## 1 Introduction  
Computer vision syndrome (CVS), also referred to as digital eye strain, describes a group of eye and vision-related problems (i.e., visual discomfort, visual fatigue, blurred vision, eye strain, dry eyes, irritated eyes, double vision, vertigo/dizziness, polyopia, headaches, neck pain, and difficulty refocusing the eyes) that result from prolonged computer, tablet, e-reader and cell phone use [1, 2, 3, 4]. Among these problems, visual discomfort and visual fatigue are core symptoms. The prevalence of CVS has been reported to range from 25% to 93% of computer users depending on the instrument employed, sample examined and methodology used [3]. Despite the varied prevalence of CVS, as many as 90% of digital device users experience symptoms of CVS [4], nearly 60 million people are estimated to suffer from CVS globally, and new cases will increase at a rate of 1 million each year [5]. Multiple factors contribute to the severity of CVS, which can roughly be classified into three clusters: eye-related, environment-related and device-related [4]. Environment- and device-related factors are easily self-determined by users and, thus, should be prioritized in intervention studies. Among all the influential factors, ambient illuminance (which is categorized as an environment-related factor) and screen luminance (which is categorized as a screen-related factor) seem to be the most influential [1, 2] and have attracted much attention in previous studies; thus, they are the focus of the current study. Visual discomfort and visual fatigue have been studied in the light and lighting fields during the last half century [6, 7, 8]. Light is not only a prerequisite for vision but also a beneficial factor in improving visual health if used in the correct manner (see review [9]). Despite the extensive research performed on this topic, previous findings have been diverse, and it is difficult to obtain a consensus. For example, some studies have found that compared with low (200 lx) and high ambient illuminance (800 or 1000 lx), medium illuminance (500 or 600 lx) is most beneficial for improving visual performance [10; 11], but other research has revealed no significant visual performance improvement with ambient illuminance [12; 13]. Compared with high ambient illuminance (700 lx), low illuminance (200 lx) has been shown to be slightly more preferred and better for visual recognition performance [14]. Although subjective visual fatigue was shown to be higher under 200 lx than 500 lx in one study, the objective visual fatigue evaluated by critical flicker frequency (CFF, which is an objective indicator of visual fatigue) was not as severe under the 200 lx condition as that under the 500 lx condition [15]. Moreover, visual comfort remains stable when the illuminance level achieves a comfortable threshold [16], and high illuminance (i.e., 1000 lx) can even cause dissatisfaction and discomfort and increase visual fatigue [17; 18]. All these results suggest that relatively low or medium levels of ambient illuminance may be beneficial for enhancing visual comfort and minimizing visual fatigue. In addition to ambient illuminance, screen luminance is another vital factor influencing visual discomfort and visual fatigue. Studies investigating the effects of screen luminance on visual work efficiency have revealed that compared with low screen luminance, high screen luminance significantly improves visual display quality and task performance [19]. High screen luminance has been shown to visually reduce image distortion and improve image quality, which is beneficial for task performance [20]. However, several studies have shown that working under high screen luminance for a long period elicits more visual fatigue (see review [2]). [PERSON] and [PERSON] (2015) investigated the optimal luminance under extremely low illuminance levels (less than 1 lx) and found that the optimal levels were 10 cd/m\({}^{2}\) for the initial viewing condition and 40 cd/m\({}^{2}\) for the continuous viewing condition [21].