Precise Evaluation of Spectacle Lenses
With the aid of an instrument developed by the National Bureau of Standards, it is now possible to measure both central and marginal powers of a spectacle lens with accuracy.
This instrument, known as the Spectacle Lens Tester, was designed by Dr. F. E. Washer of the NBS staff in connection with a study1 of eyeglass lenses for the Veterans Administration.
In operation it simulates the refractive effect of the various parts of the lens under study upon the human eye. Although the Spectacle Tester is basically a research tool, its essential principles have been utilized to design marginal-power attachments2 for the standard vertex-power instrument used by practicing opticians.
The earliest spectacle lenses had spherical surfaces only. However, when the existence of astigmatism was established, it was found that a combination of spherical and cylindrical surfaces improved vision, particularly in the axial region. Thus lens prescriptions specify a correction in terms of both spherical and cylindrical power measured in diopters.
This power is what the layman refers to when he speaks of the "strength" of his glasses.
The problem of achieving simultaneous accurate correction at all points on a lens is complex.
When only spherical power is involved, it is possible to determine the proper curvature of front and rear surfaces to yield zero marginal astigmatism for a specified object distance over a definite range. But for lenses having both spherical and cylindrical power, the problem of simultaneous marginal and axial correction is more difficult. An almost infinite number of lens shapes is possible. However, in practice a selected series of base curves has been found adequate for visual correction.
Each year the Veterans Administration purchases large quantities of spectacle lenses. To insure that the lenses received provide as excellent correction of vision as practicable, the VA requested the Bureau to develop practical specifications with respect to actual performance of both center and edge and the degree of marginal variation that might reasonably be allowed.
While the accuracy of a spectacle lens can be determined rather easily at the optical center, the determination is much more difficult 1 For further technical details, see "Instrument for Measuring Marginal Power of Spectacle