## Methods
### Data Analysis  
All analyses were performed using GraphPadPad Prism 8. First, the descriptive data (mean and standard error) on screen brightness, visual comfort with screen luminance, visual fatigue, and illuminance satisfaction were analyzed. A 5 (Ambient Illuminance: 0, 25, 50, 75, and 100 lx) \(\times\) 6 (Screen Luminance: 3.87, 21.47, 42.74, 64.12, 84.77 and 106.7 cd/m\({}^{2}\)) repeated analysis of variance (ANOVA) was applied to all subjective evaluations using SPSS version 21.0 (IBM, USA). All post-hoc tests were Bonferroni corrected for multiple comparisons. Note that all the results of ANOVA analysis were not included in the main text but could be found in the Supplementary Materials.  
Second, the mean ratings on screen brightness, visual comfort with screen luminance, visual fatigue, and illuminance satisfaction were subjected to the \"Nonlinear regression (curve fit)\" module. Curve fitting has been used in previous studies on visual comfort [53; 54] and brightness perception [55]. The general rule of curve fitting was to maximize adjusted R\({}^{2}\) without overfitting the model. \"Polynomial\" was selected, and \"First order polynomial (straight line)\", \"Second order polynomial (quadratic)\", \"Third order polynomial (cubic)\", \"Fourth order polynomial\", and \"Fifth order polynomial\" were tested sequentially. After each test, the curve fitting results were checked to determine if the model was overfitting. If the model was overfitting, the process of curve fitting was stopped, and the best function being tested was selected for curve fitting.  
Figure 3: Overview of the experimental procedure.  
Third, after the determination of the best function, the curves were fitted for each dependent variable. For screen brightness, a rating of \"4\" means \"neither too dark nor too bright\", and, thus, a straight line across \"4\" was fitted to determine the optimum screen brightness. The intersection between this straight line and the best-fit straight line for each ambient illuminance condition was used to determine the optimum screen luminance for each ambient illuminance condition. For visual comfort with screen luminance, the max value for visual comfort was determined after the best-fit curve was plotted for each ambient illuminance, and the corresponding screen luminance was the best one for this specific ambient illuminance condition. The minimum value for visual fatigue was determined after the best-fit curve was plotted for each ambient illuminance, and the corresponding screen luminance was determined as the best one for this specific ambient illuminance condition to maintain visual fatigue at a relatively low level. The max value for illuminance satisfaction was determined after the best-fit curve was plotted for each screen luminance, and the corresponding ambient illuminance was determined as the best one for this specific screen luminance condition. To determine the best ambient illuminance for visual fatigue, the mean ratings for visual fatigue were modeled again based on ambient illuminance. The minimum value for visual fatigue was determined after the best-fit curve was plotted for each screen luminance, and the corresponding ambient illuminance was determined as the best one, for this specific screen luminance condition, to keep visual fatigue at a relatively low level.  
After step 3, the optimum screen luminance for screen brightness, visual comfort and visual fatigue under different ambient illuminance conditions was determined, and was used for curve fitting. Moreover, the optimum ambient illuminance for satisfaction and visual fatigue under different screen luminances was also determined, and was utilized to fit the curve.