Abstract
The estimation of optimal print size for reading is often essential in clinical treatment and/or universal design; however, it is not known how to calculate the proper letter size for reading with maximum efficiency. Psychophysics studies have revealed psychometric functions of reading that exhibit a hill-like shape with a plateau of maximum speed and a downfall beyond Critical Print Size (CPS). To control the magnification rate of visual aids for patients with visual impairments, CPS that can indicate the boundary of maximum efficiency is now becoming a noteworthy index to determine optimal letter size. In addition to reading, word searching is also an important task for our living. However, the CPS of word search tasks has not been examined yet. We estimated the CPS of word search from the results of two experiments focused on searching for words in Chinese characters and Japanese alphabet (Katakana: square forms) in Japanese. The functions of the searching tasks showed a hill-like shape almost identical to the reading tasks but with elevated speed, and the CPS were stable around 0 logMAR in both the reading and the searching tasks. Hence, CPS is unsusceptible to tasks and can function as a robust marker for the smallest print size with maximum speed. This finding indicates that CPS is the threshold of proficiency (maximum/reduced) beyond the threshold of vision (visible/invisible). CPS can be a meaningful index to achieve the appropriate control of print size and subsequently help people with visual problems.
