Use of the yellow filter on slit-lamp examination
• Fluorescein sodium is not effective for conjunctival staining due to reduced contrast of the fluorescein stain over the conjunctival surface
• The mechanism of this is thought to be multifactorial.
• First, the scattering of blue light from the sclera reduces the contrast between the surface and the background.
• Second, the background fluorescence in the conjunctiva and sclera, and penetration of fluorescein sodium also further reduces the contrast of the fluorescein staining.
• However, these limitations can be overcome by use of a yellow barrier filter.
• Fluorescein sodium absorbs and is excited by light between 465 and 490 nm, and it has an emission peak between 520 and 530 nm.
• The yellow filters, which transmit above 495 nm, can be useful in the detection of conjunctival fluorescein staining, and the combination of a cobalt blue exciter filter and a yellow barrier filter provides a high contrast of fluorescein staining against the back-ground.
Image from Rajan Eye Care Hospital
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