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Measurements of the colour sequences in positive visual after-images
VisionRrs. Vol. 8, pp. 93Y-949.Pergamon Press1968.Printed in Circa1 Britain.
COLOUR
MEASUREMENTS OF THE SEQUENCES IN POSITIVE AFTER-IMAGES C.
VISUAL
A. PADGHAM
Department of Ophthalmic Optics, The City University, London (Received 23 February 1968) INTRODUCTION VISUAL after-images have been studied extensively since the time of Aristotle, and there is a vast literature which has been thoroughly surveyed up to 1922 by BERRY (1922). Although much qualitative work has been reported on the colour-sequence of visual after-images, little quantitative data has appeared. A number of workers however have measured the decay curves of apparent brightness of after-images (PA-S, 1947; ALPERN and BARR, 1962; PADGHAM, 1965). This work is an extension of the use of binocular matching technique (P-HAM, 1953) to measure continuously the colour of positive after-images as they decay. In this method an after-image is induced in the left eye by exposing a stimulus of high luminance. A comparison patch of light is then projected into the dark-adapted right eye, suitably displaced so that binocular fusion does not occur. The colour and luminance of this comparison light is then varied continuously by the observer to match the colour sequences of the after-image. THE APPARATUS
The design of the binocular calorimeter
Figure 1 shows the optical layout of the apparatus. It consists of two optical systems. The first provides the inducing stimulus for the after-image in the left eye. In this Hr is a 6V, 108W ribbon filament lamp, the light from which is collimated by lens L, ; D is an iris diaphragm combined with a photographic shutter. An Snitely distant image of D is viewed by the left eye Er through the lens L2 and the reflecting prisms Pt and P2. It is necessary to displace the optical systems by means of these prisms since they must be placed further apart than the normal interocular separation in or&r to accommodate the two optical systems side by side including the lamp housings. The image of D is exposed for a known time using the photographic shutter, and this provides the stimulus for the after-image for Er. The second optical system is used to project a comparison stimulus into the right eye Ez. In this system the lamp Hz is similar to Hr. The light from it is collimated by lens L3, and passes through a variable neutral density wedge W, the optical density of which varies from 0.07 to 4.0. This is followed by a vertical slit S and by two spectrum interference filters F, which provide the coloured comparison stimulus, the details of which are given below. The image of the lamp filament is then focused by lens L.t on to the trans939
C. A. PAD~HAM
940
lucent front surface of a mirror integrating box M, which mixes the coloured light from the filters F. This is followed by a diaphragm R which defines the comparison stimulus, and this is viewed by the right eye Ez through the lens Lg. A headrest is used to position the observer’s head.
I____-----__ -- ---
L,
_--_---yp _-_-_--.
A,
L*
qr-_-
-W
1.---dl,~# -- 0 S
_-
M
F
$
--_ --
H, - =,
L,
FIG. 1. Diagram of apparatus. The two interference filters F are continuous running spectrum filters designed to pass the colours of the visible spectrum from one end to the other. These filters are depicted in Fig. 2 in a plane perpendicular to the direction in which the light passes through them. The two filters are arranged to be adjacent, but reversed, so that the upper F1 runs from red on the left to violet on the right, whereas the lower F2 runs from violet on the left to red on the right. They are so placed that the green region of the spectrum on each filter corresponds vertically. Only the right-hand section of each filter is used. RED
VIOLET
GREEN
VIOLET
GREEN _
RED H
FIG.2. The arrmgement of the colour filters. The filters are mounted together in a frame which can be translated on kinematic slides in both vertical and horizontal directions. The movement of them is accomplished by a “joy stick” held in the right hand of the observer. This can be moved in all directions in a plane, thus allowing both vertical and horizontal motions to be transmitted to the filter frame at will. The observer can therefore vary the colour of the comparison patch using a single manual control. The other hand controls a knob which varies the position of the neutral wedge W (Fig. 1). This controls the luminance of the comparison patch. The principle of operation of this filter system is as follows. If the filters are moved so that the fixed slit S occupies the positions A, E or C (Fig. 2), the corresponding colours on the C.I.E. chromaticity diagram are shown at the single point A’ C’ E’ on Fig. 3.
Measurements of the Colour Sequences in Positive Visual After-Images
941
(The position of E in Fig. 2 is intended to be intermediate between A and C but is shown slightly displaced for clarity of the diagram. Similarly G is intermediate between B and D.) If the filters are now moved horizontally so that the slit is at B, the colour varies from A’ to B’. The corresponding positions C to D give colours along the line C’ D’. If however the filters are placed so that the slit straddles both filters, for example the positions E, I and G (Fig. 2) the corresponding colours transmitted fall on the greenwhite-purple line E’ I’ G’ (Fig. 3). Therefore any colour within the triangle A’ B’ D’ can be produced.
FIG. 3. The colour points produced by the slit positions indicated on Fig. 2.
