gloss investigations using reflected images of a target pattern

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Page 1: Gloss Investigations Using Reflected Images of a Target Pattern

APRIL, 1936

Gloss Investigations Using Reflected Images of a Target Pattern*

RICHARD S. HUNTER, National Bureau of Stdndards(Received March 18, 1936)

A target pattern of concentric rings varying from finelines to broad bands has been placed in the open face of adesk lamp. This luminous target is useful in the study ofthe gloss characteristics of the more glossy surfaces. Thelines and bands of various sizes in the target provide meansfor studying surfaces of a wide range of "distinctness-of-reflected-image" gloss. Records may be made of whichlines and bands are visible by reflection from differentsurfaces. Such records serve as permanent gloss values for

the different surfaces studied. The dark areas of the targetimmediately adjacent to the luminous areas provide idealconditions for the identification of surface "bloom." Thebest gloss differentiations are made when the lamp is usedin a darkened room so that the luminous pattern is the onlysource of light illuminating the surfaces inspected. Photo-graphic records of gloss and unusual gloss effects arediscussed.

I. INTRODUCTION

IN a recent circular' of the Scientific Sec-tion of the National Paint, Varnish and

Lacquer Association, the author described aninstrument to measure "distinctness-of-reflected-image" gloss and to study other gloss effects. Inthis gloss comparator two identical, illuminatedtargets consisting of concentric rings varyingfrom fine lines to wide bands were used. Thenarrowest lines which could be seen after re-flection from any given surface were used as thecriterion of the distinctness-of-image gloss ofthat surface. A measuring device which gavevariable amounts of diffusion to one of the targetswas used to arrange these distinctness-of-imagecomparisons on a continuous scale.

Shortly after this gloss comparator was intro-duced, it was realized that perhaps the luminoustarget itself, without the accompanying instru-ment, might be useful in gloss-control work andstudies of gloss differences. Accordingly, anenlargement of the gloss-comparator target wasprinted on a photographic plate and mounted inthe face of a desk lamp, as illustrated in Fig. 1.This lamp has proved so useful in studies of glossthat it is thought worth while to describe itsuse in detail.

* Paper presented before the Optical Society of America,New York City, February 21, 1936. Publication Approvedby the Director of the National Bureau of Standards ofthe U. S. Department of Commerce.

1 Scientific Section Circular No. 493 of the NationalPaint, Varnish and Lacquer Association, Washington,D. C. (October 1935). In this circular are described thegloss comparator and also the different appearance effectsidentified with glossiness. A bibliography on gloss-meas-uring methods is included.

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FIG. 1. Target pattern mounted in desk lamp, two panelsin position for comparison, and camera to photographimages of target pattern reflected in panel surfaces.

II. APPARATUS AND METHOD

In Fig. 2 a 90° quadrant of the target is repro-duced to full scale. Each of the lines and bandsin the target is designated with a letter by whichit may be identified. The advantages which thedevice offers when used for studies of gloss andgloss differences are: (1) It affords reproducibleconditions for the study of glossy materials;(2) the lines and bands of varying size in thetarget provide means for studying surfaces overa wide range of distinctness-of-image gloss;(3) permanent records of this type of gloss maybe obtained by identification of the rings andbands which are visible after reflection from anygiven surface, and (4) the dark areas in thetarget may be used for determining the differ-ences in "bloom" between glossy surfaces.

The design of the circular target was reachedthrough a trial-and-error development duringthe building of the earlier instrument.' Rec-tangular and straight-line targets were tried

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J. . S. A. V OL UM E 2 6

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Page 2: Gloss Investigations Using Reflected Images of a Target Pattern

GLOSS INVESTIGATIONS

FIG. 2. One quarter circle of

first, but it was discovered that the imagesreflected by many surfaces varied as the surfaceswere rotated in their own plane. The circulartarget, of course, gives the imagiig powers fromsource lines in all directions. The widths of thelines and bands have been graded to give fairlyeven steps in distinctness-of-image reflection.To illustrate the differentiations thus madepossible, three pairs of typical target imageshave been photographed with the combinationofr'camera and lamp shown in Fig. 1. Thesephotographs are reproduced in Fig. 3.

