# Visual Comfort of Tablet Devices under a Wide Range of Ambient Light Levels
## 3 Results
### Visual Comfort  
The psychophysical experiment results determine the visual comfort interval scales using the Thurstone case V method [31]. According to the Thurstone case V method, the higher interval scale value means that the observers felt more visually comfortable reading the text-background lightness combination.  
The pair comparison data are analyzed first by comparing visual comfort interval scales between the three illuminance levels of ambient lighting conditions. Figure 4a-c illustrate the Pearson correlation coefficient between the judgments under the three illuminance levels. It is found that the results under the 150 lx conditions are similar to those under the 1500 lx conditions.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Color** & **(R, C, B)** & **(x, y)** & **Luminance(cd/m2)** & **Lightness (L\({}^{\star}\))** \\ \hline Black & (6, 6, 0) & (0.295, 0.285) & 0.744 & 1.65 \\ \hline Dark gray & (64, 64, 63) & (0.317, 0.329) & 18.2 & 25.14 \\ \hline Medium gray & (118, 118, 118) & (0.314, 0.326) & 78 & 50.82 \\ \hline Light gray & (182, 182, 185) & (0.314, 0.324) & 197 & 75.00 \\ \hline White & (255, 255, 255) & (0.315, 0.326) & 408 & 100 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Colorimetric characteristics of the five achromatic colors.  
For understanding how the background color of the text-background combination influences the visual comfort, the visual comfort interval scales for five background colors are plotted against the text-background lightness difference, as shown in Figure 5a-c for 150 lx, 1500 lx, and 15,000 lx, respectively. For ambient illuminance of 150 lx and 1500 lx, the visual comfort interval scale value increases when the lightness difference becomes larger until it reaches approximately 50. It appears to decrease as the lightness difference keeps rising, as shown in Figure 5. On the other hand, the visual comfort interval scale value of 15,000 lx improves with a larger lightness difference until it reaches about 75 and keeps constant when the lightness difference increases continuously. The results indicate that the observers tend to prefer a high text-background lightness difference combination under the 15,000 lx condition. Considering the same lightness difference, the observers perceive the stimulus more comfortable with the black background than the white background for the ambient illuminance below 1500 lx, as shown in Figure 5a,b. However, the white background seems to have a larger visual comfort interval scale value than the black background compared with the ambient illuminance of 15,000 lx from Figure 5c.  
According to Figure 5a,b, it is evident that the observers tend to select the combination of the black text with the medium-gray background (i.e., \(L^{*}\)background = 50.82; \(L^{*}\)text = 1.65) as the most comfortable text-background combination under 150 lx and 1500 lx. The combination regarded as the most comfortable under 150 lx and 1500 lx also performs the secondary comfort for the 15,000 lx. Moreover, the observers choose the combination of the black text with the light-gray background (i.e., \(L^{*}\)background = 75; \(L^{*}\)text = 1.65) as the most comfortable text-background combination under 15,000 lx. Refer  
Figure 4: Correlation r between different ambient illuminance: (**a**) 150 lx vs. 1500 lx; (**b**) 150 lx vs. 1500 lx; (**c**) 1500 lx vs. 15,000 lx.  
ring to Figure 5a-c, the most obvious differences of the illuminance levels with a 1500 lx baseline are the combinations with the white or black backgrounds.