PO39
COLOR MATCHING CONSIDERATION ON THE EFFECT OF
IPRGC FOR COLOR REPRODUCTION ON DISPLAY DEVICE
Kota Akiba et al.
DOI 10.25039/x47.2020.PO39

Paper accepted for the 5th CIE Symposium on Colour andVisual Appearance

The paper was selected by the International Scientific Committee (ISC) for presentation at the 5th CIE
Symposium on Colour and Visual Appearance, Hong Kong, CN, April 21–22, 2020, which, due to thecorona pandemic, could not take place. The paper has not been peer-reviewed by CIE.

 CIE 2020
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized
in any form or by any means, electronic or mechanical, including photocopying and microfilm, without
permission in writing from CIE Central Bureau at the address below. Any mention of organizations or
products does not imply endorsement by the CIE.
This paper is made available open access for individual use. However, in all other cases all rights arereserved unless explicit permission is sought from and given by the CIE.
CIE Central Bureau
Babenbergerstrasse 9A-1010 Vienna
Austria
Tel.: +43 1 714 3187
e-mail: ciecb@cie.co.atwww.cie.co.atAkiba, K. et al. COLOUR MATCHING CONSIDERATION ON THE EFFECT OF IPRGC FOR COLOUR …

PO39

COLOUR MATCHING CONSIDERATION ON THE EFFECT OF IPRGC FOR COLOUR
REPRODUCTION ON DISPLAY DEVICE
1

Akiba, K.1, Tanaka, M.2, Horiuchi, T.1
Graduate School of Science and Engineering, Chiba University, Chiba, JAPAN2
College of Liberal Arts and Sciences, Chiba University, Chiba, JAPAN
midori@chiba-u.jp

Abstract
At the beginning of this century, intrinsically photoreceptive retinal ganglion cells (ipRGCs), newphotoreceptors different from cones and rods, were discovered. The aim of this study was toexperimentally verify the influence of ipRGCs on the c olour reproduction of a display. In theexperiment, we performed perceptual colour matching between colour patches and displays.
Although colorimetric reproduction of the colour patch was used for the initial image of thedisplay, the colour matching results were largely out of colorimetric colour reproduction. Since
this result suggests that ipRGCs may affect colour reproduction of displays, a correction
equation that accounts for the influence of ipRGCs was derived. Applying this correction formula
improved the results. It is considered possible to accurately display the c olour of a real object
on a display using a correction equation that accounts for the effect of ipRGCs.
Keywords: ipRGC, colour reproduction, colour perception, display

1 Introduction
At the beginning of this century, cones and photoreceptors other than rods were discovered in
the retina of mammals and were named “intrinsically photosensitive retinal ganglion cells”(ipRGCs) [1]. The ipRGC is a special ganglion cell containing the visual substance melanopsin.
The spectral sensitivity function [2] shown in Figure 1 was defined by the Commission
Internationale de l´Eclairage (CIE) in 2018 and had a peak at around 490 nm. The ipRGC affects
non-imaging functions, such as regulation of the circadian rhythm and pupillary light reflection,compared to imaging functions that form the image of an object, such a s cones and rods [3].
However, recent studies have reported that ipRGC affects human visual perception throughchanges in the amount of stimulation for visual perception. For example, to verify the effect ofipRGC on visual perception, we used the silent-substitution method, which changes only theamount of stimulation to ipRGC without changing the amount of stimulation to the cone bymanipulating the shape of the spectral distribution [4]. Considering a method for stimulating
only melanopsin cells, colour stimuli were created using a light-emitting diode (LED) mounted
on an integrating sphere as a light source, and experiments were performed. Fig ure 1 shows
the sensitivity of the long, medium, short-wavelength (LMS) cone and the ipRGC overlap. All
these reported experiments were performed in a tightly controlled environment.
In current displays, colour reproduction is performed colorimetrically based on the perception
amount of the LMS cone in photopic vision. However, previous studies have suggested thatipRGC may affect visual perception. If ipRGCs influence c olour perception, in addition to theamount perceived by the LMS cone, it is necessary to consider the effect of ipRGC on the c olour
reproduction of display devices. In our previous study [5], we attem pted to analyze afundamental colour matching experiment in which patches and display reproduction c olours
were visually juxtaposed. It indicated that ipRGCs might contribute to the c olour vision pathway.
However, since the stimuli were visually placed side by side, observing them with a single eye
was necessary.
The aim of this study was to verify the effect of ipRGCs on the display in an experimental
environment where patches and display reproduction colours were physically juxtaposed.
Furthermore, when it was suggested that the colour reproduction of a display is different fromAkiba, K. et al. COLOUR MATCHING CONSIDERATION ON THE EFFECT OF IPRGC FOR COLOUR …

that of a conventional display, we constructed a model for evaluating the c olour reproduction
accuracy of a new display constructed using the values obtained from the experimental results.

