PURPOSE: To measure a beam of monochromatic light having a plurality of wavelengths in one operation by a method wherein a plurality of monochromatic filters are arranged at the rear of a rotary analyzer on the same plane perpendicular to a polarization optical axis and a beam of light which has been transmitted through the filters is photodetected individually by a two-dimensional photodetector.

CONSTITUTION: A beam of light from a white light source 10 is made parallel by a lens 11. It is changed to a beam of linearly polarized light by a polarizer 13. The beam of light is reflected by a sample 14 and changed to a beam of elliptically polarized light. The beam of light is incident on a rotary analyzer 14. The intensity of a beam of light which has been transmitted through an analyzer 15 is expressed by a digital signal of I(θ)=I0 (1 + cos 2θ cos 2θ + sin β cos Δ sin 2θ). The beam of light is divided into a plurality of beams of monochromatic transmitted light by a plurality of monochromatic filters 2 in an interference filter 1, and the intensity of each beam of light is weakened. However, only I0 is made small, and the relationship of (, Δ) is not changed. While the anlyzer 15 is turned once, a two-dimensional photodetector 3 convertes each beam of monochromatic transmitted light into a digital signal. An electronic computer Fourier-transforms I regarding θ for every beam of monochromatic light, finds (, Δ) and specifies the film thickness and the refractive index of the sample on the basis of it.