Three interferometric methods for measuring the thickness of layers of thetear film

The thickness of different layers of the tear film has been measured by thr ee types of interference method, namely, wavelength-dependent fringes (WDFs ), thickness-dependent fringes (TDFs), and angle-dependent fringes (ADFs). This review begins with a discussion of characteristics which are common to all these methods-high-, intermediate-, and low-index layers, phase, optic al path difference, and contrast. For each of the three methods, we present a figure showing constructive and destructive interference, derive equatio ns for calculating tear layer thickness, describe a typical optical system, and show representative results. The particular advantages and limitations of each method are discussed. Given the clinical importance of the tear fi lm in dry eye syndrome and contact lens wear, it is unfortunate that there are considerable discrepancies among the results of interference ic and oth er methods for measuring fear film thickness; further development of these noninvasive, interferometric methods should help to provide a clearer pictu re of the thickness of different layers of the tear film, in normal and dry eyes, and in contact lens wear.