CaliBall, a calibration artifact and mount for use in the Random Ball Test

Interferometric optical test precision is primarily limited by the interferometer transmission sphere quality. The CaliBall is a means of in-house transmission sphere calibration using the Random Ball Test. The CaliBall is designed so that it can be used immediately before or after a critical test of an optic without changing anything in the test setup other than inserting or removing the CaliBall itself.

Transmission spheres are factory calibrated, but shipping, handling, thermal and other environmental effects degrade the factory calibration precision. The CaliBall eliminates these uncertainties by calibrating the transmission sphere in the test set up, an assurance that the interferometric test results are precise.


The CaliBall uses averaging of random patches of the CaliBall to calibrate transmission spheres exactly the same way surface roughness interferometers are calibrated by averaging random patches of a plane surface.


CaliBall I is for either horizontal or vertical interferometer set ups but it requires an x-y-z stage to position the CaliBall center at the transmission sphere focus.

CaliBall II is used in horizontal interferometer set ups and is held in a Zygo type 4” bayonet mount that provides the adjustments to align the CaliBall II to the transmission sphere.