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Perception of tone differences from film transparencies
Photogrammetria - Elsevier Publishing Cempany, Amsterdam - Printed in The Netherlands
PERCEPTION OF TONE DIFFERENCES FROM FILM TRANSPARENCIES J. CIHLARI AND R. PROTZ Department of Land Resource Science, University o/ Guelph, Guelph, Ont. (Canada)
(Accepted for publication September 7. 1972) ABSTRACT Cihlar, J. and Protz, R., 1972. Perception of tone differences from film transparencies. Photogrammetria, 8(4):131-140. The threshold value of detectable tone differences is of importance to various problems related to information gathering by means of photo interpretation. This is because photo interpretation cannot proceed unless a tone difference is detected. The purpose of this study was to determine thresholds for the perception of tone differences from black-and-white (BW) and color aerial transparencies. The magnitudes of differences were subjectively estimated by five observers as well as measured using densitometric and colorimetric procedures. An effort was made to approximate closely the conditions of photo interpretation. A statistical analysis showed that tho reproducibility of the subjective estimates of tone differences was better for BW than for color as expected, but surprisingly better between sessions than within sessions. Within the range of tone differences encountered on color transparencies, the re'lationship Izetween estimated and measured differences was linear. The resulting threshold values were considerably larger than those characteristic for optimal conditions of viewing. Also, the thresholds substantially exceeded magnitudes of smallest differences detectable by instruments. These findings suggested that ordinary conditions of interpretation are not conducive to maximum extraction of tone information from aerial imagery. The possibility of using instruments to perform this task is briefly discussed. INTRODUCTION Capabilities of the visual m e c h a n i s m influence the a m o u n t a n d quality of i n f o r m a t i o n extracted from aerial photographs by h u m a n interpretation. N o inform a t i o n can be o b t a i n e d without first detecting a tone difference. C o n s e q u e n t l y , the threshold of a detected tone difference is of i m p o r t a n c e in photo interpretation problems, such as a c o m p a r i s o n of interpretative values of various imageries, the choice of o p t i m u m filters in film processing, the optical c o m b i n a t i o n and e n h a n c e m e n t of multispectral images, etc. T h e p u r p o s e of this study was to establish, for a limited range of tones, threshold values for tone differences perception from B W ( b l a c k - a n d - w h i t e ) a n d color aerial transparencies. Since m a g n i t u d e s of the thresholds are i n h e r e n t l y d e p e n d e n t u p o n experimental conditions, an effort was made to a p p r o x i m a t e the conditions of actual photo interpretation.
1 Present address: Dept. of Geography, The University of Kansas, Lawrence, Kans., U.S.A.
132
J. CIHLAR AND R. PROTZ
MATERIALS
Aerial photographs used for the experiment were obtained on May 1, 1969, in Brant County, southern Ontario. A Kodak Ektachrome MS Aerographic Film 2448 was processed to a negative, and color transparencies were exposed on SO-193 Color Print Film. A Kodak X X Aerographic Film type 2405 Estar Base was used to obtain BW negatives; the diapositives were prepared using a Kodak Aerographic Duplicating Film 4427, Estar Thick Base. Three transects were chosen: two crossing bare fields, and one crossing a hay field. METHODS
Perception of tone differences Five observers (four males, one female, aged 25-60 years) participated in the experiments; all had worked with aerial photographs before. According to Ishihara's (1964) test, their color vision was normal. Apart from RP, all observers had previous experience in similar tests. Tests were conducted in a dark room where the light table, illuminated from below with seven light tubes (General Electric F30.Tl2.CW.R.S.Coolwhite 3), was the only source of light. The total energy level was approximately 2.7 milliwatts per cm'-', and the maximum of spectral energy distribution curve was at 580 nm. In the training phase, the observer first examined tone differences (i.e., differences of various magnitudes on the transparency) using a Casella pocket stereoscope with one blackened lens (magnification 2.257(), and a black paper mask (Fig.l). After the examination, the observer was asked to establish a sub-
BLACK FIBER
B~LACI~ ~//PAPER
.85~,8
Fig.1. Paper m a s k used for outlining visual fields.
jective numerical scale of tone differences with an optional number of steps, and keep this scale in memory for the whole session. Prior to the testing phase, a
PERCEPTION
OF TONE DIFFERENCES
133
film transparency was fixed on the light table previously covered by a matt acetate sheet 0.08 mm thick. The paper mask was then positioned over the transparency so that only a known part of the transect, consisting of two visual fields, could be seen. The rest of the mask and transparency were covered by a sheet of the black paper (transmission density above 4.0). The stereoscope was then placed over the visual fields and the observer used the eye he preferred (the same for all sessions). The size of each of the visual fields, restricted by the paper mask and the black fiber, wash chosen at 1.85 mm square for the following reasons: (a) the aperture 1.85 mm, together with the stereoscope, created a visual field approximately 1 ° by ! ° (2 ° by 1 ° for two visual fields viewed simultaneously) which was compatible with the arrangement used to define the 1931 C.I.E. (Commission Internathmale de l'Eclairage) Standard Observer: (b) the uncertainty of estimates decreases with increasing size of visiual field (Judd and Wyszecki, 1963). In previous tests, the average coefficient of variation decreased from 84% to 48% when the size of the field was changed from 1.4 mm to 1.85 mm square (Cihlar, 1971). To estimate a tone difference, the observer rated the difference between left and right visual fields on his numerical scale. In cases where textural variations existed within visual fields, he was asked to estimate the difference after averaging out variations within each field. The paper mask was then shifted along the transect at a distance equal to the length of the visual field (1.85 ram), while the observer viewed a part of the light table not covered with the black papcr sheet as a reference light level. For each observer and film, two sessions were held. During each session, approximately one hour long, 85 tone differences were estimated. After each session, original TD 1 estimates were transformed to a uniform scale 0-5. The transformation coefficient was 5/c, where c was the average of five highest TD estimates at the given session.
