Re (d). The influence of neighbouring light sources

This influence has already partly been dealt with in the foregoing sectionfor cases of light sources arranged at such small distances that they canbe considered as acting together like a single composite light unit. When,however, we come to consider, e.g. the Calvert configuration of approachlights, then we have to deal with rows of lights. The centre line of thisconfiguration and also the rows of runway lights are seen under a sheeringangle, thus at very short apparent spacings of the lights in the row especial-ly during the' last phase of the landing. The row will certainly not beseen as a single light unit but on the other hand some sort of "mutualassistance" of the lights in the row can be expected. By observing differentrows of lights in the arrangement sketched in fig. 4 it has been possibleto gather some information about this influence of neighbouring lightsources.. Two kinds of rows were built up; one with circular light sources eachwith a diameter of 1'3' andthe other with light sources having a diameterof 3'9'. The lights had all the same brightness and were set at equal spacingsin each row. This spacing has been varied. The total length of the row:was the same in every case and subtended at an angle of 10 degrees to theobserver's eye. All observations have been carried out at a' backgroundbrightness equal to 300 cd/m2.

236 J. B. de BOER

The results of these observations have been given in fig. 7 where theyare expressed by a "row factor", i.e. the eye-illumination produced atthe observer's eye byeach fitting in the row and required for perceivingthe row, divided by the eye-illumination required for perceiving a singlefitting in the row if only this fitting were present in the observer's fieldof view. For these observations the observers were asked to indicatethe moment at which they perceived a row of lights of whatever length.A simple calculation shows that the slope of the-curves of fig. 7 for valuesof dj D between O'5 and 1 confirms very well the statement in the fore-going section according to which the total candle power of compositelight sources of rectangular shape with restricted dimensions determinesthe visibility of those light ?ources.

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66342

Fig. 7. "Row factor" as a function of the ratio of the diameter d to the spacing D of lightsof equal size and brightness placed in a row. The "row factor" is defined as the eye-illumi-nation produced byeach fitting in the row required for perceiving the row oflights, dividedby the eye-illumination required for perceiving a single fitting in the row if only thisfitting were present in the observer's field of view. These observations have been carriedout at a background brightness of 300 cdJm2•

Re (e). The influence of the colour of the lights

Many data have been published on the visibility of lights of differentcolour but in several cases the information given is contradictory. Itis known 'that the influence of colour depends on the background bright-ness and on the manner of observing. If only foveal observations arecarried out other results will be found than in the case of parafovealor mixed observations. For the problem of approach lighting the resultsof foveal observations seem to be the most valuable. For this reason weconsider Hill's results 7) as a good basis for this problem. The curves givenin fig. 8 for red, green and yellow light obtained by placing-colour filters


in front of incandescent lamps show the colour factor defined as the eye-illumination required for perceiving these kinds of light relative to theeye-illumination required for perceiving "white" incandescent light.

1.

a5 .' g....,~ -l"'-a I)'ellql'o

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~ points: Author's results (or red

1 - - - -

2.

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---tl» Background brightness in cd! m2 663U

Fig. 8. "Colour factor" as a function of background brightness derived from foveal obser-vations obtained by Hill 7). The colour factor is the ratio of the eye-illumination requiredfor seeing a coloured light to the eye-illumination required for seeing a white light.

These curves have been calculated from Hill's results obtained with pointsources. Here we confine our considerations to these rough indicationsof the colours. For more detailed information reference is made to theoriginal publication 7). As already mentioned, Hill carried out his ohser-vations with point sources only. In the set-up given schematieally infig. 4 some observations were carried out also at brightnesses of 10 and300 cdjm2 with white and red light sources having a diameter varyingfrom I' to 50'. The light emitted from the white sources was incandescentlight with a colour temperature of 2850 OK, the red light was obtainedby putting a Schott colour-filter type OG 3 in front of the white sources.No systematic influence of the size of the light source on the visibility ofthe red sources compared with that of the white ones could be ascertained.The average of our observations (100 at each background brightness)obtained from 4 observers with normal eyes is given by the two pointsin fig. 8.

The results represented in fig. 8 justify the conclusion that for theproblem of approach lighting the influence of the colour of the lights onits visibility is negligible. To a certain extent this conclusion is confirmedby the results so far obtained from experiments which are still proceedingat the Amsterdam airport (Schiphol) where a number of lights of different

238 J. B. de BOER

colour and brightness are being observed by meteriologists wheneverhaze or fog deminishes atmospheric transmission. Although the accuracyof these experiments is restricted owing to many uncertainties in theatmospheric conditions so far no systematic difference has been found inthe visibility of fittings provided with "white" incandescent lamps, sodiumlamps, neon lamps and mercury lamps all of the same luminous intensity.

