Patent US 5,483,079

A position detecting apparatus detects a matching state of a position of a surface of a light-transmissive object with respect to a fiducial object plane of an objective optical system. Light transmitted through one-half of a pupil plane of the objective optical system is reflected by a front surface of the object through an opposite half of the pupil plane to a photoelectric detector. Light reflected from a rear surface of the object through the opposite half of the pupil plane is intercepted and prevented from reaching the detector.