# Effects of Viewing Displays from Different Distances on Human Visual System
## 4 Discussion  
The subjective visual discomfort, relative \(\beta\) power of EEG, and \(\alpha/\beta\) power ratio of EEG are used to evaluate the effect of watching videos on 2D and 3D displays associated with viewing distances  
Figure 4: Effect of viewing distance by display type interaction on relative \(\beta\) power (%). Figure 5: Effect of viewing time by display type on \(\alpha/\beta\) power ratio. (3H vs. 6H) over a viewing time of 60 min. In terms of the visual discomfort, it is revealed that watching the 3D display clearly caused more visual discomfort than watching the 2D display, regardless of the viewing distance. These results are consistent with those of previous studies [9; 33; 38; 42; 47; 48; 49]. The mental responses of the participants were analyzed while watching different films on 2D and 3D displays over sixty minutes from long and short viewing distances. The results show that the relative \(\beta\) power of the EEG decreases slightly with the increase in the viewing time. Moreover, watching the 3D display contributes more to the decrease in the relative \(\beta\) power of the EEG than watching the 2D display, which causes more visual fatigue than that evident in the case of the 2D-display. These results are consistent with the study conducted by [PERSON] et al. [6], who found that for most participants watching the 3D TV, the energy in the \(\beta\) frequency bands significantly decreases. Moreover, when the viewing distance was switched from 3H to 6H, the relative \(\beta\) power increased in the 3D-display case, while it decreased in the 2D-display case. In addition, the same levels of visual fatigue were observed when watching videos at 2D or 3D from a long viewing distance (6H). These results show that watching the 3D display from a short viewing distance resulted in a high visual fatigue. Viewing the 2D display from a short viewing distance resulted in a low visual fatigue as the viewing distance gets longer, resolution will be higher (pixels on the display will be imaged on a smaller region on viewer's retina, making the image harder to resolve), field of view will be smaller (hence the sense of immersion gets weaker), and binocular disparity will get smaller (even though the disparity is the same on the screen, the angular disparity with respect to the viewer will be smaller--therefore, the viewer will experience smaller amounts of vergence-accommodation conflict). Moreover, the type of display was found to have the most contribution to the visual fatigue compared to other variables (12.7%), followed by distance*display (11.8%), time (5.9%), and distance (1.4%). The results of the \(\alpha/\beta\) power ratio show that watching the 3D display contributes to increase in the \(\alpha/\beta\) power ratio over viewing time while it remains stable when the participants watched the 2D display over time. Moreover, the type of display has the most contribution to the visual fatigue compared to other variables (13.1%), followed by time (12.3%), and time*display (8.1%). It can be concluded that when watching videos in 3D display, visual fatigue increased between ten and twenty minutes. Subsequently, it increased significantly for 10 min (at 30 min of watching). Thereafter, the increase was gradual for the next 10 min (at 30 min of watching), and then further increases gradually for the next 20 min (at sixty min of watching). Moreover, watching the 3D display contributes to increase the \(\alpha/\beta\) power ratio more than watching videos in 2D display. This result was in agreement with that obtained by [PERSON] and [PERSON] [10], where the increase in \(\alpha/\beta\) was associated with visual fatigue. The results of this study showed that there were certain discrepancies between the subjective measure of visual fatigue and the objective metric. Because the subjective measure is one of the conventional methods which might be affected by the personal variation, practical experience, and familiarity, has a significant effect on conscious experience with both positive and negative consequence.