Color Corrected Optics for Multi-Wavelength White Light Interferometers

Interferometers with short coherence length light sources show white light interference patterns only if the group delays are equal for the different paths in the optical system. When white light interferometry is combined with multi-wavelength interferometry in the same instrument, its optical system requires a special color correction: In addition to the conventional color correction, which reduces the wavelength dependence of the normal aberrations, the balancing of the group delays is necessary. The sum of the group delays of all components in each path may still depend on the wavelength, but it must be the same for the different paths. In the air spaces the group delay is independent of the wavelength, and for visible light the group delay decreases with increasing wavelengths for all usual optical glasses. In the infrared materials with both signs of the dispersion of the group delay are available. With a careful selection of crown and flint glasses the normal color correction and the delay of the group delay can be achieved simultaneously.