Luminescence: Emission of light from any source other than high temperature.
Fluorescence: Luminescence that is maintained only by continuous excitation.
Phosphorescence: Luminescence where the emission continues long after the excitation has stopped.
BASIC PRINCIPLES OF FLUORESCEIN ANGIOGRAPHY
- Sodium fluorescein is the fluorescent dye used for performing fundus fluorescein angiography. It absorbs light energy between 465 nm to 490 nm (blue) wavelength and emits light of 520 nm to 530 nm (green) wavelength. Thus blue light is used to excite the sodium fluorescein. It absorbs the light and emits green light.
- A blue filter is placed in front of the flash of the fundus camera. This allows only the blue light to enter the eye.
- Sodium fluorescein dye is injected intravenously. Eighty percent of the dye is bound to plasma protein and is not available for fluorescence; the remaining 20% of unbound fluorescein is responsible for the fluorescence seen during the angiography procedure.
- The blue light from the fundus camera strikes the fluorescein molecules circulating in the retinal and choroidal blood vessels. It is absorbed and reemitted as green light.
- In addition to the green light, part of the blue light which is reflected from the fundus structures leaves the patient's eyes and travels back to the fundus camera.
- The reflected blue light is prevented from striking the film by placing a barrier filter in front of the film. This barrier filter allows only green light to pass through. Thus light that strikes the film is only true emitted fluorescent light.