Re (f) and (g). The character of the light and the time available for observing

In the foregoing the light has been presumed to be steady and all theinformation given is valid for infinite time of observation. Unsteadylights have very restricted possibilities of application for approach andrunway lighting. Probably they can only serve as "attention getters",e.g. when used for very strong flashing lights at the beginning of theapproach lighting system; when only flashing lights are used the informationthese convey to the pilot is very confusing.

Judging from the results of the experiments so far carried out withrespect to the visibility of periodically flashing lights with a flash durationshorter than about 0'02 sec, visibility is expected to be the same as thatof a steady source with a candle power equal to t-1 J ldt, where I is thecandle power of the flashing light source at a particular moment an~ tis the duration of one period in seconds. However, these experimentshave not been carried out with light sources giving such extremely shortflashes as those of modern gas-discharge flashlamps. Further experimentsare needed in order to gain more insight into this, but for the purposeof this study such information is not of such great importance.Observation time is certainly not unlimited during an approach. The

influence of this factor ca~ only be investigated with great difficulty.It can be considered as belonging to the general conditions of observation,which taken together cause a very great increase of the threshold valuesof eye-illumination required for seeing a light.

Re (h). The general conditions of ohservation

The threshold values of the eye-illumination mentioned in figs 1 and 2have been determined in the laboratory under conditions whereby theattention of the observers was fully concentrated on their task, good adap-tation to the background brightness involved in the tests was insured,unlimited or sufficient observation time was allowed, and so on. Thepilot, however, has to see the lights at moments of very great mentalstress, his mind being engaged with a number of flight-technical detailsdemanding most of his attention while his eyes are in many cases badlyadapted to the background brightness (e.g, coming down from clear orbright atmosphere into thick fog absorbing more and more of the daylight


the nearer he approaches ground, whilst, furthermore, observation timeis not by any means unlimited). Allowance for all these factors can onlybe made by applying a roughly estimated, "safety factor" of the orderof 100.Bouma 3) took the same factor for the purpose of black-outs. Whenwe apply this factor to Hill's curve given in fig. 2 it will be found that thevalues indicated by Rutgers 0), viz. 10-3 lux for daylight and 10-6 luxfor dark and clear nights, seem to be reasonable for the minimum eye-illumination required for seeing a light clearly under practical conditionsof flight during the approach to a runway.

Conclusions

The conclusions that may be drawn from the data collected in thispaper can be formulated as follows:

The least favourable conditions for seeing approach lights are presentat daytime in fog. Under these conditions the lights have to produce thehighest illumination at the pilot's eye. Under such atmospheric conditionsand practical conditions of flight a point source can be seen clearly whenit produces at least 10-3 lux at the pilot's eye. For circular sources offinite size this value for the eye-illumination should be multiplied by a"size factor" according to fig. 3. In the case of rectangular sources orgroups of lights arranged to form a composite unit of this shape a "shapefactor" according to fig. 5 can be applied. With such composite light unitsit is the total luminous intensity of all individual sources together whichdetermines the visibility as a whole. Lights arranged in a row are morereadily visible than single lights. Fig. 8 shows the "row factor" that canbe applied in such cases. The influence of the colour of the light is negligible.

Eindhoven, February 1951

REFERENCES

1) E. S. Calvert, Visual aids for landing in bad visibility with particular reference to thetransitionfrominstrumenttovisualHight, Trans. Ill. Eng. Soc. (London)I5 (1950) 183.

2) G. M. Kevern, Effect of source size upon approach light performance, Ill. Eng.45 (1950) 96.

3) P. J. Bouma, Physiologisch-optische Grundlagen für die Probleme der Luftschutzverdunklung, Physica 8 (1941) 398.

4) B. Schönwald, Das Ricco'sche Gesetz und die Sehschärfe, Das Lichtll (1941) 15.6) H. A. Knoll, R. Tousey and E. O. Hulburt, Visual thresholds of steady point

sources of lights in fields of brightness from dark to daylight, J.O.S.A. 36 (1946) 480.6) H. R. Blackwell, Contrast thresholds of the human eye, J.O.S.A. 36 (1946) 624.7) N. E. G. Hill, The measurement of the chromatic and achromatic thresholds of

coloured point sources against a white background, Proc. Phys. Soc. London 59(1947) 574.

8) R. G. Weigel and O. H. Knoll, Neue Untersuchungen über Schwellenwerte, DasLicht 10 (1940) 179.

0) G. A. W. Rutgers, Zichtbaarheid van signaallichten, Electrotechniek 26 (1948) 36.

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