This is a yellow dye used during an eye exam. This dye is an organic pigment similar to the pigments used in a highlighter … at least in the sense that it glows under a blue or “black” light. This glowing property is useful for checking eye pressure during applanation tonometry. Fluorescein is also useful for detecting small scratches on the cornea (see corneal abrasion) or foreign bodies that are otherwise hard to see. These irritants may not be obvious to the doctor at first glance, but they glow brightly under the microscope with the help of fluorescein dye. Fluorescein is used for many applications outside of ophthalmology. For example, the yellow dye is used in bubble levels attached to work tools.