Legibility: Back to the Basics 2011-01-0597

The objective for this study was to revisit some of the known factors that affect legibility including font characteristics, as well as, contrast polarity, luminance contrast, and color contrast under high ambient conditions as specified in SAE J1757. The study focused on older drivers due to their increased visual needs and limitations.

The study was conducted in 2 phases: 1) a study of font characteristics; character height, character width, and stroke width using a central composite design. Subjects read a group of letters and numerals displayed on a laptop display using occlusion goggles. The reading time (Total Shutter Open Time or TSOT), reading errors, and a subjective Readability Rating (using a 4 point scale "Very Easy," "Easy," "Difficult," "Very Difficult") were recorded. Licensed drivers in three age groups, 25 to 44 yrs, 45 to 59 yrs, and 61 to 91 yrs participated. The response surfaces were generated and compared to the character sizes recommended in ISO 15008. Results showed that a wide range of characters heights can be legible as long as character width and stroke width were carefully chosen.

The second study investigated legibility under SAE J1757 defined daylight conditions, Overcast and Direct sunlight. The study was run in the daylight simulation lab at Ford Motor Co. capable of simulating near full daylight illuminance levels, both diffuse and direct sunlight. 32 combinations of character and background colors were tested by varying color contrast, luminance contrast, and contrast polarity. 15 younger (≺ 60yrs) and 15 older (≥ 60yrs) drivers participated. Participants read a group a letters and numerals from each color and luminance combination under each ambient condition while reading time and reading errors were recorded along with a subjective readability rating (Unacceptable, Minimally acceptable, Preferred). The subjective data indicated that the contrast ratios participants considered minimally acceptable were close to those listed as minimally acceptable in ISO15008. The ISO15008 minimum acceptable contrast ratios resulted in reading accuracies of 98% (Overcast) and 94% (Direct Sunlight). Color difference was found to play a minor role in legibility under daylight ambient conditions.