Measurement of tone differences On BW diapositives, optical densities were obtained using a Macbeth transmission densitometer TD 203 A M for the visual fields previously observed. The area measured was delineated by a 1.85 mm square mask placed over the aperture of the densitometer. Absolute values of density differences between adjacent visual fields were taken as the counterparts ef the estimated differences. To measure tone differences on color transparencies, the spectral transmittances, colors, and color differences were determined in sequence. Spectral transmittances were measured on a Carl Zeiss P M Q - I I single-beam spectrophotometer (Cad Zeiss, 1969). To locate the visual fields accurately, the exit aperture of the integrating sphere was covered by a paper mask 1.85 mm square, and the film transparency was fixed on a microscope stage screwed to the right-hand side
1 TD designates tone difference.
134
J. C I H L A R
A N D R. P R O T Z
of the sphere attachment. Spectral transmittances were measured between 400 nm and 700 nm, at 10-nm intervals. The 1931 C.I.E. color coordinates were calculated using a computer program based on the method of weighted ordinates, with the following input: color matching functions (Wyszecki and Stiles, 1967); spectral energy distribution of light from the light table measured with ISCO spectro-rad!ometer model SR; absolute spectral reflectances of the integrating sphere coating (Hofert and Loof, 1964); and measured spectral transmittance values, corrected for transmittances between 710 nm and 750 nm using a parabolic function as suggested by MacAdam (1953). From the C.I.E. coordinates, Munsell renotations were computed? To calculate tone differences between adjacent visual fields on color transparencies, three color difference formulas were incorporated into computer programs: 1964 C.I.E. formula (hereafter designated CIE) (Wyszecki, 1963); cuberoot formula (CR) (Glasser et al., 1958); and Munsell renotation formula (MR) Wyszecki, 1968). RESULTS
AND DISCUSSION
Although tones from BW diapositives could be evaluated similarily as color tones, i.e., in a color space, it was considered more practical to analyze optical densities. These densities ranged from 0.26 to 1.12 (Table I). Colors from the TABLE
I
RANGES OF OPTICAL DENSITIES AND MUNSELL COLOR COORDINATES FOR VISUAL FIELDS ON THE THREE TRANSECTS
Transect
1 4 6
BW diapositives
C:~lor ;ransparencies
optical density
hue
value
chroma
0.26-0.90 0.28-0.69 0.85-1.12
0.97 Y-1.97 Y 0.32 Y-3.67 Y 3.24 Y-9.52 Y
4.37-7.63 4.59-6.48 3.95-5.75
3.41-7.80 3.77-6.88 2.89-5.60
three transects had a limited range of chromaticities. In the Munsell system, range of hues was restricted to approximately 10% of the hue circle, while the range of values and chromas was substantially larger (Table I). The histograms (Fig.2, 3) show that for both BW and color tones, small differences dominated. Repeatability of T D estimates
To evaluate the repeatability of TD estimates, ten pairs of visual fields were viewed twice during each session. These observations provided a measure of i Computer program was provided courtesy of Radiation Optics Section, National Research Council, Ottawa, Canada.
135
PERCEPTION OF TONE DIFFERENCES
'°l 30
L---~MEAN 20
I0
i 0
i
0.05
0 .ll0
B&W TONE DIFFEIZENCE
Fig.2. F r e q u e n c y visual fields.
t
~
0.15
0.20
(optical density
distribution
i , 0.25
units)
of B W tone differences
measured
between
adjacent
40
30
~2o
I0
I
r
1 i 5
t
[-"7 , ]
i0
15
C O L O R TONE D I F F E R E N C E
I
' 20
25
(CIE units)
Fig.3. F r e q u e n c y distribution of color tone differences m e a s u r e d between adjacent visual fields.
variability of estimates within sessions. Similarily, the same estimates from two sessions served as a measure of variation between sessions. An analysis of these limited data (Table II) indicated: (1) within sessions: correlation coefficient R ranged from 0.59 to 0.95 and was higher for BW tones for all but one observer; (2) between sessions: in all cases, R was higher for BW than for color tones; the differences between R's were smaller, however, than those within sessions (Table H). The magnitudes of T D estimates were compared using the least-square straight line which was fitted to pairs of estimates. With 1 : 1 correspondence between the estimates, slope of the line m should equal 1. In all four combinations tone/session (Table II) the deviations of m from 1.0 could not be explained by the fatigue effect (Judd, 1930) since the estimates were repeated at three different times during a session. Apart from a smaller spread for BW than for color tones,
136
J. C I H L A R AND R. P R O T Z
TABLE II REPEATABILITY OF TONE DIFFERENCES E S T 1 M A I E S
Observer
[Vi:h.)l D'2ssions BW
B e t w c ~ :: ,','c,;.';.':ms "~FI7 -,,
,_'o.:::."
