An ellipsometer, polarimeter and the like system operating in the infrared spectral range (0.75 μm to 1000 μm), utilizing a tunable quantum cascade laser (QCL) source in combination with dithering capability to reduce speckle and standing wave effects, dual-rotating optical elements, a single-point detector, as well as optional means of reducing the size of the probe beam at the measurement surface and optional chopper for lock-in detection.