A versatile interferometer has been designed and built to evaluate infrared windows as large as 51 cm in diameter. Transmitted wavefront measurements may be made with an accuracy of 0.05λ peak-to-valley at 10.6 µm. Variations in refractive index of transmitting materials as small as 5 x 10-6 may be measured and mapped. Through the use of an auxiliary mirror, systems or components requiring a spherical wavefront at f/6 or slower may also be tested.
Albert F. Slomba,
"Interferometric Measurement Of Large Aperture Infrared Windows", Proc. SPIE 0153, Advances in Optical Metrology I, (15 December 1978); doi: 10.1117/12.938217; https://doi.org/10.1117/12.938217