Optical Perspectives Group LLC has developed the CaliBall to calibrate phase-measuring interferometers. The 25.4-mm-diameter silicon nitride ball and its kinematic mount determine the errors in the transmission-sphere reference surface. The device is located with its center at the focus of the interferometer transmission sphere and aligned to better than one fringe. For calibration, a measurement is taken, and the ball is picked up, arbitrarily rotated and replaced in its mount. The center of the ball returns to its original position as a result of the kinematic mount. The process is repeated about a dozen times, and then the interferograms are averaged. The average represents the errors in the transmission sphere and interferometer. A calibration can be done in 10 min. It is best used with low f number transmission spheres.