Visually impaired observers require a larger window than normally sighted observers to read from a scroll display

As text is magnified, the number of letters that can be displayed with a magnifier or other reading aid at any one time (window size) is reduced. There is a great deal of evidence that reading rates increase as the number of letters displayed increases. The minimum number of proportionally spaced letters necessary to read at maximal rates from a scroll display, in which the text is continuously panned across a television screen, is not known.

Visually impaired (VI) and normally sighted (NA) observers were asked to read either sentences or random words scrolled across a computer screen. The window size was varied so that from one to 12 characters were visible, on average, at any one time. Each subject read with six different window sizes within the range of one to 12 characters visible.

For the NA group, a window of four to five characters was necessary to read at maximal rates. VI subjects, as a group, required a significantly larger window of six to seven characters to reach maximal reading rates.

The window size requirement for the NA group is close to the size other investigators have reported for a fixed-width font. However, the VI group required a larger window than did the NA group, which has not been previously reported. This requirement could result from the additional stimulation available with a larger window to entrain the eyes' motion when reading from a passively scrolled display.