Accuracy of optical radius measurements

The widespread availability of interferometers in optical workshops leads to an increasing use of these instruments for radius measurements of spherical surfaces. The interferometer must be equipped with a lens system for spherical surface figure testing. The emitted light is focused under interferometer control first to the center of the test surface and then to its surface vertex. The distance between both positions is the radius of the sphere. For a similar method the interferometer is replaced by an auto-collimator. The influence of several aberrations and alignment tolerances of the test set-up on the radius accuracy is analysed. A central item is the discussion of the spherical aberration of the focusing lens and its de-focusing. Both errors lead to an overestimation of small radii for convex and concave surfaces.