We study near point distance as a metric of eye fatigue in the reading task; the goal is to develop an objective scale for creating readable Electronic Paper. Near point distance can be measured by moving a target mark toward and away from subjects as they try to focus on it (Decrement method and Increment method). The correlation between the increase in the near point distance and subjective answers is evaluated in 120 minute reading tasks by using the two methods of the near point measurements. The measurement procedure based on near point decrement showed superior correlation: it is regarded adequate as a quantitative index of eye fatigue. The Decrement method is used to measure eye fatigue with 270 minute reading tasks on three media: paper, electronic paper (book reader using electrophoretic display), and conventional display (LCD). Only the display evidenced an increase in near point distance. This suggests that the electronic paper used in this study is superior to the conventional display with regards to avoiding fatigue in reading tasks.
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