5.1.1 Signal perception. The legend on light signals,
when energized, shall be readable in a 10,000 fc environment with the source
illumination striking the face of the display at an angle of incidence of 90
degrees. When not energized, the legend shall not be perceptible in the
presence of high ambient illumination.
18.104.22.168 Light signal luminance. The luminance level of
dedicated transilluminated signals and legends shall be measured by
averaging the readings within small sub-areas of the signal or legend.
Readings on an illuminated letter which is a portion of a word or legend,
may be averaged if a minimum of three readings taken on separate locations
on each letter make up the average luminance. Warning light signal utilizing
opaque letters and a translucent background may be divided into a minimum of
five approximately equal subareas for determination of luminance compliance.
Each subarea (containing at least three separate readings, which may be
averaged) shall individually meet the luminance requirements. All legends
shall be capable of being read in bright sunlight. The average contrast
ratio of any lighted legend to the background shall be 2.0 minimum when the
display is subjected to ambient illumination of 10,000 fc at 5,000 ± 500
degrees Kelvin color temperature. The contrast ratio of any unlighted legend
to the background shall be less than 0.1 for any legends, which are slightly