DISPLACEMENT MEASUREMENT DEVICE OF OPTICAL TYPE

PURPOSE:To make only one condenser means and one displacement detector suffice by receiving the light coming back from a surface to be measured by changing the planes of polarization by turns by means of a polarization plane conversion means. CONSTITUTION:A half-wave plate 30 is driven with a motor 37 to rotate its plane of polarization selectively to switch by turns between P polarized light and S polarized light of the light transmitting through the half-wave plate 30. Synchronous with the switching, the outputs of a subtraction device 35 for the P and S polarized light are stored in a memory 36 under the control of a synchronous circuit 38. The output of the subtraction device 35 for the P polarized light and the output of the subtraction device 35 for the S polarized light stored in the memory 36 are subtracted by means of the subtraction means 39. Because unlike conventional devices, two sets of condenser lenses and two- part displacement detector are not used but the displacement of a surface 28 to be measured can be measured likewise with one condenser lens 33 and a two-part displacement detector 34, the number of components can be reduced.