An instrument to measure inhomogeneity of optical materials in the 3 to 5 um as well as in the 8 to 12 pm region is described. It measures the deviation caused by inhomogeneous areas when a light beam passes through. A line spread function is formed with an auxiliary lens, and in its slope a fixed measurement slit is positioned. Deviation caused by inhomogeneities will shift the spread function, and the measurement slit will detect variations in intensity. With this technique the gradient of the refractive index, as well as the variation of the refractive index, can be measured. Typical measurement results are shown.