color
R
t;:
R
m
R
,~7
R
m
JC JEG Jk GP RP
0.95 0.82 0.82 0.91 (/.74
1.01 0.50 0.67 1. ) 7 0.53
0.79 0.69 0.89 0.81 0.59
0.76 0.45 0.89 1.25 0.51
0.96 0.91 /).94 0.69 0.90
0.96 1.03 0.60 0.68 1.24
0.88 0.83 0.86 0.61 0.88
(/.78 1.42 1.3~ 0.99 0.61
Average
0.85
0.77
0.76
0.77
0.88
0.90
0.81
1.02
m is the slope of a least-square straight line Y = m X ÷ b, where X and Y are tone differences estimates from the first and second session (for calculating R and m between sessions) or first and second estimates within sessions (for calculating R and m within sessions). R is the correlation coefficient between X and Y. the m values for i n d i v i d u a l observers did n o t seem to show any systematic relationship. A v e r a g e values of m ( T a b l e II) i n d i c a t e d that a 1 : 1 c o r r e s p o n d e n c e of pairs of estimates was b e t t e r b e t w e e n sessions than within sessions, i.e., the average values of m are closer to 1.0 for between-sessions c a m p a r i s o n . T h e results thus s h o w e d the r e p e a t a b i l i t y was: (a) better for B W than c o l o r tones; a n d (b) b e t t e r b e t w e e n t h a n within sessions. T h e first t r e n d was a p p a r e n t l y related to the n u m b e r of d i m e n s i o n s of the tones (one for B W , three for color) which influenced the T D estimates. A s to the s e c o n d trend, o n e w o u l d n o r m a l l y e x p e c t r e p e a t a b i l i t y to be b e t t e r within sessions. T h e result suggests that the between-sessions v a r i a t i o n was a s e c o n d a r y source of variability. Because of the differences existing b e t w e e n an o b s e r v e r ' s two sets of T D estimates of either tone, a statistical test was used to d e t e r m i n e w h e t h e r the two sets were, in fact, f r o m one p o p u l a t i o n . A c o m p a r i s o n of s a m p l e means of p a i r e d o b s e r v a t i o n s (Steel a n d T o r r i e , 1960) r e v e a l e d that differences between the two sets were n o t significant at the 5 % p r o b a b i l i t y level with one e x c e p t i o n (observer R P , B W tones). T h e r e f o r e , T D estimates f r o m two sessions by all o t h e r o b s e r v e r s c o u l d be a v e r a g e d since they were f r o m the s a m e p o p u l a t i o n . T h e s e a v e r a g e d estimates were used in s u b s e q u e n t analyses as they p r o v i d e d b e t t e r m e a s u r e s of e s t i m a t e d tone differences.
Linearity of the relationship of observed and measured TD A n e x a m i n a t i o n of the e x p e r i m e n t a l d a t a i n d i c a t e d that the classical n o r m a l linear regression m o d e l ( H u a n g , 1970) c o u l d be used. M e a s u r e d tone differences were c h o s e n as the d e p e n d e n t variable. L i n e a r i t y of the r e l a t i o n s h i p b e t w e e n e s t i m a t e d a n d m e a s u r e d tone dif-
137
PERCEPTION OF TONE DIFFERENCES
ferences was tested by regressing the first three terms (linear, quadratic, cubic) of T D estimates against measured differences individually for BW and color tones. Color tone differences, calculated using the 1964 CIE formula, ranged from 0.0 to 20.9 C1E units. BW tone differences for observer RP were tested separately for each session. A stepwise multiple linear regression program BMD02R was used (Dixon, 19671. The analysis showed that only in one case (observer JEG, BW tones, cubic factor) a nonlinear term was also significant at the 5% probability level; in other cases, only linear terms were significant at this level of probability. The results thus suggested that for given experimental conditions, the relationship of the observed and measured tone differences was linear. This linearity was used in subsequent calculations.
Perforrna~we of three color-difference ]ormzdas Hunt (1967) found the 1964 CIE formula satisfactory for use in color photography. In general, however, no single formula gives optimum results for an arbitrary set of experimental conditions (Billmeyer, 1970). The C.I.E. Committee on Colorimetry recommended four formulas (Wyszecki, 1968). Table Ill T A B L E Ill PERFORM'kNCE OF THREE COLOR DIFFERENCE FORMULAS
Ohs'erver
JC JEG JL GP RP
R 2 /or formula CIE
CR
MR
0.73 0.39 0.62 0.56 0.61
0.72 0.39 0.62 0.55 0.58
(t.74 0.38 0.62 0.56 0.61
R" (coefficient of determination) is equal to the proportion of total variation of measured color differences accounted for by the tone differences estimates.
shows a comparative performance of three of these formulas for TD's frcm color transparencies. The criterion (R'-') measures the ability of a formula to account for the variation of observed TD's. It can be seen that the three formulas were nearly equal for all observers. If the R" values are added, CIE and MR are exactly equal, and CR formula is slightly lower. Thus it appears that using different formulas would not result in an improved fit to the observed data. The total proportion of explained variability, R ~, was rather small for all formulas (Table III). In addition to other influences related to color differences perception (Optical Society of America, 1963), factors specific to this experiment included small size of the visual field, use of subjective scale of tone differences, and the necessity of averaging textural variations within some visual fields. The tests were performed at different times of the day (and consequently possibly dif-
138
j. ClHLAR AND R. PROTZ
f e r e n t f a t i g u e l e v e l s ) as it w a s f e l t t h a t p h o t o
interpreters
work
under
similar
constraints.
Magnitudes of the just perceptible tone difference Means
of the JPTD's l and 95%
confidence
limits were calculated on the
b a s i s o f t h e l i n e a r - r e g r e s s i o n m o d e l w i t h a l i n e a r t e r m o n l y , i.e., Y ~ The observed value of the JPTD
a X ÷ b.
for each session and observer was obtained
as
an average of the five lowest estimates during the session. Table IV presents the T A B L E IV JUST P E R C E P T I B L E TONE DIFFERENCES STATISTICS FOR B W AND COLOR TONES
Observer
Statistic
BW tonesl
Color tones~ CIE
CR
MR
JC
mean CLL" CLU
0.04 0.04 0.05
2.0 1.4 2.6
2.3 1.5 3.1
1.2 0.8 1.5
JEG
mean CLL CLU
0.05 0.04 0.05
2.7 1.8 3.6
3.2 2.1 4.3
1.6 1.1 2.2
JL
mean CLL CLU
0.06 0.05 0.07
3.5 2.8 4.1
4.1 3.3 4.9
2.1 1.7 2.5
GP
mean CLL CLU
0.05 0.05 0.06
2.9 2.1 3.6
3.4 2.5 4.3
1.7 1.2 2.2
RP
mean CLL CLU mean CLL CLU
0,04 * 0.04 0.05 0.04 ** 0.04 0.05
2.0 1.2 2.8
2.4 1.4 3.3
1.2 0.7 1.6
Avcrage
mean CLL CLU
0.05 0.04 0.06
2.6 1.9 3.3
3.1 2.2 4.0
1.6 1.1 2.0
1 Units: BW tones - optical density; color tones - specific to each formula; -~ CLL (CLU): lower (upper) 95% confidence limits. * based on the first session; ** based on the second session. statistics calculated
for color tones
(by three
different formulas)
and
for BW
tone differences. The following facts were observed: (1)
The mean value of JPTD
f o r all o b s e r v e r s c a l c u l a t e d b y t h e 1 9 6 4 C I E
1 JPTD designated a just perceptible tone difference; the just perceptible difference was defined (O.S.A., 1963) as a difference large enough to be perceived in almost every trial.
