Table 1.

Summary of four letter confusability experiments.

References	Procedure	Presentation	Font/size	Case
Townsend (1971)	The stimulus exposure sequence was white pre-stimulus with fixation point, stimulus with a random letter, and post-stimulus field with fixation point. Each letter was black against a white background. The luminance for all stimuli was 5.6 fL. The subject specific stimulus durations were chosen to produce 50% correct performance	Tachistoscope	Typewriter font/8×11 in.	Upper case
Gilmore et al. (1979)	Each letter was green in colour against a dark background. The fixation point was presented for 1 s at the beginning of each trial, and the stimulus letter was briefly presented 1 s after the fixation point disappeared. Then, it was followed by a blank screen. The stimulus durations for subjects ranged from 10 to 70 ms to obtain the correct response rate of .5	Computer	5×7 Dot Matrix	Upper case
Loomis (1982)	The visual stimuli were the transparent characters subjected to optical low-pass spatial filtering prior to viewing. The visual stimuli were viewed with the right eye in a darkened room. The stimulus duration was 2 s. The correct recognition performance was about .6	Shutter	Degraded Helvetica Extra Light/5.7 mm height	Upper case
van der Heijden et al. (1984)	Each letter was white against a dark background and subtended a visual angle of .32°×.48°. The fixation point was displayed until the subjects clearly saw the fixation point. After that, the stimulus letter was briefly presented and the subjects had to respond with a letter name within 500 ms. The stimulus durations for subjects ranged from 3 to 18 ms to obtain the correct response rate of .5	Computer	Sans Serif Roman	Upper case