The tear layer in the eye has a volume of approximately 4 to 10 millionths of a liter. Not a very large amount and difficult to measure. One of the standard clinical evaluations used in diagnosing dry eye syndrome is evaluating the size of the tear meniscus. Tear meniscuc refers to the crescent curved shape that a small wedge of tears forms along the lower lid margin. With the addition of fluorescein dye to the tear film of the eye, the tear meniscus becomes much more visible under magnification. As a routine part of dry eye evaluation this is a useful but very subjective test. The width of the teat meniscus is normally estimated and not measured. Perhaps the most useful observations are when the tear wedge is extremely thin, or when it narrows out to a very slender pattern as it moves away from the center of the lower eyelid. Some research has been done utilizing optical coherence tomography, a method of appling a low intenstiy light to form a 3D image of biological tissues. This may be a great tool for monitoring improvements of dry eye syndrome with future treatments. In the picture below you can see the bright yellow strip of dye picked up in the tear wedge on the lower and upper eyelid. In the dry climate of Northern Colorado our optometric examinations frequently find eye issues are related to dry eyes.
Tear Film Testing For Dry Eyes