The lamp makes possible the finest distinctionsin gloss when it is used in an otherwise darkenedroom. However, since the ordinary glossy appear-ance of any surface varies with the surroundingfield of view and with other illuminations, it is,in many cases, advisable to make gloss com-parisons with outside light present in order torealize more closely the actual conditions of glossobservation. It is best to obtain flat panels ofthe materials to be studied as curved surfacesreflect distorted images, which are more difficultto evaluate. A mild example of this distortion is

the target pattern reproduced full scale, with letters to identifythe different rings and lines.

seen in Fig. 3, where the panel a-I possesses aslight curvature.

For detecting gloss differences, the two panelsto be compared are placed side by side andviewed so that part of the image of the face ofthe lamp is seen reflected from each surface. Thedistance of the surfaces from the lamp and theangle of view are always important and shouldbe noted where permanent records are to be kept.If the test panels are moved toward the lamp,finer lines in the target may be distinguished.Also, a shift of the surfaces toward the lamp willmake their lightness and color characteristicsmore pronounced. When the surfaces are movedaway from the lamp, the finer lines in the patterndisappear, and color and lightness becomes moredifficult to recognize. As the angle of viewapproaches grazing incidence, images reflected inthe surfaces become more distinct. Thus manyof the very flat, matt surfaces will reflect dis-tinguishable images at near grazing incidence.

When the lamp is used as a source for ob-taining the distinctness-of-image gloss ratings ofindividual surfaces, instead of simply for com-

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Page 3: Gloss Investigations Using Reflected Images of a Target Pattern

RICHARD S. HUNTER

paring two surfaces, these surfaces must, inturn, all be placed in the same position relativeto the lamp and should be viewed from the sameposition. The exterior lighting conditions mustalso be constant and the following factorsspecified in presenting the results:

1. Distance from lamp to test surface2. Distance from observer to test surface3. Direction of view (unless otherwise specified, approxi-

mately 450)4. Amount of light present from-sources outside the

lamp.

When these factors are recorded, reproducibleconditions for the identification of the lines ofthe target are established, and observations madeat separate times may be used as records of thegloss of the surfaces tested.

The observer, who wishes to describe theappearance of the target image reflected in someparticular glossy surface, names and describesthe appearance of the least distinct ring. That is,he records the letter which identifies this ringand perhaps a modifying letter to indicatewhether this ring is sharp, fairly plain, or in-distinct; also whether it appears as a continuousring, or shows only irregularly from certainparts of the surface. In order of descending"distinctness-of-image," the letters identifyingthe rings in the target are: A1, A 2, A, B, C, D,and E. In addition, there are two auxiliary rings(D1 and X) which are occasionally used indescribing the appearance of surfaces. Thefollowing modifying letters are suggested for theabove designations: g-good definition, f-fairdefinition, p-poor definition. This system givesa distinctness-of-image gloss scale of seven letterratings and twenty-one steps, three steps foreach rating. Three letters, which serve to indi-cate the appearance of a surface aside from itsdistinctness-of-image gloss, are suggested: n-nonuniform, w-wavy, s-specks and scratches.Other characteristics of surfaces, such as bloom,particular types of texture, and unusual appear-ance may also be designated.

In the innermost band of rings, the letter A,represents resolution by the eye of the two ringsclose together just outside the center of thetarget. A2 represents resolution of the third ringas separate from the other two, but A2 wouldmean. when reported, that the observer could

FIG. 3. Comparison of three pairs of panels using thegloss-inspection lamp. a, olive-green enamels; b, white,resinous paints; and c, rubbed furniture finishes. Distancesof camera to surface and lamp to surface both about 30 cm;room dark.

not, under the given conditions, resolve the A1pair as separate and distinct rings. The letter A

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Page 4: Gloss Investigations Using Reflected Images of a Target Pattern

GLOSS INVESTIGATIONS

indicates identification of the three rings as asingle band without their separate resolution.

