The thickness of the human precorneal tear film: Evidence from reflection spectra

PURPOSE. Interferometric methods have considerable potential for studying t he thickness of layers of the human tear film and cornea because of their a bility to make noninvasive, accurate, and rapid measurements. However, prev ious interferometric studies by Prydal and Danjo yielded tear thickness val ues near 40 and 11 mu m, respectively, considerably greater than estimates made by invasive methods of 4 to 8 mu m. Using a modified version of Danjo' s method, interference effects from the tear film and cornea were studied, with the aim of correlation with known structure and optical properties of the cornea and hence determining the most probable value of tear film thick ness. METHODS. Reflectance spectra from the human cornea were measured at normal incidence. These spectra show oscillations whose maxima correspond to const ructive interference between light reflected from the air surface and from some deeper surface. The frequency of these spectral oscillations is propor tional to the thickness of the layer between the air surface and the second surface. Therefore, Fourier analysis of reflectance spectra can be used to determine the thickness of layers of the tear film and cornea. In the main experiment, 36 low-resolution spectra were obtained from six normal eyes f or measuring thickness up to 100 mu m. Control experiments included measure ments of the time course of thickness changes and high-resolution spectra f or measuring thickness up to 1000 mu m RESULTS. For the main experiment, in the thickness range 1 to 100 mu m, the strongest peak in the Fourier transform was near 3 mu m (range, 1.5-4.7 mu m) beneath the air surface. In the range 20 to 100 mu m, the strongest pea k was near 55 mu m (range, 50-59 mu m) for all 36 spectra; none were in Pry dal's range near 40 mu m. This 55-mu m peak is consistent with a reflection from the basement membrane of the epithelium. Time course measurements aft er a blink show that the 3-mu m peak is not an artifact. High-resolution sp ectra gave a peak near 510 mu m, corresponding to the complete thickness of the cornea (plus tear film). This peak had a contrast similar to that of t he 3-mu m peak. CONCLUSIONS. These studies did not confirm Prydal's estimate of approximate ly 40 mu m. Nor were there prominent peaks near Danjo's value of approximat ely 11 mu m, except in cases of probable reflex tears. Because the reflecti on at the aqueous-mucus boundary would be expected to be weaker than that f rom the epithelial surface, the 3-mu m peak is unlikely to correspond to th e aqueous layer (rather than the complete tear film). The proposal that the 3-mu m peak corresponds to a reflection from the front of the cornea is su pported by the demonstration of a peak of similar contrast from the back of the cornea. Thus, the current evidence consistently supports a value of ap proximately 3 mu m for the thickness of the human precorneal tear film.