PERCEPTION OF TONE DIFFERENCES
139
formula (2.6 units) was 13 times higher than the value of a color difference barely detectable under optimal viewing conditions (Robertson, 1967). This indicated that, in terms of optimal conditions of viewing, the given experimental conditions were far from ideal, and (indirectly) that conditions of human photo interpretation are not conducive to maximum extraction of tone information from aerial photographs. (2) The mean value of JPTD for BW diapositives was 0.05 density units which is in excellent agreement with an estimate by Anonymous (1965). (3) A comparison of statistics for various observers revealed that larger differences in J P T D existed for color than for BW tone differences. For example, the range of means for BW tones (0.04-0.06) represented 50% of the smallest mean. Considering statistics for the CIE formula, the comparable values were 2.0-3.5, and 75%. (4 I} There was no difference in the magnitudes of JPTD between observer RP who had no previous experience in similar tests and the remaining observers. This supports previous observation by Robertson I (personal communication) that the value of J P T D does not decrease with increased training. The measurements indicated that instruments could be used to obtain more accurate tone information. For example, Table IV shows that for BW diapositives, the minimum observed tone difference was five times the smallest subdivision of the densitometric scale. (It should be noted, however, that the precision of most densitometers is not smaller than 0.02). Tests of precision of spectral transmittance measurements with the PMQ-II spectrophotometer indicated that the average color differences between repeated measurements was 0.5 CIE units, a value which could be further reduced to about 0.2-0.3. Again, this is approximately one fifth of the JPTD for human observers (Table IV). CONCLUSIONS
The threshold values of tone differences resulting from this study are given in Table IV. In addition, supplementary statistical analysis of the data indicated that: (1) The repeatability of tone differences estimates was better for BW tones and between sessions than for color tones and within sessions. (2) The relationship between observed and measured tone differences was linear for the range of differences encountered. (3) The comparatively large values of the just perceptible tone differences suggested that ordinary conditions of photo interpretation are not conducive to maximum extraction of tone information from aerial photographs. While this study was based on aerial photographs, it is apparent that the 1 Dr. A. R. Robertson, Radiation Optics Section, National Research Council, Ottawa, Ont. (Canada).
140
J. CtHLAR AND R. PROTZ
results are applicable to imagery from other remote sensors recorded on photographic film. ACKNOWLEDGEMENTS T h e authors wish to t h a n k Carl Zeiss C o m p a n y Ltd. ( T o r o n t o , O n t a r i o ) for permission to use the P M Q - I I s p e c t r o p h o t o m e t e r and Mr. K. L. R. M a h l e r for his assistance in the course of m e a s u r e m e n t s . Dr. G. Wyszecki ( R a d i a t i o n Optics Section, National Research Council, Ottawa, O n t a r i o kindly offered his advice in preparing the experiment. The financial s u p p o r t of the C.D.A. G r a n t No.9097 is gratefully acknowledged. REFERENCES Anonymous, 1965. Control Techniques in Film Processing. Society of Motion Picture and Television Engineers, New York, N.Y., 181 pp. Billmeyer, F. W. Jr., 1970. Optical aspects of color, 16. Appropriate use of color-difference equations. Opt. Spectra, 4:63-66. Cihlar, ]., 1971. Color Aerial Photography in Soil Mapping. Thesis, Univ. of Guelph, Guelph, Onl., 195 pp. Carl Zeiss, 1969. PMQ-II Spectrophotometer. Carl Zeiss, Oberkochen, 31 pp. Dixon, W. J. (Editor), 1967. BMD Biomedical Compnter Programs. University of California Press, Berkeley-Los Angeles, Calif., 600 pp. Glasser, L. G., McKinney, A. H., Reilly, C. D. and Schnelle, P. D., 1958. Cube-root color coordinate system. J. Opt. Soc. Am., 48:736-740. Hofert, H. J. and I_,oof, H., 1964. Magnesium oxide powder as a white standard. Farhe, 13:53-62. Huang, D. S., 1970. Regression and Econometric Methods. Wiley, New York, N.Y., 274 pp. Hunt, R. W. G., 1967. The Reproduction o/ Color. Wiley, London, 500 pp. Judd, D. B., 1930. Precision of color temperature measurements under various observing conditions: a new color comparator for incandescent lamps, Bur. Std. J. Res., 5:11611177.
Judd, D. B. and Wyszecki, G., 1963. Color in Business, Science, and Industry. Wiley, New York, N.Y., 140 pp. lshihara, S., /964. Tests /or Color Blindness. Kanehara Shuppan. Tokyo, .. pp. MacAdam, D. L., 1953. Truncated weighted ordinate integrations in colorimetry. J. Opt. Soc. Am., 43:622-623. Optical Society of America, 1963. The Science o/ Color. Optical Society of America, Washington, D.C., 385 pp. Robertson, A. R., 1967. Colorimetric significance of spectrophotometric errors. J. Opt. Soc. Am., 57:691-698. Steel, R. G. D. and Torrie, J. H., 1960. Principles and Procedures of Statistics. McGraw-Hill, New York, N.Y., 481 pp. Wyszecki, G., 1963. Proposal for a new color-difference formula. J. Opt. Soe. Am., 53: 1318-1319. Wyszecki, G., 1968. Recent agreements by the Colorimetry Committee of the Commission lnternationale de l'Eclairage. J. Opt. Soc. Am., 58:290-292. Wyszecki, G. and Stiles, W. S., 1967. Color Science. Wiley, New York, N.Y., 628 pp.
