Optimization of color reproduction on CRT-color monitors

Optimization of ColorReproduction on CRT-ColorMonitors

J. R. Jimenez,* J. F. Reche, J. A. Dıaz,L. Jimenez del Barco, E. HitaDepartamento de Optica, Facultad de Ciencias, Universidad de Granada, Spain

Received 17 January 1998; accepted 5 November 1998

Abstract: In the present experimental study, we quantify theinfluence of the brightness and contrast levels of a CRT-color monitor in the color reproduction of 60 Munsell chipsdistributed throughout the chromatic diagram. The imageswere captured by two CCD cameras, and the color differ-ences were evaluated after reproducing the chips on a colormonitor (the experiment was performed with 3 differentmonitors) for 9 combinations of brightness-contrast levels.We evaluated the color differences with 3 different formu-las: CIELAB, CIELUV, and CIE94. The results indicate thatthe optimal settings of a monitor, to minimize the colordifferences, is a medium or minimum brightness level incombination with a maximum contrast level. This combina-tion ensures a more faithful color reproduction with respectto the original image.© 1999 John Wiley & Sons, Inc. Col Res Appl,

24, 207–213, 1999

Key words: color-monitor colorimetry; brightness and con-trast control; color-monitor performance; CCD cameras

INTRODUCTION

One of the most critical issues in color-reproduction sys-tems is fidelity with respect to the original image. For thepreservation of maximum information from the chromaticobject, the image reproduced should differ colorimetricallyas little as possible from the original. One of the color-reproduction devices most commonly used and colorimetri-cally characterized is the CRT-color monitor.1–4 Thesemonitors are widely used to display images captured byCCD cameras, generated, scanned, or digitalized. Regard-less of how the image is captured or managed, it is finally

reproduced on a monitor, which limits fidelity and thereproduction quality.

The image on a monitor is controlled primarily by 2parameters—brightness and contrast—which require onlysimple adjustment. In addition, these are the only two pa-rameters controlled by most users, and adjustment is fre-quent. Prior studies on monitors5 have examined these pa-rameters in relation to such aspects as the hypothesis ofconstant-channel chromaticity in the calibration of monitorsand the color gamut of the CRT display, the latter aspectbeing crucial by limiting the range of color stimuli that canbe reproduced independently from the original image.

Despite the importance of brightness and contrast levelsin the image on the color monitor, the impact of theseparameters on fidelity to the original image has not beencompletely quantified from a colorimetric standpoint. Suchinformation would be extremely useful in designating theoptimal settings for operating CRT-color monitors in colorreproduction, and for improving the chromatic performanceof these monitors.

In the present study, to quantify these parameters wedesigned the following experiment. First, we determined theluminance and chromaticity coordinates of a set of 60Munsell chips differing in hue, chroma, and value. Thesechips were illuminated in a light booth under a metamericspectral power distribution to the CIE daylight standardIlluminant D65. Images of the chips were captured with aCCD camera and reproduced on a CRT-color monitor atdifferent contrast and brightness levels, whereupon thecolor differences were calculated with respect to the chips inthe booth for each contrast and brightness level.

The set of Munsell chips was selected to cover a widerange of hue, chroma, and value to test the behavior of themonitors in different zones of the chromatic diagram, be-cause each color is obtained as a combination of the re-sponse of 3 phosphors and, thus, colorimetric variation of

* Correspondence to: J. R. Jimeńez; e-mail: [email protected] grant sponsor: Direccioń general de Investigacioń Cientıfica y

Tecnica Ministerio de Educacioń y Ciencia (Spain); Contract grant num-ber: PB96-1454

Volume 24, Number 3, June 1999 CCC 0361-2317/99/030207-07 207

the chips increases the number of phosphor combinations.Studies have been made for TV monitor picture tubes6,7 inwhich the color reproduction is evaluated for samples withskin tones illuminated with Illuminant D65. These studiesproved limited in terms of hue range.

A CCD camera was used to capture the image. Althoughwe did not orient the study specifically to the camera, so thatthe results would be independent from this aspect, we usedtwo different color CCD cameras (one with 1 sensor andanother with 3 sensors), operated in order to minimize theinfluence of the camera (see Method), from a colorimetricviewpoint, in the final image on the monitor.

