Compensated diopter gauge

Claim 1

A diopter gauge for measuring the surface power of a curved lens and for compensating for nonlinearities in the relationship of lens sagitta to surface power to provide an accurate reading of the measured power in diopters, comprising: dial means including a dial body, linear scale means on a rotatable face portion, associated indicator needle means, and a nonrotatable face portion, said scale means having uniformly spaced markings numbered from zero and indicating predetermined uniTs of diopter measurement; a base member fixed with respect to said dial body, for contacting the surface of the lens, said base member including a contact ring having an inner cylindrical wall, an outer cylindrical wall and a lens-contacting edge which is circularly convex in radial cross-section through said contact ring; a nonlinear compensating scale also located on said rotatable face portion of said dial means and fixed with respect to said linear scale, said nonlinear scale including a zero mark coincident with zero on said linear scale and a plurality of variably spaced scale divisions consecutively numbered to correspond to diopter values indicated on said linear scale, the location of any of said scale divisions measured from said zero mark being given by the expression: E (0.53/D + OR - r) - ((0.53/D + OR - r)2 - (Y + OR r)2)1/2 - DX1 where E an error value in an uncompensated diopter reading, the error value being represented by the distance of a particular one of said scale divisions measured from said zero mark in the same units as are used for said linear scale but in a negative direction with respect to said linear scale, D a diopter value corresponding to the particular scale division, r the cross-sectional radius, in meters, of said lenscontacting edge, X1 an incremental sagitta value, in meters, corresponding to an incremental diopter value of unity on said linear scale, and Y the radius, in meters, of said inner cylindrical wall, for convex surface measurements, or said outer cylindrical wall, for concave measurements, the value of Y being selected to yield a zero error value, E, for a particular value of D; fixed reference means on said non-rotatable face portion of said dial means; and means for moving said linear and nonlinear scales on said rotatable face portion with respect to said fixed reference means to compensate for the nonlinear relationship between the surface power of said lens and the motion of said plunger means; whereby a compensated reading of lens surface power in diopters is obtained by first obtaining an uncompensated reading on said linear scale with said reference means pointing to said zero mark, then moving said rotatable face portion until the reading of said reference means on said nonlinear scale is equal to the reading of said indicator needle means on said linear scale, and the error value E is thereby subtracted from the uncompensated reading.