PERCEPTION OF TONE DIFFERENCES FROM FILM TRANSPARENCIES J. CIHLARI AND R. PROTZ Department of Land Resource Science, University o/ Guelph, Guelph, Ont. (Canada)
(Accepted for publication September 7. 1972) ABSTRACT Cihlar, J. and Protz, R., 1972. Perception of tone differences from film transparencies. Photogrammetria, 8(4):131-140. The threshold value of detectable tone differences is of importance to various problems related to information gathering by means of photo interpretation. This is because photo interpretation cannot proceed unless a tone difference is detected. The purpose of this study was to determine thresholds for the perception of tone differences from black-and-white (BW) and color aerial transparencies. The magnitudes of differences were subjectively estimated by five observers as well as measured using densitometric and colorimetric procedures. An effort was made to approximate closely the conditions of photo interpretation. A statistical analysis showed that tho reproducibility of the subjective estimates of tone differences was better for BW than for color as expected, but surprisingly better between sessions than within sessions. Within the range of tone differences encountered on color transparencies, the re'lationship Izetween estimated and measured differences was linear. The resulting threshold values were considerably larger than those characteristic for optimal conditions of viewing. Also, the thresholds substantially exceeded magnitudes of smallest differences detectable by instruments. These findings suggested that ordinary conditions of interpretation are not conducive to maximum extraction of tone information from aerial imagery. The possibility of using instruments to perform this task is briefly discussed. INTRODUCTION Capabilities of the visual m e c h a n i s m influence the a m o u n t a n d quality of i n f o r m a t i o n extracted from aerial photographs by h u m a n interpretation. N o inform a t i o n can be o b t a i n e d without first detecting a tone difference. C o n s e q u e n t l y , the threshold of a detected tone difference is of i m p o r t a n c e in photo interpretation problems, such as a c o m p a r i s o n of interpretative values of various imageries, the choice of o p t i m u m filters in film processing, the optical c o m b i n a t i o n and e n h a n c e m e n t of multispectral images, etc. T h e p u r p o s e of this study was to establish, for a limited range of tones, threshold values for tone differences perception from B W ( b l a c k - a n d - w h i t e ) a n d color aerial transparencies. Since m a g n i t u d e s of the thresholds are i n h e r e n t l y d e p e n d e n t u p o n experimental conditions, an effort was made to a p p r o x i m a t e the conditions of actual photo interpretation.
1 Present address: Dept. of Geography, The University of Kansas, Lawrence, Kans., U.S.A.
132
J. CIHLAR AND R. PROTZ
MATERIALS
Aerial photographs used for the experiment were obtained on May 1, 1969, in Brant County, southern Ontario. A Kodak Ektachrome MS Aerographic Film 2448 was processed to a negative, and color transparencies were exposed on SO-193 Color Print Film. A Kodak X X Aerographic Film type 2405 Estar Base was used to obtain BW negatives; the diapositives were prepared using a Kodak Aerographic Duplicating Film 4427, Estar Thick Base. Three transects were chosen: two crossing bare fields, and one crossing a hay field. METHODS
Perception of tone differences Five observers (four males, one female, aged 25-60 years) participated in the experiments; all had worked with aerial photographs before. According to Ishihara's (1964) test, their color vision was normal. Apart from RP, all observers had previous experience in similar tests. Tests were conducted in a dark room where the light table, illuminated from below with seven light tubes (General Electric F30.Tl2.CW.R.S.Coolwhite 3), was the only source of light. The total energy level was approximately 2.7 milliwatts per cm'-', and the maximum of spectral energy distribution curve was at 580 nm. In the training phase, the observer first examined tone differences (i.e., differences of various magnitudes on the transparency) using a Casella pocket stereoscope with one blackened lens (magnification 2.257(), and a black paper mask (Fig.l). After the examination, the observer was asked to establish a sub-
BLACK FIBER
B~LACI~ ~//PAPER
.85~,8
Fig.1. Paper m a s k used for outlining visual fields.
jective numerical scale of tone differences with an optional number of steps, and keep this scale in memory for the whole session. Prior to the testing phase, a
PERCEPTION
OF TONE DIFFERENCES
133
film transparency was fixed on the light table previously covered by a matt acetate sheet 0.08 mm thick. The paper mask was then positioned over the transparency so that only a known part of the transect, consisting of two visual fields, could be seen. The rest of the mask and transparency were covered by a sheet of the black paper (transmission density above 4.0). The stereoscope was then placed over the visual fields and the observer used the eye he preferred (the same for all sessions). The size of each of the visual fields, restricted by the paper mask and the black fiber, wash chosen at 1.85 mm square for the following reasons: (a) the aperture 1.85 mm, together with the stereoscope, created a visual field approximately 1 ° by ! ° (2 ° by 1 ° for two visual fields viewed simultaneously) which was compatible with the arrangement used to define the 1931 C.I.E. (Commission Internathmale de l'Eclairage) Standard Observer: (b) the uncertainty of estimates decreases with increasing size of visiual field (Judd and Wyszecki, 1963). In previous tests, the average coefficient of variation decreased from 84% to 48% when the size of the field was changed from 1.4 mm to 1.85 mm square (Cihlar, 1971). To estimate a tone difference, the observer rated the difference between left and right visual fields on his numerical scale. In cases where textural variations existed within visual fields, he was asked to estimate the difference after averaging out variations within each field. The paper mask was then shifted along the transect at a distance equal to the length of the visual field (1.85 ram), while the observer viewed a part of the light table not covered with the black papcr sheet as a reference light level. For each observer and film, two sessions were held. During each session, approximately one hour long, 85 tone differences were estimated. After each session, original TD 1 estimates were transformed to a uniform scale 0-5. The transformation coefficient was 5/c, where c was the average of five highest TD estimates at the given session.
