Tower Optical Corp. (Boynton Beach, FL) has released a Transmission Flat, TxF, that is used in interferometers to verify the surface flatness of plano elements such as mirrors, windows, and prism surfaces. The transmission flat provides a reference wavefront which is compared to the returning, reflected light from the test object. The transmission flat has a surface with a reflectivity of 4% and another surface of 0.25%. This enables them to measure surfaces with 0.1% to 40% reflectivity. The 4-inch TxF mounts directly into the bayonet mounts of most interferometers.