Enhancing Color Representationfor Anomalous Trichromats onCRT Monitors

Gabor Kovacs,1* Itala Kucsera,2Gyorgy Abraham,1 Klara Wenzel21 Department of Precision Mechanics and Optics, Budapest University of Technology and Economics, Muˆegyetem rkp.3-9, Budapest, H-1111 Hungary

2 Coloryte Hungary Inc., Ko¨zuzo u. 8, Szentendre, H-2000 Hungary

Received 20 August 1999; accepted 22 November 1999

Abstract: Cathode ray tube (CRT) displays are probably themost widely used color imaging devices, and include TV setsand computer monitors. Although the CRT color represen-tations are adequate for people with normal color vision,color-deficient users would be expected to experience prob-lems of color identification and discrimination with CRTimages, just as they do with “real life” objects. A theoret-ical method is proposed for combining specially designedfilters with the unique spectral emission characteristics ofthe CRT phosphors for enabling certain types of coloranomalous trichromats to see the color of the displayedimages as a person with normal color vision would see theCRT. Given the spectral emission curves of the CRT phos-phors and the sensitivity curve of the color-deficient eye, therelative sensation levels of the L, M, and S cones can becalculated. Applying an optimized color filter and a neutraladaptation background illumination, the L, M, and S sen-sation ratios can be modified to achieve the normal values.A numerical method is presented to design the filter trans-mission curve and to verify the L, M, and S cone sensationratios.© 2000 John Wiley & Sons, Inc. Col Res Appl, 26, S273–S276,

2001

Key words: color vision; color deficiency; filter; eyeglass;CRT; monitor; color enhancement

INTRODUCTION

In this article, we propose a theoretical method that can beused by anomalous trichromats for improving their colorvision in viewing CRT monitors and video displays. Severaldifferent methods have been published and marketed forcorrecting color deficiency, but none of them has actuallyproved to be successful so far.1,2 The optical research groupat the Department of Precision Mechanics and Optics of theTechnical University of Budapest has been working in thefield of color vision and color correction for several years.Together with the company Coloryte Hungary, a new pat-ented method has been developed for improving the colorvision of anomalous trichromats by special filter glasses.Here we demonstrate the application of this method in thespecial case of cathode ray tube monitors with a P22 phos-phor. However, this algorithm for designing optimized fil-ters can be used with any image displaying technique thatutilizes discrete primary colors.

METHODS

CRT displays are probably the most widely used color-image display devices, including TV sets and computermonitors. Although the CRT color representations are usu-ally adequate for people with normal color vision, color-deficient users experience the same problems of color iden-tification and discrimination with CRT images that theyhave with “real life” objects.3,4 During the testing of generalcolor correction glasses in our department, we recognizedthat some of the glasses worked much better on CRT basedpseudo-isochromatic tests than on the regular printed Ishi-hara plates. This led us to the conclusion that special CRTfilters could be designed for anomalous trichromats.

* Correspondence to: Ga´bor Kovacs, Department of Precision Mechan-ics and Optics, Budapest University of Technology and Economics,Muegyetem rkp. 3-9, Budapest H-1111, Hungary (e-mail: kovgab@mail.fot.bme.hu)© 2000 John Wiley & Sons, Inc.

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Figure 1 shows the spectral emission curves of a typicalCRT display with P22 phosphor. The computations provedthat this unique shape of the three primary colors together withspecial interference filters enables anomalous trichromats toimprove color discrimination and identification. The two nar-row spectral-emission bands of the red phosphor make itespecially suitable for correction. The method is quite simple.Let us calculate the L, M, and S cone signal responses for theindividual primary phosphor emissions.5 For example:

Lr 5 kl E ER~l! L~l! dl. (1)

Equation (1) shows the effect of the red phosphor on the Lcone, whereER(l), is the monitor red primary emissionfunction, andL(l), is the L cone fundamental responsefunction. In the same way, we can calculate the effect ofeach phosphor for each cone:

Lg 5 kl E EG~l! L~l! dl

Lb 5 kl E EB~l! L~l! dl

Mr 5 km E ER~l! M~l! dl

Mg 5 km E EG~l! M~l! dl (2)

Mb 5 km E EB~l! M~l! dl

Sr 5 ks E ER~l!S~l! dl

Sg 5 ks E EG~l!S~l! dl

Sb 5 ks E EB~l!S~l! dl.

If we suppose that the adaptation to the filter is not extreme,and the maximum emissions of the phosphors result in anacceptable “white,” thek values can be computed so thatthey result in a neutralL, M, S response (equal to 1). Thisway we can create the monitor color matrix:

F Lr Mr Sr

Lg Mg Sg

Lb Mb Sb

G , whereO L 5 O M 5 O S5 1. (3)

This matrix explicitly describes the relationship between thephosphor emissions and the cone excitations.

Due to the different cone response functions, anomaloustrichromats have different monitor color matrices. If we usea color-correction filter, we can modify the individual coneresponses. The filtered cone responses are calculated withthe following equations:

L*r 5 kl E ER~l! L~l! F~l! dl

L*g 5 kl E EG~l! L~l! F~l! dl

L*b 5 kl E EB~l! L~l! F~l! dl

M*r 5 km E ER~l! M~l! F~l! dl (4)

M*g 5 km E EG~l! M~l! F~l! dl

M*b 5 km E EB~l! M~l! F~l! dl

S*r 5 ks E ER~l!S~l! F~l! dl

S*g 5 ks E EG~l!S~l! F~l! dl

S*b 5 ks E EB~l!S~l! F~l! dl.