The color images captured were reproduced on 3 moni-tors that differed in the color temperature of the white point,gain, and offset (controlled by the contrast and the bright-ness levels, respectively), in order to obtain the most gen-eralized results possible.

We evaluated the differences in color reproduction using3 color-difference formulas: the classical CIELUV andCIELAB8 formulas, which are widely applied and recom-mended for evaluating experimental color differences; andthe CIE94,9,10 an updated modification of the CIELABformula useful for industrial color-difference evaluation (forexample, color objects under daylight illumination). Ourcriterion for ascertaining the best monitor settings for colorreproduction was the combination of brightness and contrastlevels that rendered the least color difference (mean of thecolor differences of the 60 chips) with respect to the chipsmeasured in the light booth. These differences were notanalyzed in relation to the chromatic-discrimination thresh-olds of human observers, because our aim was only toestablish which combination of brightness and contrast lev-els minimized the color differences, regardless of whetherthe color differences were clearly perceptible or not undercertain experimental conditions.

METHOD AND EXPERIMENTAL DEVICE

Chips and Illumination System

A total of 60 chips were selected from theMunsell Bookof Color (glossy finish), covering a wide range of hue,value, and chroma. We chose 6 chips (varying value andchroma) for each hue value (we chose 10 main valuesdistributed over the entire color space: R, YR, Y, GY, G,BG, B, PB, P, and RP).

The chips were placed in a VeriVide CAC 120 lightbooth illuminated by a fluorescent lamp that provided ametameric spectral power distribution of the CIE standardIlluminant D65 (simulating daylight). The Munsell chipswere positioned in the center of the light booth so that allchips were uniformly illuminated. To eliminate reflectedspecular light, which could distort the measurements, weilluminated the chips at 45°, with the spectroradiometersituated at 90° (45/0 condition), as recommended by theCIE (Commission Internationale de l’Eclairage).11 Thechromaticity coordinates and luminance were measured bymeans of a PR-704 spectroradiometer (from Photo Re-

search) with a field size of 1° and used the CIE-1931standard observer. The spectroradiometer had an error of60.003 in the chromaticity coordinates and 4% in lumi-nance, referring to the CIE Standard Illuminant A. Thechromaticity coordinates of the chips, represented in theCIE-1931 chromaticity diagram, are shown in Fig. 1. Todetermine the chromaticity coordinates and luminance ofthe illuminant used as an achromatic reference stimulus,needed in the color-difference formulas, we used the RS-1reflectance standard, which consists of a barium sulfateplate that has a diffuse reflectance of nearly 100%. With thesame spatial arrangement as above, we captured the imagesof the Munsell chips with the CCD color cameras.

CCD Cameras and CRT Color Monitors

The two CCD cameras used, JVC TK-1270 (1 CCD,7523582 effective pixels, working color-temperature rangeof 2850–7000 K, resolution of 460 lines) and IKEGAMIHC-340 (3 CCD, 7563581 effective pixels, color-temper-ature range of 3000–5600 K, resolution of 750 lines) hadcomposite video signals output. We used a digitalized card(with 24 real color bits with an composite video signalinput) Screen Machine II from Fast Electronic MultimediaGmbH installed in an Intel-pentium 90-MHz computer. Asoftware package, SM-Camera II, was used to store andreproduce the images. As indicated in the Introduction, weuse two cameras in this type of experiment to ascertainwhether the use of two technologically different camerasmight influence our results, or whether we can disregardsuch influence. Currently, it is common to use a CCD colorcamera connected to a computer for professional as well asnonprofessional processing tasks. In the former case, thefull calibration of the CCD camera should be required, but

FIG. 1. Chromaticity coordinates of the 60 Munsell chipsrepresented in the CIE-1931 chromaticity diagram.

