Apparatus for measuring thickness of layer on substance

Claim 1

A surface layer thickness measuring apparatus comprising a reference light ray source for radiating light rays, a mirror for reflecting the light rays coming from the reference light ray source against the surface of a layer applied to the surface of a substance, an opening or closing means located between said mirror and said reference light ray source to perform the screening operation and the opening operation between them, a plane total reflection mirror located between said mirror and the measured surface layer to be measured to perform the screening operation and the opening operation between them, a light ray sensor for detecting the light ray amount from said total reflection mirror or the light ray amount reflected on said substance without being totally absorbed by said surface layer to be measured, a mode discriminating device for discriminating the combination of said opening or closing means and said plane total reflection mirror, a signal processing device for calculating the signals so that the signals showing only the light ray amount either absorbed by said surface layer to be measured or reflected by a substance without being absorbed by said surface layer may be drawn out from the light ray sensor output, and a surface layer thickness converting device for converting the signals from said signal processing device into the surface layer thickness.