GB1382523A - Holographic method and apparatus - Google Patents

Abstract

1382523 Testing tyres K VOCKENHUBER and R HAUSER 15 March 1972 12130/72 Heading B7C [Also in Divisions G1 H4 and G2] Changes in the surface structure of an object or differences between two superficially similar objects are investigated by a holographic technique which involves storing a first holographic image pattern, representing a first object or the test object before the change occurs, producing a second similar but not identical pattern, representing a second object or the test object after the change has occurred, effectively superimposing the second on the first (stored) pattern, and scanning and detecting the superposed patterns to develop modulated electrical signals which are analysed so that differences in the patterns may be displayed at different brightness levels as a television picture. By "effectively superimposing" is meant either physically superimposing the second image on the first image stored by a television camera screen or scanning the two images at different times, the video signal obtained from the first scan being stored on video tape and then compared with a video signal generated during the second scan. As shown, light from laser 1 is formed into reference beam 5 and object beam 4 by beam splitting partial reflector 3, the beams having optical system 14, 6. Light reflected from the object 7 is passed through optical system 9 which has an adjustable stop 10. The combined beams are formed as a hologram in the plane of the screen of television camera 12. The arrangement is such that the reference beam appears to come from the centre of optical system 9, and since aperture 10 is adjustable, the spatial frequencies in the hologram can be made to correspond with the camera resolving power. The hologram of the original object 7 is stored by the screen of the camera which is, e.g. a Vidicon. The change in the object occurs or is effected or the object replaced and the resultant hologram superposed on the stored pattern. The video output is analysed by one of three methods, the spatial intensity modulations in the pattern being represented in the video signal by time-base modulations. The signal is analysed and displayed on a TV monitor 15 with (a) less modulated image portions appearing dark and highly modulated bright, by rendering only intensity minima as bright displayed images; (b) the inverse of (a); and, preferably, (c) use of a possibly frequency selective, non-linear amplifier, to differentiate between object characteristics and the general granulation in the beam, and direct conversion of differences in modulation depth into brightness differences. A typical application is examination of a tyre which is inflated resulting in a changed hologram.