208 COLOR research and application

in most cases the user has no direct control over the gamma,white point, or specific filter of the camera. The two cam-eras operated with automatic white balance. This is the mostcommon situation, where the camera is designed to maxi-mize its color reproduction and, under ordinary operatingconditions, the user cannot vary these controls. The use oftwo cameras is typical in this kind of experiment to evaluatecolorimetric aspects.12

In the present work, we sought to minimize the influenceof the CCD camera. Thus, we verified that changing thediaphragm altered the chromaticity coordinates and the lu-minance of the standard white and the chips. Therefore, wefixed the diaphragm size to minimize the color differencebetween the reference white and that reproduced on thescreen for medium brightness and contrast levels, althoughwe verified that the diaphragm setting that minimized thecolor difference did not depend significantly on the settingsof the monitor chosen to minimize this difference. In anycase, any possible influence of the CCD cameras ended oncethe image was captured, and, therefore, it would be the samefor all levels of brightness and contrast of the monitors usedin the experiment.

To reproduce the images, we used 3 CRT color monitors(all with a resolution of 10243768 pixels) of the RGB type,with chromaticity phosphors similar to those of the PALsystem: a NEC Multisync 3D with a vertical refresh rate of60 Hz, a Sony Trinitron GDM20Se Multisync with a fre-quency of 72 Hz, and a Samsung CSD5577 Multisync witha frequency of 54 Hz.

For each monitor, we reproduced the chips in 9 differentbrightness-contrast combinations, each contrast level (min-imum, medium, and maximum) being combined with 3brightness levels (minimum, medium, and maximum).These levels were determined in the following manner: wemeasured the luminance of the white point on the monitor(the achromatic stimulus corresponding to a maximum ex-citation of the 3 channels: red, green, blue) at the maximumand minimum levels. By interpolation, we determined theluminance that the white point should have at the mediumlevels. In this way, for each monitor, we defined the 9brightness-contrast combinations to be used after completecolorimetric characterization from the data given by thespectroradiometer for the white point (color temperature,chromaticity coordinates, and luminance).

When the samples were measured in the monitor, theroom was dark. It should be taken into account that the colorreproduction of samples depends on ambient light, and eventhe brightness and contrast settings could be used for com-pensating for ambient light. Given that the ambient illumi-nation possible is highly variable, it is necessary to fix theillumination, because the results would be difficult to gen-eralize if we take into account illumination. In addition, ourwork refers only to the influence of the brightness andcontrast levels of the monitor in color reproduction, andvariations in environmental illumination would mask theresults that constitute the aim of our study.

Color-Difference Formulas

As indicated in the Introduction, we used the formulasCIELAB, CIELUV, and CIE94. This latter is a modificationof the CIELAB formula and has seldom been tested in thistype of measurement using a CRT. The expression of theCIE94 formula from CIELAB is given by the equations9,10:

DE*94 5 ÎSDL*

KLSLD 2

1 SDC*

KCSCD 2

1 SDH*

KHSHD 2

, (1)

where DL*, DC*, and DH*, are, respectively, lightness,chroma, and hue differences between a pair of chips calcu-lated with CIELAB.8 The weight functionsSL, SC, andSH

are obtained from the mean chromaC*ab as follows:

SL 5 1.0

SC 5 1.01 0.045C*ab

SC 5 1.01 0.015C*ab,

and the parametric factors functionsKL, KC, andKH are:

KL 5 KC 5 KH 5 1.

The choice of the parametric factors depends on theexperimental measuring conditions (thus, in the textile in-dustry, it is usual to choose the lightness parametric factorto 2). Our choices of parametric factors are adequate whenworking within the framework of the reference conditionsproposed for the CIE94 formula.9 However, our experimen-tal conditions differed in some aspects, for example, al-though we used a metamer of Illuminant D65, the illumi-nance in our booth was 1235 lux, greater than 1000 lux,which is recommended. In any case, we chose parametricfactors to be able to compare our results with those of otherexperiments in which the CIE94 formula is used, as sug-gested by the CIE (technical report).10

Regarding the above formulas, the CIELUV color for-mula is the most widely used for evaluating color differ-ences when working with systems such as CRT monitorsand television cameras.13,14 In addition, the CIELAB color-difference formula is particularly useful when working withcolor differences on CRT color monitors, as in the presentexperiment.15

To use the color-difference formulas, it is necessary tocalculate the hue, chroma, and lightness values for the chipsin the booth, and, afterwards on the monitor for the differentcombinations of brightness and contrast. This calculationinvolves the values of a reference achromatic stimulus. Inthe choice of this reference stimulus, there is no generalestablished criterion,8 and different criteria can be applied.Due to the nature of the experiment that we designed, weused the color-difference formulas but with 2 referencestimuli for determining the values for hue, chroma, andlightness, which obey different possibilities, thus duplicat-ing our results and testing a greater number of experimentalconditions with the idea of generalizing our results to thegreatest degree possible.

