Modeling for Hue Shift Effect of Human Visual System on High
Luminance Display
Tae-Hyoung Lee, Myong-Young Lee, Kee-Hyon Park, and Yeong-Ho Ha∗
School of Electrical Engineering and Computer Science, Kyungpook National Univ.,
1370, Sankyuk-dong, Buk-gu, Taegu 702-701, South Korea
ABSTRACT
This paper proposes a color correction method based on modeling the hue shift phenomenon of human visual system
(HVS). Observers tend to perceive same color stimuli, but of different intensity, as different hues, what is referred to as
the hue shift effect. Although the effect can be explained with the Bezold-Brücke (B-B) effect, it is not enough to apply
the B-B model on high luminance displays because most displays have a broad-band spectrum distribution and results
vary according to type of display. In the proposed method, the quantities of hue shift between a high luminance display
and a normal luminance display were first modeled by a color matching experiment with color samples along the hue
angle of the LCH color space. Based on the results, the hue shift was then modeled piecewise and was finally applied to
the inverse characterization of display to compensate the original input image. From evaluating the proposed method
using the psychophysical experiment with some test images, we confirmed that the proposed modeling method is
effective for color correction on high luminance displays.
Keywords: hue shift effect, high luminance display, color matching experiment.
1. INTRODUCTION
Most current displays are effective in terms of luminance, response time, and screen size, and produce good color
quality through the development of gamut extension and exact color reproduction. However, high luminance displays do
not consider changes in the human visual characteristic with luminance changes. Thus, when observers watch the same
chromaticity stimulus on a normal luminance display and high luminance display, they tend to perceive two
chromaticity stimuli, as the sensitivity of the human eye changes with luminance level. This can be explained by the
Bezold-Brücke effect, where a hue shift occurs when one observes the hue of a monochromatic stimulus while its
luminance changes.1 Weale (1964) explained the adaptation of human visual system occurring in the Bezold-Brücke hue
shift effect. In his paper, when one cone type was adapted to a greater extent than the other two, the probability
increased that another type, initially less sensitive to the test stimulus, would absorb light photons, and perceived a
commensurate change in hue.2 On the other hand, several results of hue shift have already been reported. One of the
earliest reports is Purdy’s in which the hue shift was represented with monochromatic stimulus in luminance by a factor
of 101. R.W.G Hunt also adopted the hue shift effect in the Hunt94u color appearance model by the color naming
method. In the model, the eccentricity factor (e) is the function of luminance during transforming the hue angle to the
quadrature hue.3 At the simulation result, the tendency of hue shift is different from the hue value; additionally, the
quantity of the hue shift is dependent on the intensity of luminance of a stimulus. Ralph W. Pridmore has searched the
hue shift effect, too. The hue shift described in his paper was a function of luminance by the color matching experiment
with the monochromatic stimulus and object color.4 Recently, he explained the hue shift occurring in real scenes5,6. An
example of the hue shift is the color of a red shirt and glass under the sun. The red shirt exposed to the sun appears
yellowish-red than under the shadow, whereas the glass also exposed to the sun appears yellowish-green under the
shadow.5 However, these models are difficult to apply to commercial displays, as most displays have a broad-band
spectrum distribution and thus a different hue shift. Accordingly, this paper examines the hue shift phenomenon for
general display devices, and models the result of a hue shift piecewise and using sinusoidal functions. The main purpose
of this modeling is to apply a hue shift model to a high luminance display, enabling compensation of the hue-shifted
color from such a display. Thus, test stimuli are first generated for the experiments. For a high luminance display,
∗
[email protected]; phone+82-53-950-5535; fax+82-53-957-1194; http://cilab.knu.ac.kr
Color Imaging XII: Processing, Hardcopy, and Applications, edited by Reiner Eschbach, Gabriel G. Marcu,
Proc. of SPIE-IS&T Electronic Imaging, SPIE Vol. 6493, 64930A, © 2007 SPIE-IS&T · 0277-786X/07/$18
SPIE-IS&T/ Vol. 6493 64930A-1
stimuli are generated with the same hue interval in LCH color space. Reference stimuli are also generated for a normal
luminance display with the same chromaticity as high luminance display stimuli, 1/4 times luminance, as recent high
luminance displays have almost 4 times luminance compared with normal displays. Additional stimuli are also
generated around the high luminance stimuli when varying the hue. The interval of additional stimuli is different for
each stimulus because the quantity of hue discrimination is different. For the visual experiment, two stimuli are
presented simultaneously, where one is the reference stimulus on a normal display, and the other is a hue-shifted
stimulus on a high luminance display. At this time, subjects change the amount of hue shift and select the hue-shifted
stimulus on high luminance display to match the reference stimulus on the normal display. This procedure is repeated
for all stimuli within the whole hue range. The results reveal the tendency of the hue shift and allow it to be formulated
as a hue shift model using a piecewise method. To confirm the effectiveness of hue shift model with real images, hueshift-compensated images and colorimetric matched images with the reference images on the normal display are
presented on a high luminance display. Subjects are then asked to judge the similarity of high luminance display images
to the reference images, and the observer’s preferences are calculated using z-score values. The results confirm that the
hue-shift compensated images match the reference images better than the colorimetric-matched images.
