LAUSR

Significance Accessibility of digital text is essential in modern society. The growing use of small mobile displays forces attention to the competing requirements for adequate print size and adequate screen real estate. As print size gets larger, the number of characters per line and the number of lines per screen shrinks, ultimately affecting reading performance. This is particularly challenging for older people who may require larger print and for the growing population with low vision. We developed a unified framework for measuring the impact of print size and display size on the readability of text. We show how these constraints reduce the range of print size, and for some display formats, prevent some readers from maximizing their reading performance. Two fundamental constraints limit the number of characters in text that can be displayed at one time—print size and display size. These dual constraints conflict in two important situations—when people with normal vision read text on small digital displays, and when people with low vision read magnified text. Here, we describe a unified framework for evaluating the joint impact of these constraints on reading performance. We measured reading speed as a function of print size for three digital formats (laptop, tablet, and cellphone) for 30 normally sighted and 10 low-vision participants. Our results showed that a minimum number of characters per line is required to achieve a criterion of 80% of maximum reading speed: 13 characters for normally sighted and eight characters for low-vision readers. This critical number of characters is nearly constant across font and display format. Possible reasons for this required number of characters are discussed. Combining these character count constraints with the requirements for adequate print size reveals that an individual’s use of a small digital display or the need for magnified print can shrink or entirely eliminate the range of print size necessary for achieving maximum reading speed.