Estimating the legibility of a single letter E viewed at different display angles

Text is often viewed not perpendicularly to its display. With nine assumptions, this study developed a new equation for estimating the legibility of a single letter E viewed at different display angles. This equation examines viewing distance, text height, strokewidth, Snellen acuity, background luminance, luminance contrast, and incident angle. This equation was derived from the existing Howett's equation based on a constant-solid-angle hypothesis tested in a lighting laboratory by reading letter 'E's using 20 young human subjects. The hypothesis holds when the incident angle 0° ≤ ξ ≤ 65.7°, but fails when 65.7° < ξ ≤ 82.8° (the maximum angle tested). Another ambient-light hypothesis was also tested in the same laboratory using 20 young subjects to verify the negligible influence (less than 9%) of ambient light on legibility of text. The new equation was accordingly improved to help ergonomists and other professionals create and maintain legible displays.