Figure 1 – Relative sensitivity of S-, M-, and L-cones and rhodopic and melanopic irradiance

2 Experiments
In the experiment, perceptual colour matching was performed between colour patches, and thecolour patches were reproduced on a display under 6 000 K by 4 500 lx LED illumination. The
spectral distributions of both colour stimuli were different. Therefore, matching results wereexpected without the smallest colour difference in the case that the ipRGCs influence colour
perception. To control the influence of rods, we used a high -brightness liquid crystal display(SHARP PN-A601) after colour calibration. Drawing papers toned by Japan Colour EnterpriseCo., Ltd. to the colour of the X-Rite ColorChecker were used as the real colour patches. Sevencolours (red, blue, moderate red, blue sky, magenta, cyan, and white) with comparatively highreproducibility from 24 colour patches were used as the colour stimuli in this experiment. Many
short- and long-wavelength components containing a colour were selected in addition toconfirming the effect at approximately 490 nm, which is the peak visibility of ipRGCs. TheCIE1976 ∆E colour difference was an average of 1.42 and a maximum of 3.15 (cyan).
In the experiment, colour matching was performed between a colour patch and a reproduced
image on a high-brightness display. Since the spectral distributions of both colours are different,
it is hypothesized that if ipRGC affects colour perception, the colour difference will not always
be minimal. Figure 2 shows the experimental environment. The subjects were six observers (5men and 1 woman) with normal colour vision. The experiment was performed twice to evaluate
the stability of the results. The viewing distance from the observer to the display and colour
patch was set at 150 cm, and the experimental environment was designed in a way that both
stimuli could be physically juxtaposed and observed with both eyes. The viewing angle of eachcolour stimulus was 3.4°. It was shielded from light by a black paper, except for the part showing
the colour of the display. The colour matching procedure was performed by independently
adjusting the hue, saturation, and lightness. The colorimetric reproduction c olour with thesmallest colour difference was displayed for each stimulus as the initial c olour. The experiment
was performed after the training process for the observers to become accustomed to toning.
The adjusted colour on the display device was measured using a spectroradiometer (KONICAMINOLTA CS-2000) after the colour matching experiment.

Akiba, K. et al. COLOUR MATCHING CONSIDERATION ON THE EFFECT OF IPRGC FOR COLOUR …

Figure 2 – Experimental environment

3 Results and discussion
The CIE 1976 Delta E colour difference between the reproduced colour and the colour patch
was calculated after colour matching. The outliers for each colour of the experimental results
were derived using the Smirnov–Grubbs test. Figure 3 shows the average statistical results oftwo experiments performed by six observers for each colour stimulus, together with the colour
difference of colorimetric colour reproduction (initial colour). The deviation in the figure
indicates the standard deviation. As shown in Figure 3, the colour difference average of theseven colours of colorimetric colour reproduction was 1.4, and it changed to 3.9 after colour
matching. In particular, the errors for the blue and white patches were large, and variations
were observed among the observers.

Figure 3 – Average colour difference among the six observers
The ratio of colour difference, i.e. ΔL*:Δa*:Δb*, was 0.26:0.30:0.44. Focusing on the blue
stimulus overlapping with the sensitivity of ipRGCs, the c olour difference ratio was0.12:0.36:0.52, and the colour difference of b* was large. This result indicates that ipRGCs may
also contribute to the colour vision pathway.

