The refractive index homogeneity of highly homogeneous glass is determined using a Fizeau interferometer. Using wavefront deformation of a laser beam with a 632.8 nm wavelength, the relative change of refractive index in a test sample is measured at dual flows through the sample.

In this regard, SCHOTT uses a Carl Zeiss interferometer, type Direct 100 with apertures from Ø = 300 mm and Ø = 500 mm, which permits detection of wavefront deviations of < 10 nm and refractive index homogeneities of < 5 · 10–7. Measurements can be carried out on blocks and round plates with diameters of up to 1500 mm and a thickness of 300 mm. Stitching software enables aggregation of individual measurements on glass pieces larger than the interferometer’s aperture into a homogeneous image.

The required optical homogeneity is derived from the requirements of the subsequent application and the dimensions of the optical component. The homogeneity distribution in the optical glass can be accounted for using Zernike analysis. The optical homogeneity generally corresponds to the maximum refractive index variation within the desired testing aperture. On request, the gradient of the homogeneity distribution can be evaluated in terms of refractive index variation per cm aperture.

SCHOTT has a large stock of selected annealed cut blanks with high homogeneities.