The vertical and horizontal movements V and H respectively of the filter frame are read off by two fixed 10042 high-resolution, wire-wound rotary potentiometers, which are actuated by nylon cords attached to the frame. These cords pass round pulleys on the potentiometer spindles. Similarly the position of the neutral wedge W is read by a third rotary potentiometer. The potentiometers are energised by a stabilised 12V d.c. supply, and the three outputs are recorded simultaneously on a 3-channel recording milliammeter, which gives a full-scale delkction on each channel of 1mA. Thus the V and H records give the instantaneous colour of the after-image, and the W record its luminance. The colour corresponding to the filter positions are found by using calibrated colour filters, and also filter combinations from a Lovibond-Schofield visual calorimeter. The type of calibration lines obtained are shown dotted on Fig. 3. Thus the recorder readings can be translated directly into C.I.E. chromaticity co-ordinates. The operation and precision of the calorimeter It was found that some practice was needed to operate the calorimeter successfully, in order that the rapidly varying colour sequences of the after-images could be followed with reasonable accuracy. It is important to avoid observational “hunting” and “overshoot”, since these would be recorded as artefacts. The intrinsic precision of this type of calorimeter is considerably less than a normal calorimeter. There are several reasons for this. First the precision of binocular colour matching is lower than monocular matching. Secondly the instrument is not designed
C. A. PAWHAM
942
primarily for high precision, but to enable rapid variations of the colour of the comparison patch to be produced with the use of only one manual control. Thirdly the rapidly varying nature of the after-image phenomena gives little time for a match to be made, and there is no possibility of any subsequent verification. From experimental calibration measurements using colour filters, the standard deviation of a static colour reading is of the order of &O-O7 to +O-10 in C.I.E. x and y chromaticity co-ordinates, under the worst circumstances. There is the further inherent difficulty that the comparison stimuli must of necessity be of low luminance in order to match the faint after-images. After-images are often highly saturated even though they may be faint. Therefore it can happen that the comparison stimulus appears less saturated than the after-image, and this is particularly so in the purple region. There was no alternative in these cases to producing a match as close as possible to the after-image colour. RESULTS The
quantitative results quoted are those obtained by the author alone, who is a normal trichromat. This ensures that the results have a greater self-consistency, although it gives no information on individual differences. It is hoped, however, later to extend this work to other observers, both colour-normal and colour-defective.
0’8
FIG. 4. C.I.E. chromaticity diagram showing the colour sequences of after-images produced by exposure to a 2-Y dia. circular white stimulus viewed foveally for 1 sec. The results of these separate experiments are shown. (Stimulus of colour temperature 3000”K, and of retinal illumination 2 x 106 trolands). Numbers on curves indicate time in seconds from beginning of stimulus exposure.
Measurements of the Colour Sequences in Positive Visual After-Images
943
The stimulus was a circular white light source of colour temperature 3000°K of retinal illumination 2 x 106 trolands. The C.I.E. chromaticity co-ordinates of the stimulus are x = 0.43, y = 0.40. The eyes were dark-adapted for 10 min prior to each exposure to the stimulus. Figure 4 shows plots on the C.I.E. chromaticity diagram of the colour sequence of the after-images formed by exposure to a 25” diameter circular stimulus viewed foveally, for 1 second. The results of three separate experiments are shown. The point S represents the stimulus colour. The numbers against the points indicate the time in seconds that has elapsed since the beginning of the stimulus exposure. It is not possible to make measurements for about 20 set after the presentation of the stimulus. This is the order of time necessary to allow the extreme sensation of glare to subside, and for the after-image colour to be perceived. The multiple exposures indicate the order of reproducibility of the measurements. The variations shown include both instrumental and observational errors, and variations in colour of successive after-images. Figure 5 shows the c&our sequences of the same stimulus exposed for 2, 4 and 8 sec.
x FIG. 5. C.I.E. chromatieity diagram showing the colour sequences of after-images produced by exposure to a 2 5” dia. circular white stimulus viewed foveally for three different exposure times. (Stimulus of colour temperature 3OOO”K, and of retinal illumination 2 x lOatrolands). Numbers on curves indicate time in seconds from beginning of stimulus exposure.
In general the colour sequences are as follows. First the colour is white, close to the stimulus colour but often slightly bluer. It then moves towards the green, remaining of
944
C. A. PADGHAM
low saturation and then returns to near the stimulus colour. The movement is next towards the red-purples passing through purple to blue-purple and finally blue. These latter phases generally are of high saturation. The general sequence is always similar, although variations in saturation occur. This colour sequence has been fairly well confirmed by a number of normal observers using a colour-naming technique, although a small minority quote a slightly different colour sequence. The longer exposure times to the stimulus give rise to similar after-image colour sequences, but with longer time scales. It is interesting that BERRY(1922) quotes similar time sequences by some earlier workers. However, the stimulus and observing conditions in a good deal of the early work were either not controlled adequately or not specified, and are thus of little value. Figure 6 shows the colour sequences in after-images produced by exposure to a similar stimulus for 1 see, but of angular diameter 5”. The results of three separate experiments are shown. These results are not significantly different from those produced with a 2.5” diameter stimulus (Fig. 4).
