Conspicuity of Aids to Navigation: Temporal Patterns for Flashing Lights

Mariners frequently have trouble picking out lighted aids to navigation in harbors and other areas that have a high density of background lights. The U.S. Coast Guard is seeking ways to enhance the conspicuity, or likelihood of being noticed, of these aids. Literature has shown that a flashing light is more conspicuous than a light that is not flashing. This investigation sought to improve conspicuity by finding the optimal flash characteristics for a light on a background of steady lights. Twenty observers searched for a flashing point source of light among backgrounds of steady lights of various numerosities on a computer controlled CRT screen. They indicated which of the five screen sectors contained the flashing target, and the computer recorded the accuracy and response time. Targets were flashed at the rates of 1, 2 and 3.85 Hz, each at duty cycles of 0.3, 0.5, and 0.8 (proportion of total time on). After a brief practice period, each observer completed 360 trials in a single one-hour session. An analysis of variance (ANOVA) showed significant effects of frequency, duty cycle, and background light density. Search time increased as the number of background lights increased. Conspicuity improved as frequency increased and as duty cycle decreased.