Optical system for measurement

An optical system for measurement, wherein first and second optical paths intersected with each other at an intersection on a sample holder are set, wherein the first and second optical paths are formed so that light from a light source is projected so as to be converged on the intersection from an incoming side beam switching mirror selectively switches a direction of the light, via one of first and second converged light reflectors,

• wherein first and second received light reflectors that projects the light to an exiting side beam switching mirror are disposed on the first and second optical paths respectively, and the exiting side beam switching mirror is capable of switching a direction of the light projected from one of the first and second received light reflectors so that the light is projected toward a detector, and
• wherein intensity of light from the sample in case of face side incidence and back side incidence to the sample can be measured therein.