Fluorescein angiography is a technique for examining the retina and the blood vessels behind the retina. It uses a fluorescent dye and a specialized camera. The dye is injected into the patient or swallowed by the patient and when it circulates to the eyes, a light to which the dye will react is shined into the eye, and the result is photographed.

The technique has the benefit of not using ionizing radiation. However, the dye can have side effects, from minor such as discoloration of the urine to an allergic reaction.

Fluorescein angiography at Wikipedia