FIO. 6. C.I.E. chromaticity diagram showing the colour sequencesof after-imagesproduced by exposure to a 5” dia. circular white stimulus viewed foveally for 1 sec. (Stimulus of colour temperature 3OOO“K,and of retinal illumination 2 x 106 trolands). Numbers on curves indicate time in seconds from beginning of stimulus exposure. ANALYSIS
OF RESULTS AND DISCUSSION
The results shown in Figs. 4 and 5 are re-plotted in Figs. 7-10. These show the values of the C.I.E. chromaticity co-ordinates X, y and z of the colour matches on the after-images as a function of time, measured from the beginning of the stimulus. The interesting common
Measurements of the Colour Sequences in Positive Visual After-Images
945
feature is the fact that the y and z components progress approximately in anti-phase, whereas the x component varies in a similar way to that of the JJ component.
0 SEC
FIG. 7. The C.I.E. chromaticity co-ordinates of after-image colours as ordinates shown as a function of time measured from the beginning of inducing stimulus. (Stimulus white, colour temperature 3000°K. circular 2.5” dia., retinal illumination 2 x lOa trolands exposed for 1 set).
0 0
100 SEC
FIG. 8. The C.I.E. chromaticity co-ordinates of aftcr-image colours as ordinates shown as a function of time measured from the beginning of inducing stimulus. (Stimulus white, colour temperature 3OOO”K, circular 2.5’ dia., retinal illumination 2 x lOa trolands, exposed for 2 set).
The results of Figs. 4 and 5 were also transformed to Kiinig-type fundamental sensations suggested by JUDD (1945). The transformation equations which were used were as follows:
R = l,OOg G = -0.46 R + 1.36 y’ + 0.10 i V=
1.002
11 = 1, r, = 13 = 0.
C. A. PADCHAM
946
0 0
100 SEC
FIG. 9. The C.I.E. chromaticity co-ordinates of after-image colours as ordinates, shown as a function of time measured from the beginning of inducing stimulus. (Stimulus white, colour temperature 3OOO”K,circular 2.5” dia., retinal illumination 2 x 10atrolands, exposed for 4 set). 1-o
0' 0
100 SEC
FIG. 10. The C.I.E. chromatic&y co-ordinates of after-image colours as ordinates shown as a function of time measured from the beginning of inducing stimulus. (Stimulus white, colour temperatunz 3000°K, circular 2.5” dia., retinal illumination 2 x 106 trolands, exposed for 8 set).
The transformed results are shown in Figs. 1l-13, for exposures of 1, 2 and 4 set where the values of R, G and V as ordinates are plotted on an arbitrary scale as a function of time. (The 8-set curves lie too close together to be plotted on this scale.) The significant feature of these curves is the similarity in the decay functions for the R and G fundamentals which are closely exponential. This agrees with previous resuits (PADGHAM, 1963). However, the violet fundamental response, whilst initially lower, predominates, after about 60 set, towards the end of the decay period. This suggests that it is perhaps related to the observations of the “positive blue” phenomenon by WRIGHT (1946; see page 252). He found that following colour adaptation of the retina to almost any radiation, a test colour chosen from the red-yellow-green region of the spectrum appears, for a minute or so, to be mixed with a positive amount
Measurements of the Colour Sequences in Positive Visual After-Images la0
o 100 SEC 0 FIO. 11. The fundamental sensation magnitudes in after-image as ordinates shown as a function of time measured from the beginning of inducing stimulus. From data of Fig. 7 (I set exposure).
0 0 100 SEC Fro. 12. The fundamentai sensation magnitudes in after-image as ordinates &own BII a function of time measured from the @inning of inducing stimulus. From data of Fig. 8 (2 = exposun).
0 100 SEC FIG. 13. The fundamental sensation magnitudes in after-image as ordinates shown as a function of time measured from beginning of inducing stimulus. From data of Fig. 9 (4 set exposure). r)
947
C. A. PADGHAM
948
of blue. Thus blue, in addition to red and green are needed to be added together to produce a binocular match on a yellow test patch. It is interesting to note that in Wright’s experiments the amount of biue needed initially fell, at the beginning of the recovery from colour adaptation, then rose to a ma~mum after approximately 60 see, and subsequently fell. This is in general agreement with the variation of the violet fundamental sensation component of the after-image recovery, although Wright’s adapting stimulus luminance and exposure times were somewhat different. Wright’s theory that a persistent after-image was responsible for the phenomenon is thus given support by these observations. The use of other fundamental sensation data will only slightly aiter the transformation, the greatest diiTerence being in the red sensation. All the suggested fundamental sensations in effect equate the violet sensation with the 2 function and thus the general conclusions will not be affected by the type of transformation used. The violet sensation anomaly, indicated by these results suggests a basic difference between the after-functioning of the violet process and those mediating the red and green sensations. Acknowlcrlgcmmrs-The apparatus was constructed in the Department of Physics, whilst the author was a member of that Department. Thanks are due to the Head, professor F. Y. POYNTON,for his great interest and help, The author also wishes gratefully to acknowledge the help, in the design of the apparatus, given by Dr. B. H. CRAWFORD, late of the National Physical Laboratory, and the help and encouragement of Professon R, J. FLETCHER and R. W. G. HUNT. Finally, thanks are due to those students who helped in some colour naming experiments on after-hnagea,
REFERENCES ALPERN,M. and BAIIR, L. (1962). Durations of the after-images of brief flashes and the theory of the Broca and Sulxer phenomenon. J. opt. Sot. Am. 52,21P-221. BERRY,W. (1922). The Aigitt of coIours in the after-image of a bright light. Psychol. Bull. 19,307-337. JUDD, D. 3. (1945). Standard response functions for protanopic and deuteranopic vision. J. opt. Sot.