III. REPRESENTATIVE RESULTS

This gloss-rating scale was used by a numberof observers to grade the three pairs of panelsillustrated in Fig. 3. The results of their observa-tions are given in Table I.

Each pair of panels was observed at twodistances from the lamp (30 and 90 cm); in eachinstance the observer's eyes were slightly fartherfrom the test surfaces than the test surfaceswere from the lamp. The direction of view was450; the room was dark.

The observer's rating, g, f, or p, is placedopposite the designation (A1, A 2 , * ) of thering which was least discernible. A mean ratingis assigned at the bottom of each set of observa-

TABLE I. Tabulation of distinctness-of-image gloss ratings bSurfaces were observed at two dista

tions. To obtain these means, it is assumed thatthe distinctness-of-image gloss scale is a uniformcontinuous scale of 21 steps from "pE," at thebottom, to "gA " at the top. Tenths of a stepwere computed from the averages of the sixobservers. Below the sharpness-of-image ratingsin the table are crosses to indicate the observers'identifications of nonuniformities, surface wavi-ness, and specks or scratches.

From the photographic record in Fig. 3, andthe gloss data recorded in Table I, the degree ofconsistency between visual and photographicrecords of gloss may be seen. Note, for instance,that panels a -I and b - r were designated"wavy" by all observers in the visual test; alsothese panels in the photograph show the typicaldistortion due to waviness. The distinctness ofthe images reproduced in the figure do not

y six observers of the three pairs of surfaces compared in Fig. 3.nces and surface characteristics noted.

DISTANCE FROM 30 cm 90 cm DISTANCE FROM 30 cm 90 cmLAMP TO SAMPLE LAMP TO SAMPLE

OBSERVERS P4 0 4 34 3 OBSERVERS P P 2 0z = U a 3

A1 f g f gfg - - - - - - A1 g g g g g g - - - - -A2 - - - - - - _ _ _ _ _ _ A2 - - - - - - - g f - - PA - - - - - - f f P P P f A - - - - - - g - - f g -

Panel a-l B - - - - - - - - - - - - Panel a-r B - - - - - - - - - - - -C ---- __ __ - - C -- ______D - - - - - - - - D - - - - - - - - - - -E - - - - - - - - - - E - - - - - - - - - - -

Mean Rating (f+0.5)A 1 (p+0.5)A Mean Rating. gAl (p-O.2)A 2nonuniformities - - - - - - - x - x - - nonuniformities - - - - - - - - - - - -waviness - x x x x x x x x x x x waviness - - - - - - x - - - - -specks scratches x - x x x x - - x - - - specks scratches x - xx x x x x x x--

A1 f p p fp - - - - - - Al p p p pp p - - - - -A2 - - - - - - - - - - A2 - - - - - - - - - - - -A - - - - - - - - - - - f A - - - - - - - - - pPanel b-l B - - - - - - f g p f f - Panel b-r B - - - - - - p p p p p -C ------ -____- C ----- ______D - - - - - - - - - D - - - - - - - - - -E - - - - - - - - E - - - - - - - - - - -

Mean Rating (p-+0.3)A1 (f+0.5)B Mean Rating pA1 (P+0.5)Bnonuniformities x - x x - - - - - - - - nonuniformities xx -xxx - x - - x-waviness - - - - - - x x x x - - waviness x x x xx x x x x x x xspecks scratches x - x x x - x - - x x - specks scratches - -x xx - - x x x - -

Al - - - - - - _ - - - - - Al - - - - - - - - - - - -A2 - - - - - - - - - - - - A2 - - - - - - - - - - - -A - - - - - - - - - - - - A - - - - - - - - - - - -

