Interest in a clinical application of the FPL procedure led Teller and her colleagues to develop the Teller Acuity Cards,[13] 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.[2] 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.