Hydroxyxanthine dyes such as fluorescein have been used for decades to aid assessment of the ocular surface, especially the cornea. Fluorescein is available topically in eye drops or impregnated strips. This water soluble dye readily enters and stains the corneal stroma in areas where the epithelium is absent (or even when the epithelial cells have loose intercellular junctions). Fluorescein is, therefore, extremely useful for detecting areas of epithelial deficiency that occur, for example, in corneal abrasions and herpes simplex keratitis. Although fluorescein staining can be seen using a white light, it is best detected with cobalt-blue filter illumination which causes the dye to fluoresce.