Panel c-l B - - - - - - - - - - - - Panel c-r B - - - - - - - - - - - -C J p f f ff - - - - - - C - - - - - - - - - - -D - - - - _ p f p p g D p p - - - - - - - -E - - - -__ - - f g f g f g P P P P

Mean Rating (f-0.2)C (p+0.5)D Mean Rating gE (p+0.5)Enonuniformities - - - x - - - - - - - - nonuniformities - - - - - - - - - - - -waviness - - - - - - - - - - - - wavinessspecks scratches x x x x x - x - x x - x specks scratches x x x x x x x - x x - -

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RICHARD S. HUNTER

exactly correspond to the 30-cm ratings given inthe table because of slight changes in reproduc-tion introduced in the various printing processes.

When it is considered that the gloss inspectionlamp and the method of rating gloss were bothnew to these observers, the agreement amongthem seems quite satisfactory. The lamp isshown to be capable of establishing twenty-onesteps on a distinctness-of-image scale. The rangeof this scale may be adjusted to give the bestgloss differentiations of any particular type ofglossy material. This adjustment is usuallyaccomplished by varying the distances of lampto test surface and test surface to observer.Glossy surfaces which reflect sharp images shouldbe compared at greater distances from the lamp,while better differentiations are made on the lessglossy surfaces by approaching them closer tothe lamp. The average observed, distinctness-of-image gloss differences, expressed in gloss stepsbetween the I and r panels of each pair at the twodistances recorded in Table I, are as follows:

Distances

Samplea pairb pairc pair

30 cm0.50.34.8

Thus it can be seen that the glossy a pair ofpanels showed an average difference of onlyone-half a step at 30 cm, while at 90 cm thisdifference was more than two steps. The con-verse is true for the c pair of low gloss panels;thus, viewed at 30 cm, the average differencewas 4.8 steps, but at 90 cm the differencerecognized was only 3.0 steps.

The preparation recently of a specification forthe gloss of automobile enamels furnishes anexample of the practical use of this flexiblegloss scale. Representative test samples of suchglossy enamels, some weathered and some fresh,were used for trial observations to determineunder what conditions the best gloss differentia-tions could be made. It was found that at a lamp-to-surface distance of one meter, a lamp-to-observer distance of 1 meters and an angle ofview equal to 30°, practically the completegloss scale could be used. The test room wasdark, the lamp was placed on a bench, the test

panels were placed on the floor, and the ob-servers viewed the images of the lamp targetreflected in the various surfaces while standing.Tentatively, the distinctness-of-image gloss re-quirements were chosen as follows: fresh. sur-faces: B fair, or higher, after 168 hours ofaccelerated weathering: C good, or higher.

In comparing the superior and inferior enamels,it was found that the better grade materials re-tained their gloss more effectively through accel-erated weathering.

IV. IDENTIFICATION OF DIFFERENCES IN

"BLOOM," OBJECTIVE GLOSS

AND TEXTURE

"Bloom," objective gloss, and surface texturewere three of the types of gloss identified in theearlier publication.' It is not possible to arrange,by means of the lamp, these factors along ascale such as used in the case of distinctness-of-image gloss. However, the lamp and target haveproved useful in identifying differences in thesethree types of surface appearance and in furnish-ing qualitative, rather than quantitative, dataon them.

When using the gloss-inspection lamp for theidentification of "bloom," one inspects the imageof the black spot in the center of the target andthe other dark areas in the target. "Bloom" isindicated as a haze in these dark areas of theimage. The two glossy olive-green enamel panelscompared in Fig. 3 for distinctness-of-imagegloss showed also a pronounced difference in"bloom." This effect is quite appreciable in theleft panel of Fig. 4 which is another print fromthe a pair negative of Fig. 3, but less dense.In the b pair of white surfaces, Fig. 3, b-ireflects the sharper images, and also shows more"bloom" than b-r. "Bloom" is shown only bysurfaces which reflect images.

