Chromaticity and Luminance on Visual Performance and Subjective Performance for Imagery on LCD

Po-Chun Chang Department of Industrial Management

National Taiwan University of Science and Technology 43 Keelung Road, Section 4, Taipei, Taiwan, R.O.C

wolf.chang@asml.com

Der-Song Lee*, Yun-Ying Yeh and Kong-King Shieh

Department of Industrial Management Oriental Institute of Technology

Banciao City, Taipei County, Taiwan, R.O.C. dersonglee@gmail.com

Abstract—This study investigated the effects of chromaticity and luminance on legibility and subjective preference for imagery presented on LCD. The results showed that chromaticity and luminance had little effect on legibility speed. Both chromaticity and luminance significantly affected subjective preference. Chromaticity seemed to be more important than luminance for subjective preference. Participants appeared to prefer medium luminance, they also showed less preference for achromatic imagery. Regression equations for legibility speed and subjective preference were constructed. Correlation analysis showed that the equations were better than Lippert's �E (CIE Y, u', v').

Keywords-Chromaticity; Luminance; Legibility; Subjective preference; LCD

I. INTRODUCTION With the progress of computer technology, people spend

more and more time on computer. Visual display terminal (VDT, or computer screen) is the major interface between user and computer. It is worthwhile to explore the design of computer screen to optimize users' visual performance and subjective preference. Chromaticity is one of the most important factors affecting visual performance and subjective preference on computer screen. It can effectively enhance the interaction between user and computer (Pastoor, 1990; Shieh and Lai, 2008) [1, 2]. It can also make computer work pleasant and acceptable. However, inappropriate use of chromaticity may result in poor visual performance, or even adverse effect like visual fatigue (Chen and Shieh, 1995; Ko, Shen and Lee 2010). [3, 4] Although there are many suggestions regarding the use of chromaticity on computer screen (Sanders and McCormick, 1993) [5], they are mostly ambiguous. There is a need for an easy, quantified, and effective index of color use on computer screen.

ANSI/HFS 100-1988 (1988) [6] recommended using Lippert (1986) [7] �E (CIE Y, u', v') to determine the legibility of chromatic imagery, or the recognition of symbols on the screen. However, there are some issues to be further investigated. First of all, Lippert used CRT (Cathode Ray Tube) to present stimuli. However, the liquid crystal display (LCD) has become the most popular computer screen in recent years. The effectiveness of Lippert �E (CIE Y, u', v') for LCD remains to be verified.

Moreover, Lippert developed the formula based on response speed. Many studies (Christ, 1975; Matthews et al, 1989) [8, 9] reported that computer users have greater preference for color display and they believe that colors may lower visual fatigue or pressure, or even enhance work performance. But there were few researches on quantitative relationship between chromaticity and subjective preference. Therefore, one of the objectives of this study was to quantify the relationship between chromaticity and subjective preference.

II. METHOD

A. Participants Forty college students (20 males, 20 females)

participated in this experiment. All participants were between 20 and 26 years of age (M = 22.8 yr., SD = 1.6 yr.) and had 20/25 corrected visual acuity or better, and normal color vision. B. Experimental design

This study assessed two independent variables: text luminance and chromaticity. There were four different levels of luminance, 15, 30, 45, and 60 cd/m2. Nine levels of chromaticity were used. The CIE chromaticity coordinates of the nine colors are shown in Table 1. Luminance was a between-subject factor, 10 participants (5 males, 5 females) were randomly assigned to each level of luminance. Chromaticity was a within-subject factor. That is, each participant completed all 9 colors in the experiment.

TABLE I. CIE CHROMATICITY COORDINATES OF COLORS USED IN THE EXPERIMENTS

x y z

1 (Orange-red) 0.420 0.310 0.270

2 (Celadon) 0.310 0.420 0.270

3 (Purplish-red) 0.357 0.247 0.397

4 (Turquoise) 0.247 0.357 0.397

5 (Purple) 0.411 0.255 0.333

6 (Cyan) 0.255 0.411 0.333

7 (Orange) 0.378 0.378 0.244

8 (Brown) 0.288 0.288 0.423

9 (White) 0.333 0.333 0.333

Background (Black) 0.312 0.365 0.323

CoordinateColor

2012 International Conference on Computer Science and Electronics Engineering

978-0-7695-4647-6/12 $26.00 © 2012 IEEE

DOI 10.1109/ICCSEE.2012.183

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C. Apparatus A Topcon SS-3 screenscope and the Standard Pseudo-

Isochromatic Charts were used to examine participants' visual acuity and color vision. The CIE chromaticity coordinates of colors were measured with a Minolta chroma meter CS-100. An Intel Intel Core i3-370M notebook computer with a TFT type 15.5 inch backlit LCD screen was used to present the experimental task. Screen resolution was 1366 x 768 pixels. The screen was frosted to reduce specular reflections.

