# Visual Comfort of Tablet Devices under a Wide Range of Ambient Light Levels
## 4 Discussion  
To better know the impact of the wide ambient illuminance, including Dark, 150 lx, 300 lx, 1500 lx, 3000 lx, and 15,000 lx, on the visually comfort scales of the observers, the analyses below consider the visual comfort interval scales in our previous work [28]. The visual data of three ambient lighting conditions, the Dark, 300 lx, and 3000 lx with a horizontal CCT of 6500 K from our previous work, are applied for a comprehensive  
Figure 5: Visual comfort interval scale of the 20 text-background combinations under: (**a**) 150 lx; (**b**) 1500 lx; and (**c**) 15,000 lx.  
discussion based on a similar apparatus and essential experimental procedures. However, the major difference of the prior apparatus is the viewing booth, in which the size is 70 cm \(\times\) 65 cm \(\times\) 85 cm for its length, width, and height. The other difference between the two booths is the LED lighting devices. A two of 14-channel and a 4-channel spectrally tunable LED device is adopted to supply the test light sources with a CCT of 6500 K individually for previous work and the current research. Additionally, the five light sources are calibrated using the same calibrated equipment (JETI specbos 1211). Table 3 summarizes the colorimetric characteristics of the five light sources measured at the same viewing point with the observer.  
Therefore, the following analysis includes the evaluations from 44 Chinese observers between 19 and 25 years old. A total of 20 observers (mean = 22, SD = 1.3) evaluate 210 paired comparisons under dark, 300 and 3000 lx conditions, and 24 observers (mean = 21.3, SD = 1.1) evaluated under 150 lx, 1500 lx, and 15,000 lx. Totally 27,720 evaluations (210 pair comparisons \(\times\) 3 illuminance levels \(\times\) 20 observers from our previous work [28] +210 pair comparisons \(\times\) 3 illuminance levels \(\times\) 24 observers in the study) are discussed.  
To study the visual comfort with a variety of ambient illuminance levels, their visual comfort interval scales are plotted against each other, as shown in Figure 6a-o. For Pearson correlation coefficient r, the results significantly show the high positive correlation of the visual comfort interval scales. The r values are higher than 0.65 for the illuminance levels above 150 lx and below 3000 lx. Conversely, except for those illuminance conditions (i.e., Dark, 150 lx, 3000 lx, and 15,000 lx), the more reduced r values present. As a result, the illuminance levels with high correlation are recommended for visual perception of comfort.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Illuminance (lx)** & **CCT (K)** & **CRI Ra** & **Duv** \\ \hline
150 & 6475 & 97 & 0.001 \\
300 & 6547 & 95 & 0.0039 \\
1500 & 6454 & 95 & \(-\)0.001 \\
3000 & 6533 & 98 & 0.0041 \\
15,000 & 6471 & 98 & \(-\)0.002 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Colorimetric characteristics of the five light sources.  
Figure 6: _Cont_.  
To investigate the influence of visual comfort on the white and black background, the visual comfort scales are plotted according to the text-background lightness difference and shown in Figure 7a-f. Figure 8a-f are the line graphs of visual comfort scales against text-background lightness differences for white and black text. Figure 7a-f indicate that the observers prefer the combination with the black background to the white background on visual comfort when reading under the illuminance level below 1500 lx (Dark, 150 lx, 300 lx, and 1500 lx). On the contrary, the combinations with the white background are more comfortable evaluated by the observers under high illuminance levels (3000 lx and 1500 lx).  
Figure 6: Correlation r between different combinations of ambient illuminance settings (Dark, 150 lx, 300 lx, 1500 lx 3000 lx, and 15,000 lx).  
Figure 8: Visual comfort interval scale of the text-background combination with white and black text under six lighting conditions (**a**) Dark (**b**) 150 lx (**c**) 300 lx (**d**) 1500 lx (**e**) 3000 lx (**f**) 15,000 lx.