discovering dry eye
Measuring Tear Film Thickness:
The King-Smith Method
BY BARBARA FINK, OD, FAAO
Practitioners could better manage their patients' problems associated with dry eye, surface disorders and contact lenses if we could increase our understanding of the thickness and structure of the tear film. However, there is still much controversy concerning the thickness of the tear film (from three to 40 microns), and several alternatives rival the traditional three-layer structure of the tear film.
The King-Smith Method
Recently, King-Smith and colleagues developed a new method to measure the thickness of the tear film, the cornea and its component layers. Using optical interference, this method has several advantages over other methods that measure tear film thickness. It is versatile, non-invasive, accurate, rapid and has a low noise level. It can measure the component layers of the tears and cornea, as well as pre- and post-lens tear thickness.
In an incident monochromatic wave reflected from the front and back of the tear film, reflections that are are in phase produce constructive interference and a relatively large total reflectance. Monochromatic light of a different wavelength is out of phase, causing destructive interference and a relatively small total reflectance. Therefore, you would see oscillations in the reflectance spectrum. The maxima of the oscillations correspond to wavelengths where there is constructive interference, while the minima of the oscillations correspond to wavelengths where there is destructive interference.
Figure 1 shows the percent reflectance versus the wave number (reciprocal of wavelength) and interference effects from four layers of increasing thickness. The lipid layer is so thin that it produces a fraction of an oscillation. Fourier Transform of the spectrum results in peaks that correspond to the depths of the reflecting surfaces behind the air surface.
Use of the King-Smith method has already demonstrated a complete tear film thickness of about 3.0µm. Following a blink, tear thickness increases about 1.3µm, then a rapid decay follows within two seconds as the tear film redistributes while the upper eyelid ascends. This method has also shown that the evaporation rate of tears is about 0.87-1.40µm/min, the thickness of the tears in front of a hydrogel contact lens is about 2.3µm, and the thickness of the tears behind a hydrogel contact lens is about 2.4µm.
Figure 1. Measured reflectance spectrum from tear film and cornea, showing contributions from four interference effects (32-year-old white male).
Dr. Fink is an associate professor at The Ohio State University and a principal investigator for the CLEK clinic.