Lensmeter with correction for refractive index and spherical aberration

Abstract

In an automated lensmeter, provision is made for estimating dispersion when high index glasses are utilized with lenses having relatively large diopter prescriptions and for eliminating error due to spherical aberration when determining the power of contact lenses. For estimation of dispersion--typically required for optical corrections in the range of 10 diopters--two discrete color light sources are used for optical measurement of the power of the suspect lens. As a result of the obtained power difference resulting from the differing colors used, dispersion can either be estimated and referenced back to a generally accepted standard or a look-up table can be utilized to identify the specific glass class or even specific glass type being measured. In the case of the measurement of power of contact lenses, differing points of measurement through the suspect contact lenses are made. Effects of possible dispersion are ignored and differences in lens power determined with different radii of measurement. The different lens powers which are a function of radii of measurement are ascribed to spherical aberration. Once the value of spherical aberration is known, it is subtracted from the total determined prescription with the end effect that the true paraxial power of the contact lenses is determined. Provision is made for the incorporation of both the dispersion correction feature and the contact lens measuring protocol into the same instrument.