2. VISUAL EXPERIMENTS
The goal of this paper is color correction of a high luminance display using a hue shift model based on the visual
experiments. Thus, to get the quantity of the hue shift, a color matching experiment is performed between a high
luminance display and a normal luminance display. A high luminance device is LG LZ-10 LCD which has about
430cd/m2 with the white stimulus and the normal luminance device is EIZO T 966 CRT which has about 110cd/m2 with
the white stimulus for the color matching experiments. For accurate color reproduction in each device, CRT adopts the
GOG model, and LCD adopts the 3D_LUT model as a characterization algorithm, respectively.7 After characterization,
the average values of ∆Eab between measured stimuli and predicted stimuli are 0.6139 and 1.1436 for CRT and LCD,
and max values of ∆Eab are 1.8215 and 2.5608, indicating that the generated stimuli on the CRT and the LCD are quite
accurate. Figure 1 shows a flow chart of the whole procedure
Stimuli generation
Visual experiment
Mathematical modeling of the hue shift
Application to display characterization
Figure 1. The flow chart of proposed color correction of high luminance display.
2.1 GENERATION OF STIMULI
Stimuli for displays are generated in the LCH color space, because at the space, the unit of stimuli is expressed by a
degree which can be controlled easily. Additionally, luminance and saturation can be fixed while hue is changed and the
quantity of hue shift is expressed by hue. 1
2.1.1 GENERATION OF LCD STIMULI
The stimuli of LCD are used to determine the quantity of hue shift. At first, lightness is separated into three levels which
is 40, 60, and 80 to observe the hue shift in whole lightness. Then 24 stimuli are generated with a 15° interval in the hue
space for each lightness level. Additional stimuli are also generated around the 24 stimuli on each lightness level when
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varying just the hue. The quantity of the hue variation for additional stimuli is not fixed, because the perceived color
discriminance is different for each stimulus. The variation is determined by visual experiments and the quantity is from
2° to 6°. Figure 1 shows the stimuli in the LCH color space, whereas figure 2 shows the measurement of the first
generated stimuli for each lightness level measured by a Minolta CS 1000 spectro-radiometer.
L
H
Figure 2. The first generated 24 stimuli for each lightness.
(a)
(b)
(c)
Figure 3. The measurement of the first generated 24 stimuli; (a) lightness is 40, (b) lightness is 60, and (c) lightness is 80
2.1.2 GENERATION OF CRT STIMULI
CRT stimuli are used as the reference stimuli in the color matching experiments. Thus, the number of stimuli is the
same as the first generated LCD stimuli and should have the same chromaticity as the LCD stimuli, but the luminance is
1/4. The method to get the same chromaticity with 1/4 times luminance is generation of the CRT stimuli which have the
1/4 times tristimulus values of LCD stimuli. As a result, the chromaticity of stimuli on the CRT and LCD are
colorimetric matched. If the generated stimuli for CRT do not have the same chromaticity as the LCD stimuli when
measured, the digital rgb for each stimulus is manipulated until they have the same chromaticity. Figure 3 is the
measurement of stimuli on the CRT and LCD to compare the chromaticity. The paired stimuli have almost the same
chromaticity.
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(a)
(b)
(c)
Figure 4. The measurement of the first generated LCD stimuli and CRT stimuli.
2.2 CIRCUMSTANCE OF EXPERIMENT
The place for color matching experiment is chosen by a dark room to prevent any illuminant. The two devices, the LCD
and the CRT, are placed on the table side by side with the same height as the subject’s eye. The height and width of
stimuli on the display are 1/5 of each display height, and the distance of observation is the 4 times the display height.
