INFRARED TECHNIQUE FOR MEASURING A PARAMETER EG THICKNESS OF A THIN FILM

摘要

1367636 Photo-electric reflection measurements INFRA SYSTEMS Inc 27 Oct 1970 [28 Nov 1969] 50979/70 Heading G1A [Also in Division G3] In a dual beam reflection measurement apparatus for measuring a parameter, e.g. thickness, of an infrared transparent film C and in which the film is irradiated at a given angle O i alternately by a beam which is strongly absorbed by the film and an unabsorbed reference beam, a photo-detector 11 is arranged to receive only the diffusely reflected components R d , reflected at angles other than O i , of the two incident beams to provide a ratio output indicative of the parameter. In this way errors due to interference in the film are removed. Most of the diffusely scattered components derive from the under surface D of the film, but there may be some such scattering from within the substrate B. The alternating beams are provided by a single source and an associated rotating filter wheel 10. A switch 18 operated in synchronism with the filter wheel directs the output of the photo-cell, e.g. a photo-voltaic all or photo-resistor, into the appropriate reference (14) or measuring (15) channel, whose outputs are combined and displayed on a meter 17. The output from the analyzer circuit 16 may also be used to control the thickness d of the film as it is applied to the substrate. For thick polyethylene films, i.e. 0À010 inch 7 d 0À0005 inch, the absorption wavelength is 2À30-2À60 Á, and the reference wavelength 2À25Œ 0À02Á. For thin polyethylene films, i.e. d 0À001 inch, the absorption wavelength is 3À45 Œ 0À02Á, and the reference wavelength either 2À65 Œ 0À02Á or 3À75 Œ 0À02Á.