Measurement of tone differences On BW diapositives, optical densities were obtained using a Macbeth transmission densitometer TD 203 A M for the visual fields previously observed. The area measured was delineated by a 1.85 mm square mask placed over the aperture of the densitometer. Absolute values of density differences between adjacent visual fields were taken as the counterparts ef the estimated differences. To measure tone differences on color transparencies, the spectral transmittances, colors, and color differences were determined in sequence. Spectral transmittances were measured on a Carl Zeiss P M Q - I I single-beam spectrophotometer (Cad Zeiss, 1969). To locate the visual fields accurately, the exit aperture of the integrating sphere was covered by a paper mask 1.85 mm square, and the film transparency was fixed on a microscope stage screwed to the right-hand side
1 TD designates tone difference.
134
J. C I H L A R
A N D R. P R O T Z
of the sphere attachment. Spectral transmittances were measured between 400 nm and 700 nm, at 10-nm intervals. The 1931 C.I.E. color coordinates were calculated using a computer program based on the method of weighted ordinates, with the following input: color matching functions (Wyszecki and Stiles, 1967); spectral energy distribution of light from the light table measured with ISCO spectro-rad!ometer model SR; absolute spectral reflectances of the integrating sphere coating (Hofert and Loof, 1964); and measured spectral transmittance values, corrected for transmittances between 710 nm and 750 nm using a parabolic function as suggested by MacAdam (1953). From the C.I.E. coordinates, Munsell renotations were computed? To calculate tone differences between adjacent visual fields on color transparencies, three color difference formulas were incorporated into computer programs: 1964 C.I.E. formula (hereafter designated CIE) (Wyszecki, 1963); cuberoot formula (CR) (Glasser et al., 1958); and Munsell renotation formula (MR) Wyszecki, 1968). RESULTS
AND DISCUSSION
Although tones from BW diapositives could be evaluated similarily as color tones, i.e., in a color space, it was considered more practical to analyze optical densities. These densities ranged from 0.26 to 1.12 (Table I). Colors from the TABLE
I
RANGES OF OPTICAL DENSITIES AND MUNSELL COLOR COORDINATES FOR VISUAL FIELDS ON THE THREE TRANSECTS
Transect
1 4 6
BW diapositives
C:~lor ;ransparencies
optical density
hue
value
chroma
0.26-0.90 0.28-0.69 0.85-1.12
0.97 Y-1.97 Y 0.32 Y-3.67 Y 3.24 Y-9.52 Y
4.37-7.63 4.59-6.48 3.95-5.75
3.41-7.80 3.77-6.88 2.89-5.60
three transects had a limited range of chromaticities. In the Munsell system, range of hues was restricted to approximately 10% of the hue circle, while the range of values and chromas was substantially larger (Table I). The histograms (Fig.2, 3) show that for both BW and color tones, small differences dominated. Repeatability of T D estimates
To evaluate the repeatability of TD estimates, ten pairs of visual fields were viewed twice during each session. These observations provided a measure of i Computer program was provided courtesy of Radiation Optics Section, National Research Council, Ottawa, Canada.
135
PERCEPTION OF TONE DIFFERENCES
'°l 30
L---~MEAN 20
I0
i 0
i
0.05
0 .ll0
B&W TONE DIFFEIZENCE
Fig.2. F r e q u e n c y visual fields.
t
~
0.15
0.20
(optical density
distribution
i , 0.25
units)
of B W tone differences
measured
between
adjacent
40
30
~2o
I0
I
r
1 i 5
t
[-"7 , ]
i0
15
C O L O R TONE D I F F E R E N C E
I
' 20
25
(CIE units)
Fig.3. F r e q u e n c y distribution of color tone differences m e a s u r e d between adjacent visual fields.
variability of estimates within sessions. Similarily, the same estimates from two sessions served as a measure of variation between sessions. An analysis of these limited data (Table II) indicated: (1) within sessions: correlation coefficient R ranged from 0.59 to 0.95 and was higher for BW tones for all but one observer; (2) between sessions: in all cases, R was higher for BW than for color tones; the differences between R's were smaller, however, than those within sessions (Table H). The magnitudes of T D estimates were compared using the least-square straight line which was fitted to pairs of estimates. With 1 : 1 correspondence between the estimates, slope of the line m should equal 1. In all four combinations tone/session (Table II) the deviations of m from 1.0 could not be explained by the fatigue effect (Judd, 1930) since the estimates were repeated at three different times during a session. Apart from a smaller spread for BW than for color tones,
136
J. C I H L A R AND R. P R O T Z
TABLE II REPEATABILITY OF TONE DIFFERENCES E S T 1 M A I E S
Observer
[Vi:h.)l D'2ssions BW
B e t w c ~ :: ,','c,;.';.':ms "~FI7 -,,
,_'o.:::."