In this case,F(l) is the spectral transmission of the filter.We have found that the color adaptation mechanismswork similarly in an anomalous trichromat and a normalobserver.6 Accordingly, the newk values should be re-

FIG. 1. Typical CRT monitor emission spectrum.

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calculated to the new adaptation conditions. The goal isto create a filter shape for the anomalous trichromat thatresults in the same monitor color matrix as for the colornormal, or in other words, the excitation ratios of the L,M, S responses generated by the individual primaries arethe same. Other properties, like the individual electrongun gain, gamma corrections are all adjustable on moderncomputer graphics subsystems to provide the optimumcolor representation in the filtered case. Our calculationshave indicated that, in most cases, significant improve-ment, even complete correction, could be achieved. Fromthe equations, we can see that only color anomaloustrichromats can be corrected with this method; dichro-mats cannot. The level of the possible correction dependson the extent of the color deficiency.

Naturally these types of color-correction filters work onlyon the three primary colors of the specified CRT and cannotcorrect the full spectrum; however, special correction filterscan be designed for other primaries, too (like other CRTphosphors, LCD displays, or color printers). The most crit-ical part of the method is the accurate specification of thecone response functions.

For the filter design, we used a computer program thatenabled us interactively “bend” the transmission curve ofthe filter, with real-time display of the desired normal andthe actual corrected monitor color matrix. An automaticcorrection method has also been developed for speeding upthe optimization.

For design purposes, we used a simple filter shape builtfrom six straight line segments. Figure 2 shows the theo-retical filter curve with the variableA B C D Epoints. In themanual optimization, we could individually alter the spec-tral position and transmission of these points. For the auto-matic algorithm, we used a damped least-squares method tooptimize the filter shape. The method is similar to the oneused in optical lens system design.7,8

We have nine cone response values, which are functionsof A, B, C, D, E point transmission and wavelength values.We are looking for solutions that minimize the merit func-

tion constructed from the normal and the filtered monitorcolor-matrix elements:

f 5 ~L*r 2 Lr!2 1 ~L*g 2 Lg!

2 1 ~L*b 2 Lb!2

1 ~M*r 2 Mr!2 1 ~M*g 2 Mg!

2 1 ~M*b 2 Mb!2

1 ~S*r 2 Sr!2 1 ~S*g 2 Sg!

2 1 ~S*b 2 Sb!2. (5)

This function is minimal, if the corresponding cone re-sponse values are equal. The merit function can be morecomplex, containing other design goals such as the maxi-mum overall transmission.

In the least-squares optimization, we calculate the partialderivatives of each cone response with respect to eachdesign parameter, and find the gradient value of the optimaldesign change. Because the solution is not a linear functionof the design parameters, we can change the variables onlyin small steps, iteratively recalculating the gradient.

As an alternative, a different method is under develop-ment, in which the variable design parameters are the trans-mission values of the filter curve at a large number of fixedpoints (for example, every two nanometers).

As with all nonlinear optimization methods, the startingcurve of the filter is very important, so it is always worth-while to spend some time manually optimizing the filter. Inthis way, we can avoid local minimum points and reachsolution more quickly.

RESULTS

For a sample computation, we used the Smith–Pokorny9

normal cone response functions and the emission curves ofa Nokia 19“ monitor with P22 phosphor. We derived animaginary anomalous trichromat model from the normalone with a 10-nm shift of the L cone (Fig. 3).

The monitor color matrices of the normal and the anom-alous trichromat models are the following:

FIG. 2. Theoretical filter curve with the variable A, B, C, D,and E points.

FIG. 3. The normal and the color-deficient model.

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Normal: F 0.24 0.14 0.020.57 0.61 0.230.19 0.25 0.76

GAnomalous:F 0.18 0.14 0.02

0.59 0.61 0.230.23 0.25 0.76

G . (6)

After some optimization we arrived at a filter curve (Figure4), which enhances the color contrast between the L and Mcones, and also balances the S cone response. This filterproduced the following monitor color matrix:

Filtered anomalous:F 0.24 0.15 0.020.57 0.61 0.250.19 0.24 0.73

G . (7)

We can see that the normal matrix is almost completelyrestored. In this fashion, the anomalous trichromatic indi-vidual experiences the same excitation ratios as a color-normal observer for each primary, provided that the post-

receptoral processes underlying discrimination behavior arenot affected by the color deficiency. The overall transmis-sion of the filter is about 50% for the monitor emissions,which is greater than for a light sunglass.

CONCLUSIONS

Knowing the exact type of the color deficiency of an anom-alous trichromat, it is possible to design a filter to be usedwith a specific CRT display, which compensates for thecolor deficiency. The efficiency of the possible correctiondepends on the extent of the color deficiency. The specifi-cally designed filter enables individuals to be employed inpositions such as CAD operator or graphics designer, whichmay not have previously been available to them because oftheir color vision. However, to be able to fully utilize thismethod, we have to do more research on defining the exactshape of the response functions of the normal and anoma-lous cones.

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FIG. 4. The optimized monitor filter with the monitor emis-sion curves.

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