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
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
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.
Ford Motor Company, Ford Motor Coompany
SAE 2011 World Congress & Exhibition
SAE International Journal of Passenger Cars - Mechanical Systems-V120-6, SAE International Journal of Passenger Cars - Mechanical Systems-V120-6EJ, Automotive Lighting Technology and Human Factors in Driver Vision and Lighting, 2011-SP-2300