color
R
t;:
R
m
R
,~7
R
m
JC JEG Jk GP RP
0.95 0.82 0.82 0.91 (/.74
1.01 0.50 0.67 1. ) 7 0.53
0.79 0.69 0.89 0.81 0.59
0.76 0.45 0.89 1.25 0.51
0.96 0.91 /).94 0.69 0.90
0.96 1.03 0.60 0.68 1.24
0.88 0.83 0.86 0.61 0.88
(/.78 1.42 1.3~ 0.99 0.61
Average
0.85
0.77
0.76
0.77
0.88
0.90
0.81
1.02
m is the slope of a least-square straight line Y = m X ÷ b, where X and Y are tone differences estimates from the first and second session (for calculating R and m between sessions) or first and second estimates within sessions (for calculating R and m within sessions). R is the correlation coefficient between X and Y. the m values for i n d i v i d u a l observers did n o t seem to show any systematic relationship. A v e r a g e values of m ( T a b l e II) i n d i c a t e d that a 1 : 1 c o r r e s p o n d e n c e of pairs of estimates was b e t t e r b e t w e e n sessions than within sessions, i.e., the average values of m are closer to 1.0 for between-sessions c a m p a r i s o n . T h e results thus s h o w e d the r e p e a t a b i l i t y was: (a) better for B W than c o l o r tones; a n d (b) b e t t e r b e t w e e n t h a n within sessions. T h e first t r e n d was a p p a r e n t l y related to the n u m b e r of d i m e n s i o n s of the tones (one for B W , three for color) which influenced the T D estimates. A s to the s e c o n d trend, o n e w o u l d n o r m a l l y e x p e c t r e p e a t a b i l i t y to be b e t t e r within sessions. T h e result suggests that the between-sessions v a r i a t i o n was a s e c o n d a r y source of variability. Because of the differences existing b e t w e e n an o b s e r v e r ' s two sets of T D estimates of either tone, a statistical test was used to d e t e r m i n e w h e t h e r the two sets were, in fact, f r o m one p o p u l a t i o n . A c o m p a r i s o n of s a m p l e means of p a i r e d o b s e r v a t i o n s (Steel a n d T o r r i e , 1960) r e v e a l e d that differences between the two sets were n o t significant at the 5 % p r o b a b i l i t y level with one e x c e p t i o n (observer R P , B W tones). T h e r e f o r e , T D estimates f r o m two sessions by all o t h e r o b s e r v e r s c o u l d be a v e r a g e d since they were f r o m the s a m e p o p u l a t i o n . T h e s e a v e r a g e d estimates were used in s u b s e q u e n t analyses as they p r o v i d e d b e t t e r m e a s u r e s of e s t i m a t e d tone differences.
Linearity of the relationship of observed and measured TD A n e x a m i n a t i o n of the e x p e r i m e n t a l d a t a i n d i c a t e d that the classical n o r m a l linear regression m o d e l ( H u a n g , 1970) c o u l d be used. M e a s u r e d tone differences were c h o s e n as the d e p e n d e n t variable. L i n e a r i t y of the r e l a t i o n s h i p b e t w e e n e s t i m a t e d a n d m e a s u r e d tone dif-
137
PERCEPTION OF TONE DIFFERENCES
ferences was tested by regressing the first three terms (linear, quadratic, cubic) of T D estimates against measured differences individually for BW and color tones. Color tone differences, calculated using the 1964 CIE formula, ranged from 0.0 to 20.9 C1E units. BW tone differences for observer RP were tested separately for each session. A stepwise multiple linear regression program BMD02R was used (Dixon, 19671. The analysis showed that only in one case (observer JEG, BW tones, cubic factor) a nonlinear term was also significant at the 5% probability level; in other cases, only linear terms were significant at this level of probability. The results thus suggested that for given experimental conditions, the relationship of the observed and measured tone differences was linear. This linearity was used in subsequent calculations.
Perforrna~we of three color-difference ]ormzdas Hunt (1967) found the 1964 CIE formula satisfactory for use in color photography. In general, however, no single formula gives optimum results for an arbitrary set of experimental conditions (Billmeyer, 1970). The C.I.E. Committee on Colorimetry recommended four formulas (Wyszecki, 1968). Table Ill T A B L E Ill PERFORM'kNCE OF THREE COLOR DIFFERENCE FORMULAS
Ohs'erver
JC JEG JL GP RP
R 2 /or formula CIE
CR
MR
0.73 0.39 0.62 0.56 0.61
0.72 0.39 0.62 0.55 0.58
(t.74 0.38 0.62 0.56 0.61
R" (coefficient of determination) is equal to the proportion of total variation of measured color differences accounted for by the tone differences estimates.
shows a comparative performance of three of these formulas for TD's frcm color transparencies. The criterion (R'-') measures the ability of a formula to account for the variation of observed TD's. It can be seen that the three formulas were nearly equal for all observers. If the R" values are added, CIE and MR are exactly equal, and CR formula is slightly lower. Thus it appears that using different formulas would not result in an improved fit to the observed data. The total proportion of explained variability, R ~, was rather small for all formulas (Table III). In addition to other influences related to color differences perception (Optical Society of America, 1963), factors specific to this experiment included small size of the visual field, use of subjective scale of tone differences, and the necessity of averaging textural variations within some visual fields. The tests were performed at different times of the day (and consequently possibly dif-
138
j. ClHLAR AND R. PROTZ
f e r e n t f a t i g u e l e v e l s ) as it w a s f e l t t h a t p h o t o
interpreters
work
under
similar
constraints.
Magnitudes of the just perceptible tone difference Means
of the JPTD's l and 95%
confidence
limits were calculated on the
b a s i s o f t h e l i n e a r - r e g r e s s i o n m o d e l w i t h a l i n e a r t e r m o n l y , i.e., Y ~ The observed value of the JPTD
a X ÷ b.
for each session and observer was obtained
as
an average of the five lowest estimates during the session. Table IV presents the T A B L E IV JUST P E R C E P T I B L E TONE DIFFERENCES STATISTICS FOR B W AND COLOR TONES
Observer
Statistic
BW tonesl
Color tones~ CIE
CR
MR
JC
mean CLL" CLU
0.04 0.04 0.05
2.0 1.4 2.6
2.3 1.5 3.1
1.2 0.8 1.5
JEG
mean CLL CLU
0.05 0.04 0.05
2.7 1.8 3.6
3.2 2.1 4.3
1.6 1.1 2.2
JL
mean CLL CLU
0.06 0.05 0.07
3.5 2.8 4.1
4.1 3.3 4.9
2.1 1.7 2.5
GP
mean CLL CLU
0.05 0.05 0.06
2.9 2.1 3.6
3.4 2.5 4.3
1.7 1.2 2.2
RP
mean CLL CLU mean CLL CLU
0,04 * 0.04 0.05 0.04 ** 0.04 0.05
2.0 1.2 2.8
2.4 1.4 3.3
1.2 0.7 1.6
Avcrage
mean CLL CLU
0.05 0.04 0.06
2.6 1.9 3.3
3.1 2.2 4.0
1.6 1.1 2.0
1 Units: BW tones - optical density; color tones - specific to each formula; -~ CLL (CLU): lower (upper) 95% confidence limits. * based on the first session; ** based on the second session. statistics calculated
for color tones
(by three
different formulas)
and
for BW
tone differences. The following facts were observed: (1)
The mean value of JPTD
f o r all o b s e r v e r s c a l c u l a t e d b y t h e 1 9 6 4 C I E
1 JPTD designated a just perceptible tone difference; the just perceptible difference was defined (O.S.A., 1963) as a difference large enough to be perceived in almost every trial.