Akiba, K. et al. COLOUR MATCHING CONSIDERATION ON THE EFFECT OF IPRGC FOR COLOUR …

The RGB colour-matching function of CIE1931 was derived from colour matching experiments
using Guild and Light. Because this colour-matching function includes negative terms, theCIE1931 XYZ colour matching function was derived by basis transformation, which assumes
that the LMS signals are independent. However, we hypothesized that the LMS signal would
reach the ganglion cells and that the LMS signal would be biased by ipRGCs in the ganglion
cells. Therefore, the correction formula of CIE XYZ obtained by correcting each value of CIE
XYZ with the ipRGC absorption rate was derived by regression with the dependent variable as“CIE XYZ of a real colour patch” and the independent variables as “ipRGC absorption rate” and“CIE XYZ of the reproduced colour on the display after colour matching.” Here, for theregression, 189 datasets obtained by removing the outliers from the XYZ data observed 12times for the seven colour stimuli were used. The ipRGC absorption rate was calculated using
the spectral sensitivity of ipRGC [2] and spectral distribution of the reproduced c olours on thedisplay. The modified XYZ values, 𝑋𝑌𝑍𝑖𝑝𝑅𝐺𝐶 , are derived by the ipRGC effect as follows:

𝑋𝑌𝑍𝑖𝑝𝑅𝐺𝐶 = −9.479 + 107.5 × 𝑖𝑝𝑅𝐺𝐶 + 0.9277 × 𝑋𝑌𝑍𝑚 – (1)where 𝑋𝑌𝑍𝑚 represents the CIE XYZ values of the reproduced colour on the display device afterthe colour matching, including the impact of ipRGCs. The variable 𝑖𝑝𝑅𝐺𝐶 is the ipRGC
absorption rate of the reproduced colour. By adopting the modified XYZ values, Figure 4
presents the average colour difference among the six observers. According to the description
in the figure, after applying the modified XYZ values, the CIE Delta E c olour difference aftercolour matching worsened to an average of 5.6. The ratio of c olour difference was
0.22:0.31:0.47.
Since the absorbance spectrum of ipRGCs has a peak on the short wavelength side, wemodified only the Z-value of the display after colour matching. Figure 5 shows the averagecolour difference among the six observers. The colour difference using the correction formula
improved to an average of 3.5 for the seven colours. In particular, improved results wereobtained for the colour stimuli containing many short wavelength components. This suggested
that the absorption component of ipRGCs affected the z -value in the tristimulus values.

Figure 4 – Average colour difference among the six observers after applying the modified
XYZ values

Akiba, K. et al. COLOUR MATCHING CONSIDERATION ON THE EFFECT OF IPRGC FOR COLOUR …

Figure 5 – Average colour difference among the six observers after applying the modified Zvalue

4 Conclusions
In the colour matching experiment using a high brightness display, colour perception of thedisplay was influenced by ipRGCs. This effect could be confirmed even when c olour matching
was performed for the juxtaposed stimuli in the same way as in the previous study that includeda single eye. We assumed that the LMS signal was biased by ipRGCs in the ganglion cells. The
study also provided a derivation of the correction formula of CIE XYZ acquired by correcting
XYZ values with the ipRGC absorption rate. In particular, by considering the CIE Z value, thecolour difference improved and suggested the necessity of ipRGCs in c olour perception ofdisplays.
Further analysis is required to derive a more appropriate correction formula while considering
the effect of ipRGCs on the perceptual model. In general, it is expected that the sensitivity ofipRGCs and cone sensitivity depend on the observer; therefore, we need to consider anexperimental design that takes this into consideration.

Acknowledgment
This work was supported by JSPS KAKENHI Grant Number 19K12038.

References
1.

D.M. Berson, F.A. Dunn, and M. Takao “Phototransduction by Retinal Ganglion Cells That Set theCircadian Clock” SCIENCE, vol 295, 8 February 2002

2.

CIE System for Metrology of Optical Radiation for ipRGC -Influenced Responses to Light(2018)

3.

S. Hattar, H.-W. Liao, M. Takao, D.M. Berson, and K.-W. Yau “Melanopsin-Containing RetinalGanglion Cells: Architecture, Projections, and Intrinsic Photosensitivity” SCIENCE, vol 295, 8February 2002

4.

T.M. Brown, S. Tsujimura, A.E. Allen, J. Wynne, R. Bedford, G. Vickery, A. Vugler, and R.J.
Lucas “Melanopsin-Based Brightness Discrimination in Mice and Humans” Current Biology,
22, pp.1134- 1141 (2012)

5.

K. Akiba, M. Tanaka, and T. Horiuchi “Experimental consideration on the effect of ipRGC
for color reproduction on display device” International Color Association, Buenos Aires,Argentina, October 2019