PURPOSE: To correctly detect the position of a ridge when a roof surface is to be measured, by inserting a shielding plate into an entering optical path of the illuminating light to the roof surface which crosses the ridge of the roof surface slantwise.
CONSTITUTION: The luminous flux from an illuminating light source 11 is expanded by a beam expander 12, reflected by a half mirror 13 and then partially reflected at a surface 14a to become a reference wave. The remaining light is reflected to be a to-be-detected wave at a roof surface 30 to be measured. The interference fringes formed when these wavefronts interfere with each other are detected by a photodetecting element 15. When a ridge 31 is close to a line and the roof surface 30 is little curved, it is difficult to detect the position of the ridge 31 from the interference fringes. Therefore, a shielding plate is inserted into an optical path between the roof surface 30 and a standard 14 in a manner to cross the ridge 31 of the roof surface 30 set in an interferometer 10 slantwise. Since the luminous flux entering the roof surface 30 is returned after being reflected twice in total at two surfaces, the shadow of the shielding plate appears symmetrically to the ridge 31. Accordingly, a bisector of an acute angle defined by the shadows indicates the position of the ridge 31.
MATSUO HIROBUMI
ONO MASAHIRO
NOGUCHI MASATO