Rm. 3!J,lPP-221. PAWHAM, C. A. (1953). Quantitative study of visual after-images. Br. J. Ophtlud. 37, 165-170. PAWHAM C. A. (1963). The role of the retinal receptors in the formation of the positive visual after-image. Vision Res. 3, 4549. PALSHAM,C, A. (1965). After-images as a means of investigating rods and cones. Ciba Formdution Sym~s~um on Pky~logy and Experiments Psyckoiogy of Colour Vision, pp. 249-264, edited by
G. E. W. WANES
and JUUE Kr+nzrr. Churchill, London.
PANNED, M. (1947). lzber den Heligkeitsverlauf positiver Nachbilder. Ophtkahnoiogicu, &set 280-289. WRIGHT, W. D. (1964). Researches on normal and defective colour vision, Kimpton, London.
apparatus is described by means of which the colour of positive visual afterimages can be measured con~u~iy as they decay, by means of binocuiar matching. Results are shown of the coiour sequence6 in the after-images formed by exposure to a highintensity white light stimulus for various times. The general sequence is white, green, white red-purple, blue-purple and blue. The results are transformed into fundamental sensation suggcsted by Judd. This shows that the after-functioning of the red and green processes are similar, but that of the violet process is quite different. Abstract-An
R6swnG-On d&it un appareil qui permet Ia me-sure, par comparaison binoculaire, de la
couleur dune image consecutive positive, d’une fawn continue tandis qu’elle s’attetme, On montn les reSultats de la suite de couleurs pour ies images consecutives P une hnni&e blanche intense de durt5e varied. En g&&al cette suite est blanc, vet?, pourpre rougeatre peu satut-6, bleu-pourprt! et bleu. On transforme ces r&Bats au moyen des fondamentales sugg&es par Judd. Cela montre que le fonctionnement consecutif des mecanismes rouge et vert sent semblables, mais que celui du violet est tout-a-fait different.
113,
Measurements of the Colour Sequences in Positive Visual After-Images Zusammenfasaang-Es wird ein Apparat bcschrieben, mit dem die Farbe der positiven Nachbilder durch binokularen Vergleiih w&rend des ZcrfaUs kontinuierlich gemessm werden kann. Die Ergebnisse zeigen Farbsequenzen in den Nachbildem, die entstehen, wenn ein weifkr Lichtreiz hoher LeuChtdiChte bei vezhiedenen Z&en dargehOten wird. Die al&me& Reihenfolge ist WeiD, Grlin, Rot-purpur, Biau-purpur und Biau. Die Ergebnisse werden in die von Judd vorgeschlam Grundvalenzen umgesetzt. Hier zeigt sich, da0 die NachablWfe der roten und grUnen Prozesse iihnlich, dejenige des Violettprozesses aber ganz anders ist. Pexortse - OmicaH annapar c nob+ollfbm ~oroporo 4Be-r nono~Tc.nbHbIx noneXOBa=JIbHblX 06pa3OB MO=? H3MCpSITbCX HenpepMBHO l’IpH HX 3amaHEiH, Eia OCHOBaHZiM 6~Ho~R~pHoro CpaBIleHEfa. OZIW,I~C~ ~~CXO~~~bH~b C RoropoZi B03HmaioT u~era 31ux 06pa30~, nouI(: 3~cxxo3~335iw R crmyny 6enoro cBeTa BbICOKOjl HHTCSHCHBHoCTEI,HO piuHO# QnIUWIbHoCTli. 0614~ 3aKOHOMCpHocTb IIoCJle~oBa~bIfoCTH IlORBJlK)UIHXCIIUBeTOB: 6CllbIti, 3eneHb&, cienbIi% KpaCliO~TOQ’plYypOBbIi8, C~HenypnypoSti Ii CHIiHti. Pf3ynbTaTbI TpaHC@OpMApoBaHhl B OCHOBHble Olll)‘LUeHUII, no ,l&ew. nOKaSX0, ‘iT0 CneBOaafi &‘HKuH% RpaCHOrO zi 3eneHoro nporreccoe cxojq8a, a CpsioneToBoro npoi4ecca coE4epUemio 0xmi’IHa.
