A high precision reflectometer is made so that even a slight change in the reflectivity spectrum can be detected. The reflectometer consists of two components; the optical and the electronics parts. The main optical part is a rotating disk equipped with two mirrors. The two mirrors, which rotate with the disk, chop the light beam so that the detector can pick up the direct (I0) and reflected (RI0) beams one after the other. The dual boxcar circuit selectively integrates the signal pulses, I0 and RI0, by the use of a gate circuit. A dividing circuit gives the ratio RI0/I0 of the two signals from the boxcar and the reflectivity (R). An example of the reflectivity measurements for n-type germanium is shown.