Volume 24, Number 3, June 1999 209

On calculating the values of hue, chroma, and lightnessfor the chips in the booth, the reference stimulus was alwaysthe illuminant in the booth, but, for the chips measured onthe CRT-color monitor, we had two options. On one hand,for measuring the chromaticity coordinates and luminanceof the chips in the color-monitor, it would be logical to usethe chromaticity and luminance of the illuminant measuredon the monitor for the different levels of brightness andcontrast tested, because it is under these conditions that wemeasured the chromaticity coordinates and luminance of thechips. However, on the other hand, with regard to a morefaithful color reproduction, the achromatic reference stim-ulus should be the illuminant in the booth, because other-wise we would not be using the same reference (i.e., thechips would have one reference stimulus in the booth andanother on the monitor). Thus, when measuring the colordifference between the sample in the booth and that repro-duced on the monitor, we could not ascertain which leveloffered a better color reproduction, because the resultswould not refer to the same reference white.

Finally, it should be taken into account that monitorsdiffer notably with respect to maximum and minimum lu-minance values provided. Although, for example, the chro-maticity coordinates of the primaries differ minimally, vari-ability in the luminance range is greater between monitors.5

In our case, the chips used had different luminances withinthe range varying in a particular monitor for each brightnessand contrast level. In addition to our aim of evaluating theoverall color differences (lightness, hue and chroma), itwould be useful to evaluate the reproduction of the monitorsfor the different brightness and contrast settings in thechromaticity variable alone, which is more related to thevariations of hue and chroma of the color-difference formu-las. Thus, we have evaluated the CIELAB, CIELUV, andCIE94 color difference formulas with and without the light-ness term. Although we know that the color perception doesnot function independently of lightness, by excluding thisterm, we seek to analyze the reproduction of hue andchroma (as we indicated above, the terms most related to thechromaticity of the stimuli), without these being masked bythe lightness term, thereby minimizing the influence of themonitor variability in the luminance range of the color-difference formulas.

RESULTS AND DISCUSSION

First of all, we examined the influence that the brightnessand contrast levels exert in the color gamut of the monitor.We verified that the triangle of primaries was smaller for themaximum levels of brightness and minimum of contrast, thebest reproduction resulting from the medium-to-minimumbrightness and maximum contrast. Figure 2(a) shows thecolor gamut for the maximum level of brightness and min-imum of contrast with the three monitors, and Figure 2(b)shows the color gamut for the maximum contrast and min-imum brightness. These figures also present the chromatic-ity coordinates of the chips in the light booth. In the firstcase, we verified the maximum level of brightness and

minimum of contrast. A large number of chips fall outsidethe triangle of the primaries, this number diminishing as weapproach the situation shown in Figure 2(b), the minimumbrightness and maximum contrast level. As a result, colorreproduction improved at settings where the color gamutassociated with the triangle of primaries was broader, thisbeing characteristic of the minimum-maximum and medi-

FIG. 2. (a) Color gamut corresponding to the maximumbrightness and minimum contrast settings for the threemonitors (smallest primary triangle). The figure includes thechromaticity coordinates of the chips in the light booth. (b)Color gamut corresponding to the minimum brightness andmaximum contrast settings for the three monitors (largestprimary triangle). The figure includes the chromaticity coor-dinates of the chips in the light booth.


um-maximum levels of brightness and contrast (we haveused the following notation: the first word indicates thebrightness level, and the second, the contrast level.). regard-less of the monitor and video camera used, in accordancewith previous results obtained in our laboratory.5

We could have restricted ourselves to choosing the chipshaving chromaticity coordinates within the triangle of pri-maries (low saturation), but, in this way, we would havelimited the validity of the experiment by failing to workwith a wide range of chromatic variables, this being theprimary aim of our study. Our greater variety of colorstimuli more closely approximates real working conditions,although we face the problem of reproduction of thesechips, due to the monitor or even the camera used.

