Digital Text Parameters for Low-Vision Readers Quantified

By Anne Harding

November 11, 2020

NEW YORK (Reuters Health) - Digital text must include a minimum number of characters per line to be read easily, and that minimum is smaller for readers with low vision, new research shows.

"We showed that normally sighted people need at least 13 characters and people with low vision need at least 9 characters to keep their fluent reading performance," study authors Dr. Nilsu Atilgan and Ying-Zi Xiong of the University of Minnesota in Minneapolis told Reuters Health in an email.

"Doctors and vision rehabilitation therapists can use these critical values to advice their low vision patients on maximizing their reading performance," they add. "Specifically, using our findings, doctors and therapists can detect the lower bounds of letter and screen size for their patients and use this information while prescribing reading aids."

Print size is an obvious factor in text legibility, and research suggests that text line length also plays a role, the authors note in their report in the Proceedings of the National Academy of Sciences. To date, the dual effect of text size and the number of characters per line on reading speed, a common way to gauge text legibility, has not been investigated.

The authors hypothesized that reading speed would slow if lines of text fell below a certain number of characters, which they term the critical character count (CCC). A person's CCC would be the smallest number at which they could read at 80% of their maximum reading speed (MRS). They also established a critical print size (CPS), or the smallest letters a person could read fluently.

They had 30 people with normal vision and 10 with low vision read stories from Grimms' Fairy Tales, looking at reading speed as a function of CCC and CPS.

In readers with normal vision, the CCC for reading on laptops, tablets and phone size was 12.8, regardless of font size or display format. For the low-vision readers, both MRS and CPS varied widely, but CCC was the same across the three reading formats, at 8.4.

The larger the print, the fewer characters per line, and on phone screens, some people with low vision had a CPS that was too large to fit into an eight-character line.

"We are currently working on a follow up paper that is more accessible to people outside of vision research field, in which we are trying to come up with a 'recipe' for optimum reading conditions," Dr. Atilgan and Dr. Xiong said. "We aim to present an accessible and straightforward calculation method for doctors and therapists where they can account for many different factors such as visual acuity, display size, font type, letter spacing, etc., so that they can advise their patients with optimum device and letter size."

SOURCE: PNAS, online November 9, 2020.