Transmission Spheres

As an example, when using an SLR camera, the camera body cannot take a picture without a camera lens attached. Similarly, an interferometer body cannot measure without a transmission sphere or transmission flat. F/# is the ratio between focal length and entrance pupil diameter where different F/#‘s are required to measure different radius optics. F/# can be simply thought of as the ratio between the radius of curvature and visible aperture for reflected wavefront measurements. A “faster” (smaller number) F/# can measure a larger area for a given radius, but the focal length is shortened, limiting the radius of curvatures you can measure with it.

The key to the accuracy of a t-sphere is the final surface of the assembly, known as the reference surface. When light from the interferometer passes through the reference, the resulting wavefront interferes with the surface(s) under test. In general, the reference surface is assumed to be perfect, and all the interference that is measured by the interferometer is assumed to be from the part under test. Torrent Photonics high precision, pitch polished reference surfaces for smooth, low irregularity wavefronts. This level of optical workmanship allows us to offer all our t-spheres (including our F/0.56) at λ/20 quality (at λ = 632.8nm) and most at λ/40 level.