A simple formula for calculating the harmonicaverage oxygen transmissibility of an optically powered RGP contact lens

Abstract

The harmonic average oxygen transmissibility is a useful predictor of the performance of a rigid gas permeable(RGP) lens. Translation of the lens at each blink mixes the postlens tear film. The amount of oxygen diffusing through different parts of the lens must therefore be averaged to arrive at the amount of oxygen available to the cornea by the diffusion process. When the contact lens has optical power outside the range -3D to +3D, an average process must be applied to the annular zones of different thickness. Campbell's formula for point-to-point lens thickness can be manipulated by the methods of calculus to arrive at a simple formula that gives harmonic average oxygen transmissibility. The formula requires only centre thickness, diameter of the optical zone, optical power, and refractive index. These quantities are used to generate a table of harmonic average oxygen transmissibilities for a variety of RGP lenses. The table shows that harmonic average oxygen transmissibility varies less with optical power than does oxygen transmissibility based on centre thickness. The clinical implication is that oxygen supply to the cornea will not be sensitive to the optical power of the lens.