A new method of measuring lens refractive index

A new clinical method for determining the refractive index of a lens is described. By measuring lens power in air and then immersing the lens in a liquid of known refractive index (n), it is possible to calculate the refractive index of the lens material (micro) by using the formula: micro = (nK (v,1) - K(v,n))/(K (v,1) - K (v,n)) where K (v,1) is the lens power determined in air K (v,n) is the lens power determined in the immersion liquid. The only materials required are a digital lensmeter and a wet cell for holding the lens in a liquid. The theoretical basis of the method is explained and a description given of the limitations. The optimal method of measuring different types of lenses is discussed. Sources of error include the thin lens theory behind the method, the use of a wetcell and the digital lensmeter. The theoretical accuracy of the results is given as 0.02 but 0.01 is usually achieved. In all cases, measuring the front vertex powers (FVP) yields a more accurate estimate of refractive index of a lens than measuring back vertex power (BVP). The author found half the lenses measured attained values within 0.005 of the known material index. This method is usually sufficiently accurate to isolate which lens material has been used in manufacturing and permit manufacturing spectacles that mimic the appearance of an earlier pair. Some suggestions for further refinement are given.