Interferometric inspection apparatus
1,024,598. Selective signalling. E. I. DU PONT DE NEMOURS & CO. Aug. 27, 1964 [Aug. 27, 1963], No. 35235/64. Heading G4H. [Also in Divisions G1 and G2] To determine the thickness of a travelling film 10<SP>1</SP> two beams of light reflected from the top and bottom faces of the film are made to constructively interfere by shortening the optical path of one beam by means of a reciprocating mirror 30<SP>1</SP>, the position of the mirror, and hence the instant in its cycle, at which interference takes place indicates the film thickness. The two beams are split by a 45-degree half-silvered mirror 32<SP>1</SP> on to the fixed mirror 311 and moving mirror 30<SP>1</SP>. At the position of the mirror 30<SP>1</SP> which compensates for the additional optical path in the film, light of one beam from the fixed mirror and light of the other beam from the moving mirror constructively interfere and initiate a pulse in the detecting circuit. The instant at which this occurs gives a measure of the thickness of the film. The timing of this interference is based upon a fixed reference pulse created during each cycle of the mirror when light from each beam individually constructively interferes. In the arrangement shown the moving mirror is offset so that the reference interference is not produced. The single pulse per cycle is superimposed upon an oscillator 70 input to a bistable multivibrator 79 and is centred upon it for the nominal thickness of the film, any time deviation by the pulse from the oscillator input indicates a change from the nominal thickness. In another arrangement (Fig. 6A, not shown) the reference pulse and the varying pulse are used to start and stop a clock-driven counter to give a measure of the thickness of the film. Instead of using a clock the counter pulses may be provided by using a source of monochromatic light to give a large number of constructive interferences per cycle of the mirror. In an alternative inferometer (Fig. 5, not shown) the additional optical path is changed by angularly moving the mirror. The main source of light delivers infra-red radiation for a lead sulphide photoresistor. (Refractive index and bi-refringence.may also be measured.)