An optical radiation processing unit directs different wavelengths of the optical radiation emitted by an optical source to an object being measured from a direction that differs from the normal of a surface being measured so that the different wavelengths focus on different heights in the direction of the normal of the surface. A possible polarizer polarizes the reflected radiation in a direction perpendicular to the normal of the surface. The optical radiation processing unit directs to a detector polarized optical radiation that received from the object. The signal processing unit determines on the basis of a signal provided by the detector from the detected radiation the wavelength on which radiation is the highest, and determines the location of the surface by the determined wavelength. When measuring an object from both sides, the thickness of the object being measured is determinable using the locations of the surfaces.