949
COLOUR
MEASUREMENTS OF THE SEQUENCES IN POSITIVE AFTER-IMAGES C.
VISUAL
A. PADGHAM
Department of Ophthalmic Optics, The City University, London (Received 23 February 1968) INTRODUCTION VISUAL after-images have been studied extensively since the time of Aristotle, and there is a vast literature which has been thoroughly surveyed up to 1922 by BERRY (1922). Although much qualitative work has been reported on the colour-sequence of visual after-images, little quantitative data has appeared. A number of workers however have measured the decay curves of apparent brightness of after-images (PA-S, 1947; ALPERN and BARR, 1962; PADGHAM, 1965). This work is an extension of the use of binocular matching technique (P-HAM, 1953) to measure continuously the colour of positive after-images as they decay. In this method an after-image is induced in the left eye by exposing a stimulus of high luminance. A comparison patch of light is then projected into the dark-adapted right eye, suitably displaced so that binocular fusion does not occur. The colour and luminance of this comparison light is then varied continuously by the observer to match the colour sequences of the after-image. THE APPARATUS
The design of the binocular calorimeter
Figure 1 shows the optical layout of the apparatus. It consists of two optical systems. The first provides the inducing stimulus for the after-image in the left eye. In this Hr is a 6V, 108W ribbon filament lamp, the light from which is collimated by lens L, ; D is an iris diaphragm combined with a photographic shutter. An Snitely distant image of D is viewed by the left eye Er through the lens L2 and the reflecting prisms Pt and P2. It is necessary to displace the optical systems by means of these prisms since they must be placed further apart than the normal interocular separation in or&r to accommodate the two optical systems side by side including the lamp housings. The image of D is exposed for a known time using the photographic shutter, and this provides the stimulus for the after-image for Er. The second optical system is used to project a comparison stimulus into the right eye Ez. In this system the lamp Hz is similar to Hr. The light from it is collimated by lens L3, and passes through a variable neutral density wedge W, the optical density of which varies from 0.07 to 4.0. This is followed by a vertical slit S and by two spectrum interference filters F, which provide the coloured comparison stimulus, the details of which are given below. The image of the lamp filament is then focused by lens L.t on to the trans939
C. A. PAD~HAM
940
lucent front surface of a mirror integrating box M, which mixes the coloured light from the filters F. This is followed by a diaphragm R which defines the comparison stimulus, and this is viewed by the right eye Ez through the lens Lg. A headrest is used to position the observer’s head.
I____-----__ -- ---
L,
_--_---yp _-_-_--.
A,
L*
qr-_-
-W
1.---dl,~# -- 0 S
_-
M
F
$
--_ --
H, - =,
L,
FIG. 1. Diagram of apparatus. The two interference filters F are continuous running spectrum filters designed to pass the colours of the visible spectrum from one end to the other. These filters are depicted in Fig. 2 in a plane perpendicular to the direction in which the light passes through them. The two filters are arranged to be adjacent, but reversed, so that the upper F1 runs from red on the left to violet on the right, whereas the lower F2 runs from violet on the left to red on the right. They are so placed that the green region of the spectrum on each filter corresponds vertically. Only the right-hand section of each filter is used. RED
VIOLET
GREEN
VIOLET
GREEN _
RED H
FIG.2. The arrmgement of the colour filters. The filters are mounted together in a frame which can be translated on kinematic slides in both vertical and horizontal directions. The movement of them is accomplished by a “joy stick” held in the right hand of the observer. This can be moved in all directions in a plane, thus allowing both vertical and horizontal motions to be transmitted to the filter frame at will. The observer can therefore vary the colour of the comparison patch using a single manual control. The other hand controls a knob which varies the position of the neutral wedge W (Fig. 1). This controls the luminance of the comparison patch. The principle of operation of this filter system is as follows. If the filters are moved so that the fixed slit S occupies the positions A, E or C (Fig. 2), the corresponding colours on the C.I.E. chromaticity diagram are shown at the single point A’ C’ E’ on Fig. 3.
Measurements of the Colour Sequences in Positive Visual After-Images
941
(The position of E in Fig. 2 is intended to be intermediate between A and C but is shown slightly displaced for clarity of the diagram. Similarly G is intermediate between B and D.) If the filters are now moved horizontally so that the slit is at B, the colour varies from A’ to B’. The corresponding positions C to D give colours along the line C’ D’. If however the filters are placed so that the slit straddles both filters, for example the positions E, I and G (Fig. 2) the corresponding colours transmitted fall on the greenwhite-purple line E’ I’ G’ (Fig. 3). Therefore any colour within the triangle A’ B’ D’ can be produced.
FIG. 3. The colour points produced by the slit positions indicated on Fig. 2.