PERCEPTION OF TONE DIFFERENCES
139
formula (2.6 units) was 13 times higher than the value of a color difference barely detectable under optimal viewing conditions (Robertson, 1967). This indicated that, in terms of optimal conditions of viewing, the given experimental conditions were far from ideal, and (indirectly) that conditions of human photo interpretation are not conducive to maximum extraction of tone information from aerial photographs. (2) The mean value of JPTD for BW diapositives was 0.05 density units which is in excellent agreement with an estimate by Anonymous (1965). (3) A comparison of statistics for various observers revealed that larger differences in J P T D existed for color than for BW tone differences. For example, the range of means for BW tones (0.04-0.06) represented 50% of the smallest mean. Considering statistics for the CIE formula, the comparable values were 2.0-3.5, and 75%. (4 I} There was no difference in the magnitudes of JPTD between observer RP who had no previous experience in similar tests and the remaining observers. This supports previous observation by Robertson I (personal communication) that the value of J P T D does not decrease with increased training. The measurements indicated that instruments could be used to obtain more accurate tone information. For example, Table IV shows that for BW diapositives, the minimum observed tone difference was five times the smallest subdivision of the densitometric scale. (It should be noted, however, that the precision of most densitometers is not smaller than 0.02). Tests of precision of spectral transmittance measurements with the PMQ-II spectrophotometer indicated that the average color differences between repeated measurements was 0.5 CIE units, a value which could be further reduced to about 0.2-0.3. Again, this is approximately one fifth of the JPTD for human observers (Table IV). CONCLUSIONS
The threshold values of tone differences resulting from this study are given in Table IV. In addition, supplementary statistical analysis of the data indicated that: (1) The repeatability of tone differences estimates was better for BW tones and between sessions than for color tones and within sessions. (2) The relationship between observed and measured tone differences was linear for the range of differences encountered. (3) The comparatively large values of the just perceptible tone differences suggested that ordinary conditions of photo interpretation are not conducive to maximum extraction of tone information from aerial photographs. While this study was based on aerial photographs, it is apparent that the 1 Dr. A. R. Robertson, Radiation Optics Section, National Research Council, Ottawa, Ont. (Canada).
140
J. CtHLAR AND R. PROTZ
results are applicable to imagery from other remote sensors recorded on photographic film. ACKNOWLEDGEMENTS T h e authors wish to t h a n k Carl Zeiss C o m p a n y Ltd. ( T o r o n t o , O n t a r i o ) for permission to use the P M Q - I I s p e c t r o p h o t o m e t e r and Mr. K. L. R. M a h l e r for his assistance in the course of m e a s u r e m e n t s . Dr. G. Wyszecki ( R a d i a t i o n Optics Section, National Research Council, Ottawa, O n t a r i o kindly offered his advice in preparing the experiment. The financial s u p p o r t of the C.D.A. G r a n t No.9097 is gratefully acknowledged. REFERENCES Anonymous, 1965. Control Techniques in Film Processing. Society of Motion Picture and Television Engineers, New York, N.Y., 181 pp. Billmeyer, F. W. Jr., 1970. Optical aspects of color, 16. Appropriate use of color-difference equations. Opt. Spectra, 4:63-66. Cihlar, ]., 1971. Color Aerial Photography in Soil Mapping. Thesis, Univ. of Guelph, Guelph, Onl., 195 pp. Carl Zeiss, 1969. PMQ-II Spectrophotometer. Carl Zeiss, Oberkochen, 31 pp. Dixon, W. J. (Editor), 1967. BMD Biomedical Compnter Programs. University of California Press, Berkeley-Los Angeles, Calif., 600 pp. Glasser, L. G., McKinney, A. H., Reilly, C. D. and Schnelle, P. D., 1958. Cube-root color coordinate system. J. Opt. Soc. Am., 48:736-740. Hofert, H. J. and I_,oof, H., 1964. Magnesium oxide powder as a white standard. Farhe, 13:53-62. Huang, D. S., 1970. Regression and Econometric Methods. Wiley, New York, N.Y., 274 pp. Hunt, R. W. G., 1967. The Reproduction o/ Color. Wiley, London, 500 pp. Judd, D. B., 1930. Precision of color temperature measurements under various observing conditions: a new color comparator for incandescent lamps, Bur. Std. J. Res., 5:11611177.
Judd, D. B. and Wyszecki, G., 1963. Color in Business, Science, and Industry. Wiley, New York, N.Y., 140 pp. lshihara, S., /964. Tests /or Color Blindness. Kanehara Shuppan. Tokyo, .. pp. MacAdam, D. L., 1953. Truncated weighted ordinate integrations in colorimetry. J. Opt. Soc. Am., 43:622-623. Optical Society of America, 1963. The Science o/ Color. Optical Society of America, Washington, D.C., 385 pp. Robertson, A. R., 1967. Colorimetric significance of spectrophotometric errors. J. Opt. Soc. Am., 57:691-698. Steel, R. G. D. and Torrie, J. H., 1960. Principles and Procedures of Statistics. McGraw-Hill, New York, N.Y., 481 pp. Wyszecki, G., 1963. Proposal for a new color-difference formula. J. Opt. Soe. Am., 53: 1318-1319. Wyszecki, G., 1968. Recent agreements by the Colorimetry Committee of the Commission lnternationale de l'Eclairage. J. Opt. Soc. Am., 58:290-292. Wyszecki, G. and Stiles, W. S., 1967. Color Science. Wiley, New York, N.Y., 628 pp.