Interferometric testing of large optics over long path lengths has been hampered by vibration in the test set-up. The precision of phase measuring interferometry has not been able to provide measurements in vibration environments due to the time required to perform the required phase shift between multiple images of the interferogram. The simultaneous phase shift interferometer (SPSI) has eliminated effects of vibration from phase measurements by creating four separate phase shifted interferograms simultaneously, viewed with four CCD cameras. The CCD cameras provide electronic shutter exposure control which effectively 'freezes' the interference patterns producing high contrast interferograms even with severe vibration. Polarization optics are used to maintain the appropriate phase relationships between the four interferograms. Four separate synchronized video digitizers are used to digitize the interferograms to a maximum resolution of 380 by 240 pixels by 8 bits per pixel. The phase at each pixel in the interferogram is calculated by a PC/486 based microcomputer which also provides complete analysis and graphics of the measurement. Averaging of multiple measurements to reduce the effects of air turbulence is done automatically.