Patent GB-1380725-A (GB)
Title: Optical gauging systems
IPC: G01J1/44
Filed: 1971-05-10
Abstract: 1380725 Photo-electric gauging apparatus INFRARED ENG Ltd 20 July 1972 [10 May 1971] 14112/71 Heading G1A In an optical gauging apparatus radiation from a source is formed into two beams having different spectral wavelengths both of which are modulated by relatively movable elements having path defining means a plurality of which simultaneously pass parts of each beam, the beams are directed to a sampling region from which the radiation is received by a detector to produce an electrical signal which is synchronously demodulated. As shown Fig.1, radiation from an IR source S is passed through a chopper B and then through one of two interference filters F and G of different spectral wavelengths presented alternately. The resulting beam is reflected from a sample J, received by an elliptical or parabolic mirror and directed on to a detector the signal from which is synchronously demodulated to produce signals corresponding to the two spectral wavelengths. These signals may be combined to produce a measure of a property of the sample e.g. moisture content, thickness, concentration of individual elements, weight per unit area, and thickness of moisture or ink films on a surface. A filter may be placed in front of the detector to reduce interference from background radiation. The chopper comprises two plates each having a large number (e.g. 50 to 150) of alternate transparent and opaque sectors, one of the plates being rotatable relative to the other so that a radiation beam of larger diameter than the widths of the sectors at the point of incidence is alternately passed and blocked as the plate sectors move into and out of alignment. The filter plate W may be rotated together with the movable chopper plate so that a series of pulses at one spectral frequency are followed by a series of pulses at the other spectral frequency. The chopper may alternatively be of the reflective type in which the sectors of the plate remote from the source are alternately absorbing and reflective. The radiation source may be a filament lamp, an exposed heating filament, a Nernst filament or a Globar. A reference radiation beam may be passed through a portion of the chopper not utilized by the main beam to obtain the switching waveform for the synchronous detectors. The chopper plates may be of opaque material on a transparent substance or may be stamped or otherwise formed from a metal or other opaque sheet material, circular strengthing bars being provided if required. In a second embodiment, Figs.3 and 4 (not shown), radiation from the source passes through fixed filters and the resulting beams are chopped at different frequencies by a single device comprising a pair of plates one of which is rotated relating to the other arranged so that one beam passes through a region of outer annular portions of the plates which have larger numbers of alternate transmissive and opaque segments than do the inner portions through a region of which the other beam passes. The sample may be investigated with respect to the light reflected transmitted or backscattered and a diffuser, which may be made of the same material as the sample, may be mounted in front of the receiver.