When red lights look yellow

Abstract

PURPOSE. Red signals are typically used to signify danger. This
study was conducted to investigate a situation identified by
train drivers in which red signals appear yellow when viewed
at long distances (900 m) through progressive-addition
lenses.
METHODS. A laboratory study was conducted to investigate the
effects of defocus, target size, ambient illumination, and surround
characteristics on the extent of the color misperception
of train signals by nine visually normal participants. The data
from the laboratory study were validated in a field study by
measuring the amounts of defocus and the distances at which
the misperception of the color of train signals was apparent
and whether these distances varied as a function of time of day.
RESULTS. The laboratory study demonstrated that small red
targets (1 min arc) can appear yellow when viewed through
small amounts of defocus ( 0.75 D) under bright illumination
(1910 cd/m2). In the field study, the defocus needed to
produce the color misperception was similar to that found in
the laboratory study. Time of day affected the color misperception,
and there was no misperception at night.
CONCLUSIONS. The color misperception is not solely associated
with progressive-addition lenses, but occurs in the presence of
small amounts of positive defocus. The potential for the
misperception to result in collisions and fatalities presents a
major safety concern.