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

摘要

1348978 Photoelectric reflection and tranmission measurements BRUN SENSOR SYSTEMS Inc 5 Nov 1970 [5 Nov 1969] 51147/70 Heading G1A [Also in Division G3] In a dual beam reflection or transmission measurement apparatus for measuring a parameter, e.g. the thickness d, of an infrared transparent film C and in which the film is irradiated alternately by a beam which is strongly absorbed by the film and an unabsorbed reference beam the beams are both divergent so that the interference pattern caused by radiation being reflected at two different sufaces, B 1 and C 1 is substantially the same for each wavelength. To achieve this, the maximum allowable difference ## between the wavelengths is given by where # is the average wavelength. The alternating beams are derived from a single source having a collimating lens system and a rotating filter wheel. The circuitry of the detector 11, which may be a photo-voltaic cell or photoresistor, is synchronized with the motion of the filter wheel to provide on a meter 17 the ratio of the two detected beam intensities. The two alternating beams need not be strictly monochromatic. If each beam consists of a band of wavelengths centred on # and #2 respectively, then to avoid interference errors the minimum wavelength spread of each band is governed by the relationship In this embodiment, the film may be irradiated at a single angle of incidence or over a range of such angles, as previously described. Interference errors in transmission measurements may also be minimised using the method of the invention Fig. 2 (not shown). The output from the analyser circuit 12 may be supplied to a process control unit for controlling the thickness of a film applied to a moving substrate B.