The VDT workstation in the experiment simulated a computer working environment. The screen was placed on a desk of 73cm height. The distance from the center of the screen to the top of desk was 23 cm and to the edge of the desk was 40 cm. The screen inclination angle was 105 . The light source was fluorescent lamp. The ambient illumination was around 400 lux. The viewing distance from the participant to the screen was 60 cm, with no glare on the screen.

D. Task and procedure There were two stages in this experiment. In stage 1,

participants were required to orally read aloud the stimulus presented on the screen as fast as possible. There were 60 stimuli, including 10 digits (0-9), 26 English capital letters (A-Z), and 24 Chinese characters. The stimuli were presented one by one under each luminance and chromaticity combination on a fixed black background. The CIE chromaticity coordinates of the background black color is also shown in Table 1. It luminance was 0.63 cd/m2.

Stage 2 was for subjective preference. First of all, all 60 stimuli were shown with a particular luminance and chromaticity combination (4 × 9 = 36 combinations totally) on one screen. At the same time, the same stimuli was shown in standard condition on a separate screen. The standard condition was white (x = 0.333, y = 0.333) with luminance 35 cd/m2. The participants were required to compare their preference of the luminance and chromaticity combination against the standard condition. The preference score for the standard condition was fixed at 50. The preference rating for the luminance and chromaticity combinations ranged from 0~100, the higher the score meant the greater preference. Each participant completed 36 preference ratings.

The dependent variables in the study included legibility speed and subjective preference. The response time from stimulus presentation to correct oral response was recorded, then transformed to reciprocal as legibility speed (responses/second). The larger the number indicated the higher the legibility speed. The data were analyzed by the method of analysis of variance and regression analysis. All calculations were made with the Statistical Analysis System (SAS). The level of significance was

III. RESULTS Legibility speed and subjective preference under levels of

luminance and chromaticity are showed in Table 2. The followings are the results of analysis of variance and regression analysis.

A. Legibility Speed Table 2 showed that legibility speed was similar under

either various luminances or chromaticities. ANOVA results showed the two factors had no statistically significant effect (�=0.05) on legibility speed.

TABLE II. MEANS AND STANDARD DEVIATIONS OF LEGIBILITY SPEED AND SUBJECTIVE PREFERENCE UNDER EACH LEVEL OF THE INDEPENDENT

VARIABLES

Independent variables legibility speed (sec) Subjective preference

Luminance (cd/m2)15 1.64 48.830 1.63 59.345 1.67 67.160 1.64 64.1

Chromaticity1 1.64 63.12 1.63 55.53 1.65 66.04 1.60 59.55 1.61 72.46 1.67 70.67 1.65 54.98 1.68 48.69 1.68 47.8

B. Subjective Preference As indicated in the analysis of variance (Table 3),

luminance had significant effect on subjective preference (F(3, 36) = 5.40, p 0.05). The paired comparison of critical difference was 5.78 as determined by LSD multiple testing method. Subjective preference went up significantly from luminance 15 cd/m2, 30 cd/m2, to 45 cd/m2, then dropped at 60 cd/m2. Chromaticity had significant effect on subjective preference (F(8, 288) = 25.34, p 0.01). Chromaticity 5 and 6 had the highest preference rating at 72.4 and 70.6 respectively, while 8 and 9 were the lowest, at 48.6 and 47.8 respectively. Chromaticity 8 and 9 were close to achromaticity (white). There seemed to have a trend that the closer a chromaticity is to white (CIE x = 0.333, y = 0.333), the lower preference rating it got.

TABLE III. THE ANOVA FOR SUBJECTIVE PREFERENCE

Source do SS MS F

Luminance (L) 3 17325 5775 5.40 *

Error for L 36 38488 1069

Chromaticity (C) 8 25375 3171 25.34 **

L × C 24 4789 199 1.59

Error 288 36048 125 * Significant at � = 0.05; ** Significant at � = 0.01.

