Application of interferometry for evaluation of the effect of contact lens material on tear film quality

The lateral shearing interferometer was applied to in vivo investigate the stability of the tear film surface covering the contact lenses. The 8 mW HeNe laser was used as the light source. The sequences of interferograms were recorded by CCD camera in real time during the inter-blink intervals at 25 fps, stored in a computer memory and numerically processed. Every frame illustrates the pattern of interference fringes that corresponds to temporal stage of prelens tear film surface. Fast Fourier Transform was used to quantitative evaluate tear film surface irregularities and the numerical measure M2 was used to obtain quantitative description of the tear film smoothness. The M2 index is the lowest for the smooth and regular surface of the tear film and its values increase if the prelens tear film begins to be unstable. The proposed way of analysis of each interferogram gives opportunity to calculate the credibility of given M2 index and automatically reject a part of a frame, that is not covered by interference fringes with an appropriate contrast due to eye movement. The tear film smoothness has been measured on different types of soft contact lenses of 4 companies. The significance differences in tear film stability have been noticed between low and high water content materials of contact lenses. The tear film was less stable on low water content materials.