Confocal interferometer - Another approach to the symmetry problem is to use components of spherical symmetry. Interferometers of this type can only produce useful interferometric modulation for a minute range of retardation. The foci of the two reflectors will match sufficiently well only very close to zero path difference. The key is the symmetry of the reflectors. [reproduced from M. Bottema and H. J. Bolle, Aspen Int. Conf. on Fourier Spectrosc., 1970 (G. A. Vanasse, A. T. Stair, and D. J. Baker, eds.), AFCRL-71-0019, p. 211 (1971)]