# Effects of Illuminance Level of Light Source on White Appearance of a Tablet Display
## 4 Discussion  
In order to better understand the effect of illuminance level on white appearance, the white appearance ratings across the four illuminance levels are compared, as shown in Figure 5 for the 3500 K light source and Figure 6 for the 6500 K light source. At both 3500 K and 6500 K, a strong correlation is evident between any two of the four illuminance levels,  
Figure 4: The u’ and v’ of the whitest stimulus judged by the observers under four illuminance levels (labeled with a blue square for the light source of 6500 K and orange for the light source of 3500 K).  
with the lowest correlation coefficient being 0.87 when comparing the results between 150 lx and 300 lx under the 3500 K light source. This seems to indicate that when the observers rate the white appearance at the same CCT, the evaluation for the whiteness percentage under different illumination is highly correlated.  
Alternatively, to better understand the impact of the CCT on white appearance, [PERSON]'s coefficient (R) is adapted to analyze the linear correlation between them. The white appearance responses at the two CCT levels are plotted against each other for each illuminance level, as shown in Figure 7. According to Figure 7, it is clear that the correlation coefficient is between 80% and 90%. When comparing the white appearance ratings of the two CCTs at illuminance levels above 150 lx, the correlation coefficients are both lower than or equal to 80%. This suggests that the CCT level has a more significant impact on white appearance rating than the illuminance level by comparing Figures 5-7.  
Based on the above results, this phenomenon is further organized in Figure 8, where the black square represents the chromaticity coordinates of the whitest stimulus rated by the observers under 150 lx, the orange cross represents the chromaticity coordinates of the whitest stimulus rated by the observers under 1200 lx at 3500 K, and the blue cross represents the chromaticity coordinates of the whitest stimulus rated by the observers under 1200 lx at 6500 K. According to Figure 8, as the illumination increases, the whitest stimulus for the 3500 K light source moves closer to its original light source, whereas the whitest stimulus for the 6500 K light source shifts towards a higher correlated color temperature.  
Figure 5: Correlation (r) between various combinations of ambient illuminance levels under 3500 K. (a) 150 lx vs. 300 lx; (b) 150 lx vs. 600 lx; (c) 150 lx vs. 1200 lx; (d) 300 lx vs. 600 lx; (e) 300 lx vs. 1200 lx; (f) 600 lx vs. 1200 lx.  
Figure 6: Correlation (r) between various combinations of ambient illuminance levels under 6500 K. (a) 150 k vs. 300 lx; (b) 150 lx vs. 600 lx; (c) 150 lx vs. 1200 lx; (d) 300 lx vs. 600 lx; (e) 300 lx vs. 1200 lx; (f) 600 lx vs. 1200 lx.  
Figure 7: Correlation (r) between the 3500 K and 6500 K light source at each illuminance level (150 lx, 300 lx, 600 lx, and 1200 lx).  
The limitations of this study lie in the limited environmental lighting parameter settings. This study only investigates the effect of four illuminance levels on white appearance at two CCTs. For further research, more illuminance levels and CCTs will be included, providing more comprehensive recommendations for lighting design applications.