## 4 Discussion  
Considering the widespread usage of LCD screens in the evening, the current study aimed to explore the optimum screen luminance for visual fatigue, visual comfort, satisfaction and brightness under varied ambient illuminance conditions at a relatively low level (ranging from 0 to 100 lx) for LCD screen users at night. Expanding on previous studies, the current study refined and employed five ambient illuminance levels and six screen luminance levels. The optimum screen luminance levels and the optimum ambient illuminance levels were determined and are reported in Tables 6 and 8, respectively. Moreover, the optimum screen brightness curve was fitted, and the high visual comfort curve and low visual fatigue curve for screen luminance, and the high satisfaction curve and low visual fatigue curve for ambient illuminance (see Figures 7 and 10), were evaluated based on subjective ratings of visual experience. To ensure that the requirements of visual comfort, visual fatigue, satisfaction and brightness were met, 13.08-62.16 lx and 20.63-75.15 cd/m\({}^{2}\) were shown to be optimal when using the LCD screen during the evening. The results did show statistically significant effects of ambient illuminance and screen luminance on subjective fatigue and visual comfort. These findings were not in accordance with the study by [PERSON] et al. (2014) [22]. The difference in the manipulations of ambient illuminance and screen luminance would partly explain these inconsistent findings. Only two ambient illuminance levels (5 and 85 lx) and two screen luminance levels (20 and 140 cd/m\({}^{2}\)) were investigated in the study by [PERSON] et al. (2014), while multiple levels of both ambient illuminance and screen luminance were employed in the current study, making it less likely to obtain significant main effects of ambient illuminance and screen luminance. Meanwhile, the current study employed subjective ratings of visual fatigue and visual comfort, while eye blink frequency was employed as an objective indicator of visual fatigue in their study. In addition to the influence of ambient illuminance and screen luminance on visual fatigue, the current findings revealed that the optimum screen luminance increased accordingly with increasing ambient illuminance levels, which was in accordance with most previous studies [23; 24; 25; 26; 27; 28]. For example, [PERSON] et al. (2013) found that the optimum screen luminance of a mobile phone was 11 cd/m\({}^{2}\) and 68 cd/m\({}^{2}\) under 0 k and 100 lx conditions,  
Figure 10: High satisfaction curve and low visual fatigue curve for ambient illuminance with different screen luminances. respectively [23]. [PERSON] et al. reported that the optimal luminance level was 55 cd/m\({}^{2}\) and 170 cd/m\({}^{2}\) under 0 lx and 150 lx conditions, respectively [25], and 58.5 cd/m\({}^{2}\) increased to 84 cd/m\({}^{2}\) when 100 lx was increased to 300 lx [26]. For absolute darkness (0 lx), both the current findings (20.63-36.2 cd/m\({}^{2}\)) and those of [PERSON] et al. (2013) (11 cd/m\({}^{2}\)) [23] fell into the range identified by [PERSON] et al. (2015) (10-40 cd/m\({}^{2}\)) [21]. However, [PERSON] et al. (2014) [25] reported a much higher level of 55 cd/m\({}^{2}\). This may be due to the differential study approach employed in the study by [PERSON] et al. (2014), in which participants were asked to adjust screen luminance subjectively to an optimal level according to their experience. Their findings also suggested that the self-adjusted optimal screen luminance levels were relatively higher than the levels automatically adjusted by the phone [25]. For the 100 lx illuminance level, the current findings were comparable to previous findings [23; 26; 28], whereas they were much lower than those of [PERSON] et al. (2017) [27]. The property of a screen may be a contributor to this discrepancy. The highest luminance was 868 cd/m\({}^{2}\) in the study by [PERSON] et al. (2017) [27], while 106.7 cd/m\({}^{2}\) was employed in the current study.