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.


Simple to Use, Easy to Calculate

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™ comes in two models


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 I One 1 Optical Perspectives Group


The CaliBall™ is used to calibrate interferometer transmission spheres by using the Random Ball Test (RBT). The Random Ball Test was used with some success in the past but the materials that the balls were made from limited the measurement accuracy. The CaliBall™ has overcome the accuracy dilemma since it is made from silicon nitride. The 25mm balls are grade 5 (round to better than 125nm), have a surface finish of ~1.3nm rms, are opaque and 11% reflective. Virtually indestructible, Caliball™ is a practical solution that can be used repeatedly in a calibration environment for years without degradation.


Details of the CaliBall™ I package:

  • A Grade 5, 1” diameter Silicon Nitride ball with a reflectivity of about 11% for use in the Random Ball Test for calibration Transmission Spheres
  • A 3 ball kinematic mount with Silicon Nitride balls for precise, repeatable positioning the CaliBall during the calibration
  • A sturdy, polypropylene carrying and storage case for the CaliBall and mount suitable for use and storage in a clean room
  • A Certificate of Compliance of a random sample of the balls produced in the lot from which the ball was taken showing the typical ball roundness and finish
  • Complete instructions for how to perform the Random Ball Test to any level of precision in a given environment


The CaliBall™ I is particularly well suited for calibrating downward looking interferometers and can also be used with horizontal looking interferometers with transmission spheres as fast as f/0.7. A customer supplied x-y-z stage is required to position the CaliBall™ I at the focus of the transmission sphere to be calibrated.



The 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. 

caliball II 2 two

Details of the CaliBall™ II package:

  • The CaliBall™ II comes with all the same components as CaliBall™ I but it is better suited for use with horizontal looking interferometers that are equipped with a Zygo style 4” bayonet mount with x-y-z adjustments
  • CaliBall™ II does not need a separate x-y-z mount because the bayonet mount performs this function


Simple CaliBall Instructions

CaliBallTM is a 25.4 mm diameter, silicon nitride ball and kinematic mount used to calibrate phase-measuring interferometers by determining the errors in the transmission sphere reference surface.

  • CaliBallTM is a 25.4 mm diameter, silicon nitride ball and kinematic mount used to calibrate phase-measuring interferometers by determining the errors in the transmission sphere reference surface.
  • The CaliBallTM is located with its center at the focus of the interferometer transmission sphere and aligned to better than one fringe.
  • A measurement of the ball is taken, the ball picked up, arbitrarily rotated and replaced in its mount.
  • The center of the ball will return to its original position due to the kinematic mount. Another measurement of the ball is then taken.
  • This process is repeated about a dozen times and the interferograms averaged.
  • The average represents the errors in the transmission sphere and interferometer because the ball is extremely round and the small, uncorrelated imperfections of the ball average to zero.
  • The calibration can be done in as little as 10 minutes and the results are difficult to dispute.


For your demonstration please contact Armstrong Optical (44 (0)1604 654220) or CLICK HERE