The color differences between the 60 Munsell chips mea-sured in the booth and images reproduced on the 9 bright-ness-contrast levels were studied for the different combina-tions: 3 monitors, 2 cameras, 3 formulas and 2 achromaticreference stimuli per formula (in one case, the illuminant ofthe booth and. in the other, the illuminant captured by thecamera and presented under each configuration of the mon-itor). For all the above combinations, the chromatic differ-ences were obtained with and without the lightness term. Inall, we had 72 means of color differences for each setting ofbrightness and contrast. Each mean represents an average ofthe color differences for the 60 chips.

For each of the 72 combinations, we identified whichbrightness and contrast settings gave the lowest mean anddetermined whether there were significant differences be-tween means. A factorial analysis of variance (ANOVA)was performed to evaluate statistically significant differ-ences between level settings.16 A confidence level of 95%(a 5 0.05) was used. In 43 of these combinations, thebrightness-contrast settings at the minimum-maximum po-sitions offered the least color difference in the reproductionof the chips. In another 23 cases, the color difference did notsignificantly differ from the settings that offered the bestreproduction (lowest mean color difference). Thus, 66 of 72combinations (91.6%) offered a color reproduction statisti-cally unsurpassed by any other combination of brightnessand contrast—in 59.75% of the cases, these levels offeredthe best color reproduction. Meanwhile, the medium-max-imum settings rendered nearly the same result, giving sta-tistically nonsignificant differences with respect to the bestcombination for the 91.6% of the cases, although thesesettings registered the least difference in 19 cases (26.38%),and in another 47 did not differ significantly from the bestcombination. For example, Table I shows the results for theSAMSUNG with the JVC camera.

These two brightness-contrast combination settings (min-imum-maximum and medium-maximum) differed sharplyfrom the others. That is, the next most effective setting wasthe medium-medium level, with 48 cases (66.6%) showingno significant differences with the best reproduction. In nocase did this setting give the least color difference.

The brightness-contrast minimum-maximum and medi-um-maximum combinations failed to offer the best colorreproduction in only 6 cases (identical for both levels), all

corresponding to the SONY monitor for the two cameras(see Table II for the IKEGAMI camera) when, in theformulas with the lightness term, the illuminant measured inthe booth was used as the achromatic reference stimulus.Under all other conditions, these two levels were in thegroup of minimum color differences, regardless of the cam-era used.

TABLE I. CIELuv, CIELab and CIE94 Color differenceformulas for the SAMSUNG Monitor and JVC Camera.Least color differences are underlined; levels not sig-nificantly different with respect to the least color dif-ferences (at 95% confidence level) are in bold. TotalCDF means Color Difference Formula including theLightness Term. I. Booth and I. Monitor indicate thatthe reference stimulus used in the CDF was the illu-minant in the booth and in the monitor, respectively.

CIELuv Brightness-Contrastb

Total C. D. F.C.D.F. Without

Lightnessa

I. Monitor I. Booth I. Monitor I. Booth

Minimum-Minimum 21.111 69.247 19.472 35.294Minimum-Medium 15.460 35.931 12.945 25.370

Minimum-Maximum 15.408 24.625 13.314 23.678

Medium-Minimum 25.201 64.988 19.671 34.397Medium-Medium 17.252 32.373 14.650 26.094Medium-Maximum 16.715 24.380 14.595 24.980

Maximum-Minimum 31.692 60.336 22.604 33.796Maximum-Medium 19.663 29.992 16.238 27.130Maximum-Maximum 18.294 25.898 15.548 26.554

CIELab Brightness-Contrastb

Total C.D.F.C.D.F. Without

Lightnessa

I. Monitor I. Booth i. Monitor I. Booth

Minimum-Minimum 17.119 64.720 15.150 27.261Minimum-Medium 13.902 31.502 11.106 18.844

Minimum-Maximum 13.844 19.540 11.447 17.789



CIE94 Brightness-Contrastb


Lightnessa


Minimum-Minimum 11.616 61.585 9.037 21.284Minimum-Medium 9.384 26.667 5.554 10.696Minimum-Maximum 8.853 13.490 5.545 10.686



a See text for more details.b With this notation, the first word indicates the brightness level

and the second the contrast level.


