PURPOSE: To provide a space filter and space filter type length measuring gage having simple constitution featuring deep optical depth and improved length measuring performance without using an imaging lens.
CONSTITUTION: This space filter 5a has the regions delineated by adjacent sine curves 5a1 which have a phase difference of 180° with each other in plan view and in which the peak part of the one curve and the valley part of the other curve are in contact or proximity with or to each other. These regions are alternately formed with the regions where light is reflected and the regions where the light is transmitted. The space filter 5a described above is used for the space filter type length measuring gage which measures the length of a moving object by allowing transmission and reflection of part of the reflected light from this moving object.
MARUYAMA YOSHIAKI
FUJITA AKIHISA
