Apparatus for measuring a gradient of a surface

Apparatus for determining the gradient at different points on the surface of an optical lens comprising, an NC table having a deck for supporting said lens, said deck being mounted for movement in one plane, a drive and controller therefor for moving said deck of said table in said plane, a light detector, a recorder having a deck upon which said light detector is mounted, said recorder having means to record the position of said recorder deck in X and Y coordinates, means for producing a beam of light and directing it normal to said deck toward a point on a surface of said corrective lens whose gradient is to be measured, a half mirror mounted at 45 degrees to the path of said light beam and said beam passing therethrough, a second mirror mounted at 90 degrees to said half mirror and laterally offset therefrom, said beam being reflected from said surface of said lens back to said half mirror where it is deflected to said second mirror and then again deflected to form a spot on said light detector, said recorder including means for moving said light detector until the center of said light detector coincides with the center of the deflected beam, said recorder including means to record the X and Y distances which said recorder deck was required to move from reference X and Y positions to bring the center of said light detector into coincidence with the center of said light spot falling on said light detector and including means for generating .DELTA.X and .DELTA.Y signals, a value .DELTA.l obtained from which signals according to the equation:

• .DELTA.l.sup.2 =X.sup.2 +Y.sup.2 is proportional to the slopes of the surface of said corrective lens at said selected point in the X and Y directions and a drive controller for causing movement of said deck of said NC table to cause said light beam to strike said lens at a different point on the surface thereof, so that the slope at said different point can be obtained upon movement of said recorder deck to a position where the center of said light spot coincides with said center of said light detector to obtain .DELTA.X and .DELTA.Y signals for said different point from which the gradient .theta. can be determined from the equation .DELTA.l =L tan 2.theta., where L is the distance which said light beam travels from said lens to said light detector.