The behavior of the SONY monitor can be explained interms of the range of the luminance means for the chips ofthis monitor in comparison with the mean of the real lumi-nance of the chips. For the SONY monitor, the averageluminance of the chips ranges from 1.43 cd/m2 (minimumbrightness and contrast level) and 50.9 cd/m2 (maximumbrightness and contrast level), while the average luminance

of the chips in the booth was 91.32 cd/m2. The averageluminance for chips in the brightness-contrast combinationat maximum-maximum was notably lower that the averageluminance of the chips in the booth. This situation occurredonly on the SONY monitor, because the other monitorsprovided higher luminances, i.e., the average luminancemean of chips was 172.4 cd/m2 for the NEC monitor atmaximum-maximum and 83.3 cd/m2 for the SAMSUNGmonitor at the same brightness and contrast level.

For the SONY monitor, there was a strong differencebetween the mean luminance of the chips in the booth andthe mean luminance of the chips on the monitor in anycombination. Therefore, we did not obtain results in whichthe best reproduction levels were minimum-maximum andmedium-maximum, and, on the contrary, for this monitorthe maximum-maximum setting gave the best reproduction,because the lightness difference predominated in the color-difference formulas. It was sufficient to eliminate the termlightness from the color-difference formula or change thereference stimulus obtained in the monitor so that the con-ditions of medium-minimum brightness and maximum con-trast were among those that give the least color difference.In any event, these results are specific to the SONY monitor,although we might expect this to occur with all the monitorsthat do not provide high luminances. The best results for theSONY monitor were obtained with the maximum-maxi-mum level, for which the option of maximum contrastprevailed as the best color reproduction.5

In analyzing the results for all the monitors tested, wehave confirmed that the results concerning chromaticity(best hue and chroma reproduction for high contrast levelsin combination with low-medium brightness levels) do notdepend on the monitor used, despite the differences in thephosphors in each case, given that these differences take theform of minor discrepancies between the chromaticities ofthe primaries.5,15 If the monitor also ensures a broad rangeof luminances, the combination described above offers amore faithful total color reproduction (chromaticity andluminance).

Concerning this point, we would like to indicate that weconfirmed the results registered by all the monitors in whichwe carried out the colorimetry and vision studies in ourlaboratories, and all the data led to the same conclusion.Specifically, we verified the results on five others monitors.When the monitor generated a wide range of luminance, theconfigurations of the brightness and contrast levels, maxi-mum-minimum and medium-minimum, gave the least col-orimetric differences. We usually tested the procedure,though of course with fewer samples than in the experimentpresented in this work, that is, with 10 samples of differenthues.

The differences in behavior of the cameras did not influ-ence the brightness-contrast combination that offered theleast color differences. Although the color differences werenot identical between cameras, the reproduction proved bestwith the brightness-contrast combination described above,despite the use of technologically different cameras, thusmaking our results independent of the camera used.

TABLE II. CIELuv, CIELab and CIE94 Color differenceformulas for the SONY Monitor and IKEGAMI Camera.Least color differences are underlined; levels not sig-nificantly different with respect to the least color dif-ferences (at 95% confidence level) are in bold. TotalCDF means Color Difference Formula including thelightness term. I. Booth and I. Monitor indicate that thereference stimulus used in the CDF was the illuminantin the booth and in the monitor, respectively.

CIELuv Brightness-Contrastb


Lightnessa





CIELab Brightness-Contrastb


Lightnessa



Medium-Minimum 14.303 58.295 10.289 22.596Medium-Medium 13.495 41.577 11.841 18.558Medium-Maximum 12.891 33.346 11.357 17.918Maximum-Minimum 27.549 39.455 18.741 23.597Maximum-Medium 23.797 30.606 17.246 21.313Maximum-Maximum 21.849 24.857 16.404 19.805

CIE94 Brightness-Contrastb


Lightnessa





a See text for more details.b With this notation, the first word indicates the brightness level

and the second the contrast level.


