Fluorescein angiography (FA) is an imaging modality used to study the circulation of the retina and choroid. About 5 mL of 10% sterile aqueous solution of fluorescein dye (or 2-3 mL of a 25% solution) is injected in the antecubital vein. The fluorescein molecules excited by a blue light (465-490 nm) emit green-yellow light (520-530 nm) (Fig. 2.1). The reflected blue light and green-yellow light are passed through a barrier filter that allows only the green-yellow fluorescent light to pass through and blocks the blue reflected light.