Objective gloss is more popularly thought of as"shininess." With the gloss lamp and target, it isrecognized by the brilliance of reflected imagesbut is not readily measurable. There are anumber of other gloss meters, however, whichgive records of specular reflectance, such as, forinstance, the instrument2 designed by the author

2 Hunter, "Tle Glossmeter," Scieiitific Section CircularNo. 456 of the National Paint, Varnish and LacquerAssociation, Washington, D. C. (April 1934).

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GLOSS INVESTIGATIONS

FrG. 4. Comparison of a pair of panels shown in Fig. 3printed from same negative as in Fig. 3, but with lowdensity to show effect of bloom on panel a-1.

in the spring of 1934. References to methods formeasuring objective gloss as well as other glossfactors are given in the previous circular.'

With regard to comparisons of surface tex-ture and the evaluation of irregularities, thislamp is useful as a source of nearly unidirectionallight. Nonuniformities, waviness, and specks orscratches were evaluated qualitatively by theobservers who compared the six surfaces andobtained the results shown in Table I. Surfaceirregularities, blemishes, and texture show updistinctly in illumination from a single directionbut are less visible in illumination which is welldiffused. Texture is most distinct at the angle ofspecular reflection, and photographs at this anglewith the camera focused on the surface tested,give, in our experience, the best records oftexture. All the surfaces photographed in Fig. 3show irregularities, but they are out of focusbecause the camera was focused on the image ofthe target.

V. VALUE OF GLOSS PHOTOGRAPHS

It can be appreciated that photographs such asare shown in Figs. 3 and 4 are better and morecomplete records of glossiness and serve betterfor comparing gloss than values obtained on anygloss scale. This is because the glossiness of asurface arises from a combination of severalfactors and these combined factors are seldomexpressible as a variation in one dimension.Because of the advantages of the photographic

method, the earlier gloss comparator' was de-signed to take a camera directly into the instru-ment. The camera is always placed in the sameposition in the instrument; no careful adjustingand focusing are necessary, and the exposures aretimed by a prepared table. With the gloss-inspection lamp, on the other hand, it is necessaryto set the camera up carefully in the desired posi-tion to photograph the effect sought. In spite ofthe greater care which is necessary, this ability touse the camera in any position is sometimes anadvantage because it gives greater latitude forthe study of gloss effects.

A 5 by 7 plate camera was used to obtain thephotographs shown in Figs. 3 and 4. This was setup as shown in Fig. 1 where the camera lens andthe lamp target are both about 30 cm from thegloss surfaces. The lens of the camera was stoppeddown to about 3 mm so that as much detail aspossible would be obtained from the out-of-focustest surfaces.

VI. GENERAL TECHNIQUE

As already implied, the observer who wishes toanalyze gloss effects must recognize that twodistinctly different modes of appearance, as wellas several appearance effects, are always possible.One mode results when the observer fixates uponthe glossy surface itself; the other when he fixatesthe image of some object reflected in that surface.Surface texture, formation, and structure areappreciated when the eye is focused upon thesurfaces being studied. On the other hand, form ofreflected images, gloss contrasts, and the distri-bution of light reflected by surfaces are usuallyappreciated when the observer fixates the imagesof objects reflected in the glossy surfaces. For thisreason, one who wishes to photograph surfacefeatures and texture focuses the camera upon theglossy surface, while one who wishes to photo-graph image reflections, "bloom," or contrasts,as in Fig. 3, focuses the camera upon the image ofthe target reflected in the test specimens. In thislatter case, pimples, irregularities, and grain inthe surfaces can be seen, but they are out of focus.

In conclusion, it may be repeated that thegloss inspection lamp is a convenient source oflight for the general study of gloss-appearanceeffects. The wide variety of ways in which it can

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