Optical measuring and testing apparatus incorporates a holographically recorded, single-frequency, optically thin phase grating. When this phase grating is illuminated by a quasi-monochromatic, spatially coherent light source, it acts as a basic common path interferometer and constitutes a highly efficient...http://www.google.com.au/patents/US4541697?utm_source=gb-gplus-sharePatent US4541697 - Ophthalmic testing devices