The vertical and horizontal movements V and H respectively of the filter frame are read off by two fixed 10042 high-resolution, wire-wound rotary potentiometers, which are actuated by nylon cords attached to the frame. These cords pass round pulleys on the potentiometer spindles. Similarly the position of the neutral wedge W is read by a third rotary potentiometer. The potentiometers are energised by a stabilised 12V d.c. supply, and the three outputs are recorded simultaneously on a 3-channel recording milliammeter, which gives a full-scale delkction on each channel of 1mA. Thus the V and H records give the instantaneous colour of the after-image, and the W record its luminance. The colour corresponding to the filter positions are found by using calibrated colour filters, and also filter combinations from a Lovibond-Schofield visual calorimeter. The type of calibration lines obtained are shown dotted on Fig. 3. Thus the recorder readings can be translated directly into C.I.E. chromaticity co-ordinates. The operation and precision of the calorimeter It was found that some practice was needed to operate the calorimeter successfully, in order that the rapidly varying colour sequences of the after-images could be followed with reasonable accuracy. It is important to avoid observational “hunting” and “overshoot”, since these would be recorded as artefacts. The intrinsic precision of this type of calorimeter is considerably less than a normal calorimeter. There are several reasons for this. First the precision of binocular colour matching is lower than monocular matching. Secondly the instrument is not designed
C. A. PAWHAM
942
primarily for high precision, but to enable rapid variations of the colour of the comparison patch to be produced with the use of only one manual control. Thirdly the rapidly varying nature of the after-image phenomena gives little time for a match to be made, and there is no possibility of any subsequent verification. From experimental calibration measurements using colour filters, the standard deviation of a static colour reading is of the order of &O-O7 to +O-10 in C.I.E. x and y chromaticity co-ordinates, under the worst circumstances. There is the further inherent difficulty that the comparison stimuli must of necessity be of low luminance in order to match the faint after-images. After-images are often highly saturated even though they may be faint. Therefore it can happen that the comparison stimulus appears less saturated than the after-image, and this is particularly so in the purple region. There was no alternative in these cases to producing a match as close as possible to the after-image colour. RESULTS The
quantitative results quoted are those obtained by the author alone, who is a normal trichromat. This ensures that the results have a greater self-consistency, although it gives no information on individual differences. It is hoped, however, later to extend this work to other observers, both colour-normal and colour-defective.
0’8
FIG. 4. C.I.E. chromaticity diagram showing the colour sequences of after-images produced by exposure to a 2-Y dia. circular white stimulus viewed foveally for 1 sec. The results of these separate experiments are shown. (Stimulus of colour temperature 3000”K, and of retinal illumination 2 x 106 trolands). Numbers on curves indicate time in seconds from beginning of stimulus exposure.
Measurements of the Colour Sequences in Positive Visual After-Images
943
The stimulus was a circular white light source of colour temperature 3000°K of retinal illumination 2 x 106 trolands. The C.I.E. chromaticity co-ordinates of the stimulus are x = 0.43, y = 0.40. The eyes were dark-adapted for 10 min prior to each exposure to the stimulus. Figure 4 shows plots on the C.I.E. chromaticity diagram of the colour sequence of the after-images formed by exposure to a 25” diameter circular stimulus viewed foveally, for 1 second. The results of three separate experiments are shown. The point S represents the stimulus colour. The numbers against the points indicate the time in seconds that has elapsed since the beginning of the stimulus exposure. It is not possible to make measurements for about 20 set after the presentation of the stimulus. This is the order of time necessary to allow the extreme sensation of glare to subside, and for the after-image colour to be perceived. The multiple exposures indicate the order of reproducibility of the measurements. The variations shown include both instrumental and observational errors, and variations in colour of successive after-images. Figure 5 shows the c&our sequences of the same stimulus exposed for 2, 4 and 8 sec.
x FIG. 5. C.I.E. chromatieity diagram showing the colour sequences of after-images produced by exposure to a 2 5” dia. circular white stimulus viewed foveally for three different exposure times. (Stimulus of colour temperature 3OOO”K, and of retinal illumination 2 x lOatrolands). Numbers on curves indicate time in seconds from beginning of stimulus exposure.
In general the colour sequences are as follows. First the colour is white, close to the stimulus colour but often slightly bluer. It then moves towards the green, remaining of
944
C. A. PADGHAM
low saturation and then returns to near the stimulus colour. The movement is next towards the red-purples passing through purple to blue-purple and finally blue. These latter phases generally are of high saturation. The general sequence is always similar, although variations in saturation occur. This colour sequence has been fairly well confirmed by a number of normal observers using a colour-naming technique, although a small minority quote a slightly different colour sequence. The longer exposure times to the stimulus give rise to similar after-image colour sequences, but with longer time scales. It is interesting that BERRY(1922) quotes similar time sequences by some earlier workers. However, the stimulus and observing conditions in a good deal of the early work were either not controlled adequately or not specified, and are thus of little value. Figure 6 shows the colour sequences in after-images produced by exposure to a similar stimulus for 1 see, but of angular diameter 5”. The results of three separate experiments are shown. These results are not significantly different from those produced with a 2.5” diameter stimulus (Fig. 4).
