Effects of viewing distance and local illumination on visual performances with a large projection screen Hsin-Chieh Wu*, Yu-Ching Cheng, Hsieh-Ching Chen, Cheng-Lung Lee Department of Industrial Engineering and Management, Chaoyang University of Technology, Taiwan * Corresponding author. E-Mail address: hcwul@cyut.edu.tw (H.-C. Wu) The usage of digital projectors has increased immensely – both in schools and companies. Compared with the visual display terminals (VDTs), the usage of a projection screen can be simultaneously watched by a large audience from different seats in a room. The use of digital projectors is becoming more popular, but studies about visual performance of digital projector users are rare. The main purpose of this study was to evaluate the effects of viewing distance (3, 6, and 9 m) and local illumination (75, 200, 450, and 750 lx) on visual performance of subjects performing visual tasks with a large projection screen. Each experimental trial included three 40-min work sessions. Each work session consisted of a 10-min proofreading task and a 30-min film watching task. Visual performance measures were obtained for 30 subjects. The results indicated that viewing distance had a significant effect on the proofreading and film-watching performances, but local illumination effect was not significant. There was a tendency to a reduced visual performance with increasing viewing distance. The findings of this study are helpful to design projected text and workplace condition for using a large projection screen. Keywords: Projector; VDT; Proofreading performance INTRODUCTION Since the rapid development of digital science and technology, the teaching aid is no longer refined to the traditional blackboard or overhead projector, it also includes the digital projector. The use of digital projectors has increased immensely – both in schools and companies. More and more people rely on a digital projector to brief plans, teach students, represent papers, demonstrate products, see films, or hold meetings and conferences. Compared with the VDT, a projection screen can be simultaneously watched by a large audience from different seats in a room. The viewing distance from the eye to the projection screen may range from 2 to 10 meter or more, which is markedly longer than those recommended for viewing from a VDT (50~70 cm). As we know, when the viewing distance is longer, the subtended visual angle of a fixed character height is smaller that may lead to worse visual performance and greater visual fatigue. Previous literature had indicated that the combination of viewing distance and character height plays an important role in VDT reading tasks (Bernard, et al., 2003; Chan and Lee, 2005; Jainta and Jaschinski, 2002; Jaschinski, et al., 1999). For reading English text from a VDT, ANSI (Human Factor Society, 1988) has specified that the minimum character height of a capital letter should be 16 min arc of visual angle (Sanders and McCormick, 1993). For reading Chinese text on a VDT, Cai et al. (2001) have suggested that the character height should be 20.39 min of arc for 100% legibility, because many Chinese characters have more complex structure and more strokes than English characters. Reasonably, a larger character size (approximately 25 min of arc) was usually adopted for reading Chinese text from a VDT (Lin, 2005; Shieh and Lin, 2000). But, the above-mentioned recommendations may not be generalizable to the use of a projector, since the optical characteristics of a projection screen is much different from the VDT. This study was thus aimed at investigating how to design an adequate character size or how to adopt suitable viewing distance for using a projection screen. In addition to character size, the ambient illumination is an important factor in VDT workplace design. The Human Factor Society (1988) recommended 200 to 500 lx is suitable for VDT tasks. As we know, higher ambient illumination would deteriorate the luminance contrast between the target and its background and thus reduce the visibility of the target on the projection screen. For this reason, we usually turn off the lights or draw the curtains to decrease the ambient illumination in order to get a clear picture on the projection screen. But it may be too dark to read or write on the table. A compromise solution to this problem is to adopt the localized general lighting method. That is to enhance local illumination only for the working table instead of improving the illumination for the whole room. Although substantial documentation exists concerning the effect of ambient illumination on visual performance for VDT tasks (Ostberg, 1980; Xu and Zhu, 1990; Shieh and Lin, 2000), there is still rare literature regarding the influence of local illumination on visual performance and fatigue for the projection screen users. In summary, the intent of this study was to investigate the effects of viewing distance and local illumination on visual