Fluorescein is a vegetable dye which can be injected into a vein in your arm and after several seconds, can be seen to circulate within the blood vessels in your eye.

Fluorescein dye first enters the eye through the choroidal (subretinal) blood vessels, the retinal arteries, and finally the retinal veins. The retina has a pigmented layer called the retinal pigment epithelium (RPE). This layer is very important for the fluorescein examination.

Since the blood vessels in the choroid are naturally leaky, fluorescein dye leaks out and (if there was no retinal pigment epithelium) they would appear bright in the light of the retinal camera.

Since pigment blocks fluorescence, and the retinal pigment epithelium lies on top of the choroid, it naturally blocks choroidal fluorescence and allows a clear view of the blood vessels within the retina.

In Wet Macular Degeneration

In wet macular degeneration abnormal "new" blood vessels are found to grow from the choroid call CNV or choroidal neovascularization. These abnormal blood vessels can grow over the retinal pigment epithelium into a position beneath the retina. New blood vessels are like wires with bad insulation. They leak blood, fats serum and fluorescein.
   

Notice the normal round optic nerve on the right, through which the retinal arteries and veins can be seen to enter and exit.
A diamond-shaped plaque of abnormal subretinal neovascularization can be seen in the center - Macula. Because it leaks fluorescein, the neovascularization appears bright.

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