Two-dimensional acceptance angles of a proustite upconverter operating in several type I phase-match conditions are measured and compared with theoretical ones. They agree reasonably well with each other.

Temperature coefficient of optical path length of solid with plane parallel end-faces is discussed. The athermal condition is generalized for oblique incidence. Useful conditions are derived in the case when vacuum surrounds the solid and in the case when gas surrounds it for reducing temperature dependence.

Periodic variations of sum-frequency output are observed in upconversion of 10.6 µm radiation from a CO2 laser in a proustite crystal pumped by a Nd:YAG laser beam and concluded to be caused by multiple reflection of infrared wave which occurs inside the nonlinear crystal.

A technique is described to compensate the difference between lateral magnifications in ellipso-interferometry with tilted image plane by using an optical fiber bundle having an obliquely cut and polished end. A major limitation of this technique also presented.