Reflectometry of the precorneal film

In order to gain information about the thickness of the oily layer of the precorneal film, a clinical method of measuring the reflectivity of the precorneal film was developed. The method entails the use of a slit-lamp photometer to measure the reflectivity at 2 selected wavelengths, 500 and 700 nm, at 20 degrees incidence. Based on a physical model of the oily layer acting as a thin dielectric film, a theory is given on the interpretation of the results in terms of the thickness of the oily layer. In 10 normal subjects a mean reflectivity (+/- SD) of 4.06% (+/- 0.83) and 3.16% (+/- 0.79) was found at 500 and 700 nm, respectively. A significant positive correlation between the measurements at the 2 wavelengths was found. It is shown, that these results are consistent with a thickness of about 40 nm, as being the most probable thickness of the oily layer. This value seems to be in accordance with other experimental data. Objective reflectometry may provide an important new tool for the study of the anteriormost part of the ocular surface.