It has been claimed in France, in support of the use of yellow headlights, that visual acuity (the ability to see fine detail) in the light from filament lamps is increased by about 8% by passing the light through a selective yellow filter in spite of a 15% loss of total light. The claim has been tested at the laboratory and a loss of about 2 and one- half percent in visual acuity obtained. When the light lost in the filter was made up by increasing the power of the lamp, visual acuity was found to be about 3% higher in the yellow light than in white light. In the laboratory's experiments the age of the observer did not appear to affect the result. It is concluded that if white headlight bulbs (or lenses) were changed to yellow there would be very little change in visual acuity (between a loss of 1% and a gain of 1%) at equal electrical power, but a small gain in visual acuity (of about 3%) at equal luminous intensity.
Abstract