FIO. 6. C.I.E. chromaticity diagram showing the colour sequencesof after-imagesproduced by exposure to a 5” dia. circular white stimulus viewed foveally for 1 sec. (Stimulus of colour temperature 3OOO“K,and of retinal illumination 2 x 106 trolands). Numbers on curves indicate time in seconds from beginning of stimulus exposure. ANALYSIS
OF RESULTS AND DISCUSSION
The results shown in Figs. 4 and 5 are re-plotted in Figs. 7-10. These show the values of the C.I.E. chromaticity co-ordinates X, y and z of the colour matches on the after-images as a function of time, measured from the beginning of the stimulus. The interesting common
Measurements of the Colour Sequences in Positive Visual After-Images
945
feature is the fact that the y and z components progress approximately in anti-phase, whereas the x component varies in a similar way to that of the JJ component.
0 SEC
FIG. 7. The C.I.E. chromaticity co-ordinates of after-image colours as ordinates shown as a function of time measured from the beginning of inducing stimulus. (Stimulus white, colour temperature 3000°K. circular 2.5” dia., retinal illumination 2 x lOa trolands exposed for 1 set).
0 0
100 SEC
FIG. 8. The C.I.E. chromaticity co-ordinates of aftcr-image colours as ordinates shown as a function of time measured from the beginning of inducing stimulus. (Stimulus white, colour temperature 3OOO”K, circular 2.5’ dia., retinal illumination 2 x lOa trolands, exposed for 2 set).
The results of Figs. 4 and 5 were also transformed to Kiinig-type fundamental sensations suggested by JUDD (1945). The transformation equations which were used were as follows:
R = l,OOg G = -0.46 R + 1.36 y’ + 0.10 i V=
1.002
11 = 1, r, = 13 = 0.
C. A. PADCHAM
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0 0
100 SEC
FIG. 9. The C.I.E. chromaticity co-ordinates of after-image colours as ordinates, shown as a function of time measured from the beginning of inducing stimulus. (Stimulus white, colour temperature 3OOO”K,circular 2.5” dia., retinal illumination 2 x 10atrolands, exposed for 4 set). 1-o
0' 0
100 SEC
FIG. 10. The C.I.E. chromatic&y co-ordinates of after-image colours as ordinates shown as a function of time measured from the beginning of inducing stimulus. (Stimulus white, colour temperatunz 3000°K, circular 2.5” dia., retinal illumination 2 x 106 trolands, exposed for 8 set).
The transformed results are shown in Figs. 1l-13, for exposures of 1, 2 and 4 set where the values of R, G and V as ordinates are plotted on an arbitrary scale as a function of time. (The 8-set curves lie too close together to be plotted on this scale.) The significant feature of these curves is the similarity in the decay functions for the R and G fundamentals which are closely exponential. This agrees with previous resuits (PADGHAM, 1963). However, the violet fundamental response, whilst initially lower, predominates, after about 60 set, towards the end of the decay period. This suggests that it is perhaps related to the observations of the “positive blue” phenomenon by WRIGHT (1946; see page 252). He found that following colour adaptation of the retina to almost any radiation, a test colour chosen from the red-yellow-green region of the spectrum appears, for a minute or so, to be mixed with a positive amount
Measurements of the Colour Sequences in Positive Visual After-Images la0
o 100 SEC 0 FIO. 11. The fundamental sensation magnitudes in after-image as ordinates shown as a function of time measured from the beginning of inducing stimulus. From data of Fig. 7 (I set exposure).
0 0 100 SEC Fro. 12. The fundamentai sensation magnitudes in after-image as ordinates &own BII a function of time measured from the @inning of inducing stimulus. From data of Fig. 8 (2 = exposun).
0 100 SEC FIG. 13. The fundamental sensation magnitudes in after-image as ordinates shown as a function of time measured from beginning of inducing stimulus. From data of Fig. 9 (4 set exposure). r)
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C. A. PADGHAM
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of blue. Thus blue, in addition to red and green are needed to be added together to produce a binocular match on a yellow test patch. It is interesting to note that in Wright’s experiments the amount of biue needed initially fell, at the beginning of the recovery from colour adaptation, then rose to a ma~mum after approximately 60 see, and subsequently fell. This is in general agreement with the variation of the violet fundamental sensation component of the after-image recovery, although Wright’s adapting stimulus luminance and exposure times were somewhat different. Wright’s theory that a persistent after-image was responsible for the phenomenon is thus given support by these observations. The use of other fundamental sensation data will only slightly aiter the transformation, the greatest diiTerence being in the red sensation. All the suggested fundamental sensations in effect equate the violet sensation with the 2 function and thus the general conclusions will not be affected by the type of transformation used. The violet sensation anomaly, indicated by these results suggests a basic difference between the after-functioning of the violet process and those mediating the red and green sensations. Acknowlcrlgcmmrs-The apparatus was constructed in the Department of Physics, whilst the author was a member of that Department. Thanks are due to the Head, professor F. Y. POYNTON,for his great interest and help, The author also wishes gratefully to acknowledge the help, in the design of the apparatus, given by Dr. B. H. CRAWFORD, late of the National Physical Laboratory, and the help and encouragement of Professon R, J. FLETCHER and R. W. G. HUNT. Finally, thanks are due to those students who helped in some colour naming experiments on after-hnagea,
REFERENCES ALPERN,M. and BAIIR, L. (1962). Durations of the after-images of brief flashes and the theory of the Broca and Sulxer phenomenon. J. opt. Sot. Am. 52,21P-221. BERRY,W. (1922). The Aigitt of coIours in the after-image of a bright light. Psychol. Bull. 19,307-337. JUDD, D. 3. (1945). Standard response functions for protanopic and deuteranopic vision. J. opt. Sot.
