Interest in a clinical application of the FPL procedure led Teller and her colleagues to develop the Teller Acuity Cards, which are now used in eye clinics around the world to measure the visual acuity of infants and young children as well as of non-verbal older individuals. On one side of each card is a pattern of black and white stripes (square wave grating). The remainder of the card consists of a uniform gray matched to the average light level of the stripes. Each card contains a different stripe width. The tester observes the infant through a peephole in the center of the card and attempts to determine the side that contains the stripes, based on the infant's looking behavior. The smallest stripe width that elicits a reliable judgment of stripe location by the observer provides a measure of the visual acuity of the infant or young child.