The height of the CRT and LCD is 34cm and 37cm, respectively. The viewing angle is 4° from the distance and size of
a stimulus. The background on the each display is covered with dark cloth to avoid the disturbance of the leaked light.
Figure 5 shows the circumstance for color matching experiments.
-I/i = 156 en'
subject
II
5
IA•f) and vicinitor
•
ground
Figure 5. The circumstance of the visual experiments.
2.3 COLOR MATCHING EXPERIMENT
The color matching experiments were performed by 7 subjects, six men and one woman. All subjects had experience in
color matching experiments. 8 At first, subjects stayed in the darkroom for five or more minutes for dark adaptation.
After that, two stimuli were presented simultaneously One is the reference stimulus on the CRT display, and the other is
a hue-shifted stimulus on the LCD display. At this time, subjects change the stimuli already prepared and select the hueshifted stimulus on high luminance display to match the reference stimulus on the normal display. From the selected
stimulus, the quantity of hue shift can be determined by calculating the hue difference between the selected stimulus and
the colorimetric-matched stimulus with the reference stimulus. This procedure is repeated for all stimuli. All subjects
performed four color matching experiments. Figure 6 depict the color matching experiments. Because of the problem of
the angle on the LCD, subjects were in front of the LCD display.
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9 orlO patches
One patch
Figure 6. The color matching experiments.
2.4 RESULT OF COLOR MATCHING EXPERIMENTS
As mentioned above, the main result of the color matching experiments is the quantity of the hue shift on high
luminance display. Table 1 shows the result at 40 lightness. The horizontal index at the first row is the hue with 15°
interval, and the vertical index at the first column means the variation steps of the additional stimuli, not the quantity of
hue shift because the additional stimuli are made with different hue intervals for each stimulus. In addition, (+) and (-)
means the tendency of hue shift. The number of the other rows and columns means the total selected number. Table 2
and Table 3 are the same interpretation as the Table 1. The tendency and the roughly quantity of hue shift are
represented, and the max selected steps are shown for each hue as the blue square. However, there is a problem in using
the max selected step because for some hues, the max selected steps are spanned with two steps. Additionally, for each
hue, the selected numbers are distributed across some steps. The average of the distribution is used to show the quantity
of hue shift. The values of average are shown in the figure 7 with three lightness levels.
Table 1. The result of hue shift from color matching experiments with L=40.
Table 2. The result of hue shift from color matching experiments with L=60.
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Table 2. The result of hue shift from color matching experiments with L=80.
wi ,IiI* O th. high iuminuic. npiy
— L=40
L=60
L=80
22
C
0
-2
50
100
ISO
2 250
330
330
hue(degree)
Figure 7. The result graph of color matching experiments with average value.
As shown in figure 7, although the quantity of hue shift is not the same for each lightness level, the tendency of hue shift
is similar. In the red region (330°~360° and 0°~40°), the tendency of hue shift is (+), which means a shift toward the
yellow. In the yellow region (50°~90°), the tendency of shift is (-) which means shift toward the red. The biggest hue
shift is occurred In blue region (200°~250°). The tendency of the shift is toward the purple.
3. MEDELING OF HUE SHIFT
Based on the result of experiments, the hue shift model is formulated. The average value of the hue shift for three
lightness levels is used because of the inconsistency of the hue shift quantity. In figure 7, the difference of the hue shift
is not very big for each lightness level. In other words, the difference is in the range that subjects cannot perceive the
difference of hue. Thus, the average value of the quantity of hue shift can be used for hue shift model. When modeling
the hue shift, the piecewise method and sinusoidal function are adopted. The piecewise method is used to separate the
region which has a different tendency, and the sinusoidal function is used to smooth the curve of hue shift model. Eq 1.
is a hue shift model, and figure 8 shows the result of average for hue shift of three lightness levels and the result of the
hue shift model.