Rm. 3!J,lPP-221. PAWHAM, C. A. (1953). Quantitative study of visual after-images. Br. J. Ophtlud. 37, 165-170. PAWHAM C. A. (1963). The role of the retinal receptors in the formation of the positive visual after-image. Vision Res. 3, 4549. PALSHAM,C, A. (1965). After-images as a means of investigating rods and cones. Ciba Formdution Sym~s~um on Pky~logy and Experiments Psyckoiogy of Colour Vision, pp. 249-264, edited by
G. E. W. WANES
and JUUE Kr+nzrr. Churchill, London.
PANNED, M. (1947). lzber den Heligkeitsverlauf positiver Nachbilder. Ophtkahnoiogicu, &set 280-289. WRIGHT, W. D. (1964). Researches on normal and defective colour vision, Kimpton, London.
apparatus is described by means of which the colour of positive visual afterimages can be measured con~u~iy as they decay, by means of binocuiar matching. Results are shown of the coiour sequence6 in the after-images formed by exposure to a highintensity white light stimulus for various times. The general sequence is white, green, white red-purple, blue-purple and blue. The results are transformed into fundamental sensation suggcsted by Judd. This shows that the after-functioning of the red and green processes are similar, but that of the violet process is quite different. Abstract-An
R6swnG-On d&it un appareil qui permet Ia me-sure, par comparaison binoculaire, de la
couleur dune image consecutive positive, d’une fawn continue tandis qu’elle s’attetme, On montn les reSultats de la suite de couleurs pour ies images consecutives P une hnni&e blanche intense de durt5e varied. En g&&al cette suite est blanc, vet?, pourpre rougeatre peu satut-6, bleu-pourprt! et bleu. On transforme ces r&Bats au moyen des fondamentales sugg&es par Judd. Cela montre que le fonctionnement consecutif des mecanismes rouge et vert sent semblables, mais que celui du violet est tout-a-fait different.
113,
Measurements of the Colour Sequences in Positive Visual After-Images Zusammenfasaang-Es wird ein Apparat bcschrieben, mit dem die Farbe der positiven Nachbilder durch binokularen Vergleiih w&rend des ZcrfaUs kontinuierlich gemessm werden kann. Die Ergebnisse zeigen Farbsequenzen in den Nachbildem, die entstehen, wenn ein weifkr Lichtreiz hoher LeuChtdiChte bei vezhiedenen Z&en dargehOten wird. Die al&me& Reihenfolge ist WeiD, Grlin, Rot-purpur, Biau-purpur und Biau. Die Ergebnisse werden in die von Judd vorgeschlam Grundvalenzen umgesetzt. Hier zeigt sich, da0 die NachablWfe der roten und grUnen Prozesse iihnlich, dejenige des Violettprozesses aber ganz anders ist. Pexortse - OmicaH annapar c nob+ollfbm ~oroporo 4Be-r nono~Tc.nbHbIx noneXOBa=JIbHblX 06pa3OB MO=? H3MCpSITbCX HenpepMBHO l’IpH HX 3amaHEiH, Eia OCHOBaHZiM 6~Ho~R~pHoro CpaBIleHEfa. OZIW,I~C~ ~~CXO~~~bH~b C RoropoZi B03HmaioT u~era 31ux 06pa30~, nouI(: 3~cxxo3~335iw R crmyny 6enoro cBeTa BbICOKOjl HHTCSHCHBHoCTEI,HO piuHO# QnIUWIbHoCTli. 0614~ 3aKOHOMCpHocTb IIoCJle~oBa~bIfoCTH IlORBJlK)UIHXCIIUBeTOB: 6CllbIti, 3eneHb&, cienbIi% KpaCliO~TOQ’plYypOBbIi8, C~HenypnypoSti Ii CHIiHti. Pf3ynbTaTbI TpaHC@OpMApoBaHhl B OCHOBHble Olll)‘LUeHUII, no ,l&ew. nOKaSX0, ‘iT0 CneBOaafi &‘HKuH% RpaCHOrO zi 3eneHoro nporreccoe cxojq8a, a CpsioneToBoro npoi4ecca coE4epUemio 0xmi’IHa.
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