Should a baby need glasses, the prescription can be determined in multiple ways. The most common includes dilating the pupil and analyzing the light reflected from the back of the eye.
Using the Visual Evoked Potential (VEP) test, doctors record brain waves as a child looks at stripes or checks on a TV screen to determine visual acuity.
Another test called Preferential Looking uses Teller Acuity Cards (simple striped cards) to attract the child's attention. In both tests, as the stripes viewed become smaller the child eventually does not respond (with brain waves or by looking at the stripes).
