A significant amount of information about tear quantity can be gained simply from observing the heights of the upper and lower tear menisci with the slit-lamp biomicroscope.
There are two techniques used when measuring the height of tear prisms. The first measures the tear meniscus formed on the lower lid margins to give a useful guide to tear volume. This simple technique employs the slit lamp biomicroscope. Excessive or prolonged use of illumination should be avoided to prevent artificial drying of the tear prism.
The second technique is to compare the tear prism height with the illuminated slit width by setting the slit horizontally in alignment with the lower lid margin, altering the slit width until it appears to match the height of the tear prism. Heights of less than 0.2mm indicate reduced tear fluid quantity.
Observation of the meniscus profile is also extremely helpful. A regular tear meniscus is typically observed in a healthy eye while a meniscus with a scalloped edge is often associated with a dry eye.