⎧6 × sin(h / 12 + 0.9) + 2.2
⎪
⎪ 1.7 × sin(h / 27) − 1.5
⎪
0.1× h − 18.2
⎪
∆hueshift (h) = ⎨ 6 × sin(h / 19 + 2) + 2.2
⎪
sin(h / 8)
⎪
⎪ 3.3 × sin(h / 8) − 3
⎪
⎩ 2.8 × sin(h / 8) + 6
0 o ≤ h < 60o
60o ≤ h < 160o
160o ≤ h < 200o
200o ≤ h < 265o
265o ≤ h < 300o
300o ≤ h < 340o
340o ≤ h < 360o
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(1)
hue shIft on ths high Iumlnsncs dies ply
-—
Hue shift model
—
0
50
icx
ISO
O 260
3)0
360
Average value of hue shift
400
hue(degree)
Figure 8. The average value of hue shift for three lightness level, and hue shift model.
4. APPLICATION OF HUE SHIFT MODEL IN DEVICES
The hue shift model can be applied to the characterization because hue can be calculated from input in devices. The
procedure is that the CIELab is calculated from the input XYZ (tristimulus) value, and then LCH is calculated. After
appling hue shift model to the hue in LCH, the modified CIELab is calculated, and last, the digital RGB is estimated by
the 3D_LUT characterization. Figure 9 shows a diagram for applying hue shift model.
input
XYZ
_____
Inverse characterization
La*b*
output
RGB for display
Additional procedure
Figure 9. Diagram for applying hue shift.
5. EXPERIMENTS AND RESULT
The effectiveness of the hue shift model is assessed by color matching experiments with real scenes in which one image
is on the CRT as the reference image, and two images – one is colorimetric matched images with reference image and
the other one is hue-shift-compensated image – are on the LCD. To compare experiments, at first, samples of natural
images are prepared for the LCD display. Then the colorimetric matched images as the reference images on CRT are
generated but have 1/4 times luminance. The procedure of CRT image generation is the same as the procedure of CRT
stimuli generation. Last, the hue-shift-compensated images are generated by applying the hue shift model. The natural
images for the experiments are a blue sky, orange, blue cloth, and red shirt images, because at the result of hue shift, the
quantity of hue shift is biggest at the blue region and red region. Figure 10 shows the images for CRT display, and
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figure 11 shows the images for LCD display and hue-shift-compensated images ([d], [e], [f], and [g]). The effect of hue
shift can be observed too.
In the experiments, the images in figure 10 are on the CRT display as the reference images, and the images in figure 11
are on the LCD display. Seven subjects performed the experiments after dark adaptation in the dark room. Then subjects
were asked to judge the similarity of the LCD display to the reference images.
I
(a)
(b)
a
a
(c)
(d)
Figure 10. Images for CRT display.
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
Figure 11. Images for LCD display. (a), (b), (c), and (d) have the same chromaticity as the CRT images. (e), (f), (g), and (h) are
the hue-shift-compensated images from (a), (b), (c), and (d), respectively.
Table 4 shows the result of experiments. Most subjects selected the hue-shift-compensated images. Table 5 shows the
subjects preferences calculated using z-score values. The result shows the color of the hue-shift-compensated images is
the more similar with the color of reference CRT images.
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Table 4. The result of experiments for the natural images.
(a)
images
(b) (c) (d)
2
Original image
Hue—shifted image
12
11
Table 5. The z-score.
6. CONCLUSIONS
This paper proposes color correction on high luminance display based on hue shift model. Although two stimuli on the
normal luminance display and high luminance display are colorimetric-matched, observers perceive that the hue of two
stimuli is different. Therefore, in the proposed method, the hue shift on high luminance display is determined by visual
experiments. For the experiments, the stimuli were generated in the LCH color space, because degree is used as the unit
of quantity of the hue shift. Additionally, the stimuli are generated on the three lightness levels for studying the hue shift
depending on the lightness levels. Color matching experiments are adopted for visual experiments. As a result, the
quantity of hue shift is determined, but the quantities for three lightness levels are different although the tendencies of
the hue shift are similar. In addition, the difference is small for human eyes, so the average value of hue shift for the
three lightness levels is used. Then, the hue shift model was formulated based on the experiments result. Additionally
the model is adopted during inverse characterization in the display system. To verify the hue shift model, natural images
were used in the last comparison experiments. Almost all subjects perceived the hue shifted images to be more similar
to the CRT reference images.
ACKNOWLEDGMENT
This work is financially supported by the Ministry of Education and Human resources Development (MOE), the
ministry of Commerce, Industry and Energy (MOCIE) and the Ministry of Labor (MOLAB) through the fostering
project of the Lab of Excellency.
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