No differences were found, either, in this behavior be-tween the color-difference formulas used. The only dispar-ity was that, in some of the 72 combinations assayed, theCIE94 formula reduced the number of combinations thatdiffered nonsignificantly from that having the least colordifference. Therefore, the use of this formula accentuatedthe difference between the various brightness-contrast com-binations, offering a more selective or restrictive color-difference criterion, although only under certain conditions.Table I shows this result; in the CIELUV, CIELAB formu-las, where the reference stimulus used was the illuminant onthe monitor, there were 5 combinations that did not differsignificantly, while there were 4 for the CIE94. In the samecase, without the lightness term in the formula, the combi-nations were reduced from 6 to 4 and 5 to 4 in the case ofusing the color-difference formula without the lightnessterm and the illuminant in the booth as the reference stim-ulus.

Finally, we performed a statistical analysis to ascertainwhether there were differences in the results according tothe tonal groups chosen. As stated in the Introduction, eachtone involves different combinations of primaries and, thus,unequal behavior could be expected. The results, with re-gard to minimizing the color differences for the maximumcontrast level in combination with medium or minimumbrightness levels, showed no differences between groupsand, therefore, this behavior is independent of the chromaticzone studied.

CONCLUSION

The present work demonstrates that to chose the brightnessand contrast settings on a monitor, the least chromaticdifferences evaluated with the CIELUV, CIELAB, andCIE94 color-difference formulas result from the maximumcontrast in combination with medium or minimum bright-ness values. This combination is useful in relation to CRTcolor monitors, as these present images managed in differ-ent manners—captured by CCD cameras or by scanners.

Although limited to CCD cameras, this study shows that,with the influence of the camera minimized, the CRT-colormonitor reproduced the image more faithfully under theconditions cited above. Therefore, this combination can beconsidered optimal for images on CRT-color monitors inorder to obtain the maximum chromatic information from areal image.

1. Cowan W. Handbook of optics I. New York: McGraw–Hill; 1995. Ch27, p 1–44.

2. Berns RS, Motta RJ, Gorzynski ME. CRT colorimetry. Part I: Theoryand practice. Col Res Appl 1993;18:299–314.

3. Post DL, Calhoun CS. An evaluation of methods for producing desiredcolors on CRT monitors. Col Res Appl 1989;14:172–186.

4. Brainard DH. Calibration of a computer controlled color monitor. ColRes Appl 1989;14:23–34.

5. Dıaz JA, Jimeńez JR, Hita E, Jimeńez del Barco L. Optimizing theconstant-channel chromaticity and color gamut of CRT color displaysby control of brightness and contrast levels. Appl Opt 1996;35:1711–1718.

6. Sproson WN. Colour science in television and display systems. Bris-tol: Adam Hilger; 1983. Ch 7, p 177–178.

7. Hisdal B. Acceptability of picture tube colorimetric reproduction.SMPTE J 1984:10;943–947.

8. Brainard D. Handbook of optics I. New York: McGraw–Hill; 1995. Ch26, p 1–48.

9. CIE Publ. 116-1995. Industrial colour-difference evaluation (technicalreport). Vienna: CIE Central Bureau; 1995.

10. Alman DH. CIE technical committee 1-29, industrial color-differenceevaluation progress report. Color Res Appl 1993;18:137–139.

11. Wyszecki G, Stiles WS. Color science: concepts and methods, quan-titative data and formulae. New York: John Wiley & Sons; 1982.

12. Corbalań M, Millan MS, Yzuel MJ. Color image acquisition bycharge-coupled device cameras in polychromatic pattern recognition.Opt Eng 1996;35:754–760.

13. Jimenez del Barco L, Dıáz JA, Jimeńez JR, Rubinõ M. Considerationson the calibration of color displays assuming constant channel chro-maticity. Col Res Appl 1995;20:377–387.

14. Hisdal B. Colorimetric evaluation of TV light sources. SMPTE J1991;1:27–32.

15. Engeldrum PG, Ingraham JL. Analysis of white point and phosphor setdifferences of CRT displays. Col Res Appl 1990;15:151–155.

16. Montgomery DC. Design and analysis of experiment. New York: JohnWiley & Sons; 1991.


Documents

Optimization of color reproduction on CRT-color monitors