C. Predicting Model of Legibility Speed and Subjective

Preference

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Stepwise regression analysis was employed to construct regression models for predicting legibility speed and subjective preference using luminance (L), chromaticity coordinate (x, y), and their interactive effect. The resulting equations for legibility speed and subjective preference were as follows:

Legibility speed 1.60 + 0.0001 L (90 - L)

- 1.7 (x - y) 2 (1) Subjective preference 18.7 + 0.015 L (100 - L)

+ 860 (x - y) 2 + 2400 y 2 (2) The coefficients of determination (R2) for legibility

speed and subjective preference were 0.11 and 0.65 respectively. The prediction for legibility speed was not good. On the contrary, the prediction for subjective preference was great. To further compare the predicting ability of equations (1), (2) and Lippert �E (CIE Y, u', v'), Pearson product moment correlation coefficients were calculated between legibility speed, subjective preference

and the above indices. The sγ were 0.33 and 0.14 respectively for equation (1) and Lippert �E (CIE Y, u', v'). Equation (1) seemed better. For subjective preference, The

sγ were 0.81 (equation (2)), greater than that of Lippert �E (CIE Y, u', v') (r = 0.28).

IV. DISCUSSION The objective of this study is to explore the effect of

chromaticity and luminance on legibility speed and subjective preference of imagery on LCD.

For legibility speed, this study revealed that the effects of either luminance or chromaticity were small. This result was consistent with the findings of Mills and Weldon (1987) [10] that chromaticity had no significant effect on legibility. However, many researches revealed legibility had significant relationship with luminance, especially luminance contrast. Snyder (1988) [11] suggested luminance contrast should be at least 0.667, and for greater legibility, should be over 0.875. This study found that the effect of luminance was insignificant. One possible reason is that the background luminance was extremely low (0.63 cd/m2). For target luminance of 15, 30, 45, 60 cd/m2, the target/background luminance contrasts were 0.958, 0.979, 0.986, 0.989, all higher than 0.875 as suggested by Snyder, and hence resulting in insignificant effect of luminance on legibility speed.

For subjective preference, both luminance and chromaticity showed significant effect. Based on regression model, the effect of luminance was luminance × (100 - luminance). This value was the greatest at medium luminance around 50. That is to say, the participants appeared to prefer medium luminance. They disliked too bright or too dark luminance.

Chromaticity had significant effect on subjective preference. Regression equation (2) showed the bigger the

chromaticity difference between x and y was, especially for bigger y, the higher the subjective preference. Besides, the participants did not prefer achromatic text. In this study, chromaticity 8 and 9 were close to achromatic (x = 0.333, y = 0.333) and they got low preference rating. Chromaticity appeared to have contrary effect on legibility speed and subjective preference. Chromaticity (away from achromaticity) enhanced subjective preference, but reduced legibility speed. Achromaticity (white) benefited legibility but deteriorated subjective preference.

V. CONCLUSION In summary, the results of this study revealed that

different factors affecting legibility speed and subjective preference for imagery on VDT. Luminance appeared to have greater effect on legibility speed. However, subjective preference was affected more by chromaticity. This study offered a regression equation to estimate legibility speed and subjective preference. However, the validity of the equation needed further verification because of limitations in this study. For example, the background color in this study was fixed as black color, more target/background color combinations should be explored in future study.

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combinations in text,” Human Factors, 32, 1990, pp.157-171. [2] Kong-King Shieh and Yen-Kung Lai, “Effects of ambient

illumination, luminance contrast, and stimulus type on subjective preference of VDT target and background color combinations,” Perceptual and motor skills, Volume: 107, Issue: 2, 2008, pp. 336-352.

[3] Ming-Te Chen and Kong-King Shieh, “ ,” 1995 [4] Ya-Hsien Ko, I-Hsuan Shen and Der-Song Lee, “Color Combinations

of Visual Display Terminal (VDT) Icon on User Preferences and EEG Response,” Perceptual and Motor Skills, 110, 2, 2010, pp. 411-428.

[5] M. S. Sanders and E. J. McCormick, “Human Factors in Engineering and Design,” McGraw-Hill, Singapore, 1993, pp. 91-131.

[6] American National Standard for Human Factors Engineering of Visual Display Terminal Workstations (ANSI/HFS Standard No. 100-1988), Human Factors Society, Inc., Santa Monica, CA, February 4, 1988.

[7] T. M. Lippert, “Color-difference prediction of legibility performance for CRT raster imagery,” SID Digest of Technical Papers, 16, 1986, pp. 86-89.

[8] R. E. Christ, “Review and analysis of color coding research for visual displays,” Human Factors, 19, 1975, pp. 542-570.

[9] M. L. Matthews, J. V. Lovasik and K. Mertins, “Visual performance and subjective discomfort in prolonged viewing of chromatic displays,” Human Factors, 31, 1989, pp. 259-271.

[10] C. B. Mills and L. J. Weldon, “Reading text from computer screens,” ACM Computing Surveys, 19, 1987, pp. 329-358.

[11] H. L. Snyder, “Image quality,” In M. Helander(Ed), Handbook of Human-Computer Interaction, Amsterdan, 1988.

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