Preferred E-mirror luminance levels for diverse ambient light conditions
Authors: Park Seonghyeok , Hwang Kitae , Yoon Sohyun , Kyung Gyouhyung 
Published: November 2019

E-mirrors comprised of cameras and visual displays are expected to replace current side mirrors. Although previous studies have extensively examined the effects of E-mirror size and location on drivers’ performance, safety, and preference, little is known about the required E-mirror luminance levels for diverse ambient light conditions that are typically involved in driving. This study examined the effects of ambient light conditions on the preferred E-mirror luminance levels.

Sixteen individuals with a mean (SD) age of 25.7 (5.8) years participated in this study. All participants were recruited from a university student population and had more than two years of driving experience. All participants reported no color deficiency. A local institutional review board approved this study. This study considered four levels of ambient light conditions, two levels of eye adaptation (light and dark), and two levels of eye adaptation phase (initial and final). The four illuminance levels simulated daytime driving (600 lux), tunnel driving (daytime and nighttime; 100 lux), nighttime driving (3 lux), and sunlight condition. The daytime, tunnel, and nighttime driving involved looking at a corresponding driving scene projected on the front screen under a specific illuminance level, which was controlled by the indoor lighting system. The sunlight condition involved looking outside through the room window instead of looking at the front screen. A driving simulator was implemented using a car seat, a gaming steering wheel, and a beam projector. Two tablet PCs with an 8.0-inch screen of 9.8 (height) × 6.13 (width) cm (Galaxy Tab A 8.0 2017, Samsung Electronics, South Korea) were used as E-mirror displays. The E-mirror brightness could be manually adjusted using the steering wheel buttons, which were connected to the two tablet PCs. The SCANeRTM studio (v1.1, Oktal, France) driving scenes corresponding to each illumination condition were used in this study. Two distinct driving scenarios were considered. The first driving scenario was for daytime ambient light conditions, and consisted of the first daytime driving (DD1), first daytime tunnel driving (DTD1), DD2, DTD2, DD3, and daytime sunlight driving (DSD). In this scenario, DTD induced dark adaptation, and DD induced light adaptation. The second scenario was for nighttime driving, and consisted of the first nighttime driving (ND1), first nighttime tunnel driving (NTD1), ND2, NTD2, ND3, and nighttime sunlight driving NSD. ND involved dark adaptation, whereas NTD involved light adaptation. Both DD1 and ND1 were included to reduce the effect of the ambient light condition outside of the experimental room.

The E-mirror brightness could increase or decrease by 10 within the range of 0-200 using the tablet PC brightness control function. The brightness of both tablet PCs was adjusted synchronously. For data analysis, the mean luminance (nit, cd/m2) of the image displayed on the left E-mirror was used. The luminance levels of the E-mirror images for the daytime, tunnel, and nighttime driving ranged from 1.5-184.4, 0-3.0, and 0-3.0 nit, respectively. The preferred luminance levels during eye adaptation were obtained every 15 s for the first 5 min and every 30 s for the remaining period of 5-30 min. The preferred luminance level after completion of each eye adaptation was measured using an ascending-descending series.

During the eye adaptation period, the preferred luminance levels for ND1 were significantly different from those for ND3, which could be largely due to the carryover effects of pre-ambient light condition. The final difference in the preferred luminance levels between ND1 and ND3 was observed at 1,215s (20.25 min). In addition, significant differences in the preferred luminance were observed during the first min between DSD and NSD. The results of nighttime driving and sunlight driving indicated that the E-mirror luminance should be adjusted during the first 1,000 to 1,200 s for dark adaptation and the first 60 s for light adaptation. Eye adaptation inside the tunnel was completed during the first 15s for DTD1 and NTD1. However, there was no significant difference between DTD2 and NTD2 except for the time of 0s. Therefore, light and dark adaptation during tunnel driving appeared to be completed within 15 s.

During the post-eye adaptation period, the effect of the pre-ambient light condition on the preferred luminance levels disappeared. The mean (SE) preferred luminance (nit) in the post-adaptation phase for the daytime, tunnel, nighttime, and sunlight driving was 126.1 (3.4), 1.5 (0.1), 0.8 (0.1), and 138.9 (3.7), respectively. The mean (SD) illuminance (lux) for DSD and NSD was 1404.7 (265.7) and 1339.9 (239.4), respectively.

Some conditions showed significant differences between the initial and final preferred luminance levels. The preferred luminance levels decreased for ND1 and ND2, whereas the preferred luminance levels increased for NSD. Dark adaptation to the nighttime driving illuminance and light adaption to the extreme illuminance both affected the preferred luminance levels. These findings support the need for gradual luminance adjustment of E-mirrors during eye adaptation. Overall, this study showed that the preferred E-mirror luminance levels were affected by previous and current ambient light conditions.

Several limitations of this study should be addressed in future studies. First, this study considered the preferred E-mirror luminance only and did not consider driving performance measures. Second, although the illuminance conditions considered in this study were carefully selected to represent distinct ambient light conditions encountered during driving, it is still necessary to examine additional illuminance conditions representing diverse driving contexts. Thus, in addition to the scenarios considered in this study, additional scenarios (e.g., different illuminance levels for daytime, tunnel, nighttime, and sunlight driving than those considered in this study as well as more rapid illuminance-changing cases) should be considered in future studies.