GB1432893A - Apparatus for evaluating holographic interferograms - Google Patents

Abstract

1432893 Holographic interferometry BBC BROWN BOVERI & CO Ltd 12 Feb 1974 [14 Feb 1973] 06290/74 Heading G1A In the investigation of an object which is deformed from one state to a second state, holographic interferometry, involving phase investigation of an interferogram of the two states, is utilized. In Fig. 1(a), a first hologram H of the object O 1 in its first state is formed using an illumination beam B(wo) of frequency wo and a reference beam R 1 (w 0 ) of the same frequency. The object is deformed (02) and illuminated at a second frequency (w2) by beam B(w 2 ), the scattered radiation and a reference beam R 1 (w 1 ) of a third frequency w 1 , being used to illuminate the hologram H, and form two wavefronts B 1 (w 1 )and B 2 '(w 2 ) which are superimposed to form an interferogram at plane E. The intensity of the image at a given point in the plane E oscillates at the beat frequency of the second and third frequencies w 1 and w 2 and the phase of these oscillations is investigated using optoelectronic detectors F 1 , F 2 (e.g. photodiodes). The former is fixed in space and provides a reference phase; the other is scanned across the interference fringes and is connected to circuitry to count fringes and to interpolate between them so providing information as to the deformation of the object O. In a second method, Fig. 2 (not shown) two exposures are made on the same hologram using same frequency w o for illuminating and reference beams. The object is deformed between the two exposures and the reference beams are spatially different for the two exposures. The developed hologram is reconstructed using reference beams of the same respective spatial characteristics as the first reference beams, but of different frequencies w 1 , w 2 . In a third method, Fig. 3 (not shown), the two holograms of the object are formed while both in position one behind the other, the reference beams being applied sequentially to their respective hologram. In the second and third arrangements, the interfering wavefronts may be produced individually by the use'of only one reference beam R 1 (w 1 ) or R2 (w 2 ) at any one time. The object may be oscillating when in the second state. The reference and illuminating beams may be derived via beam-splitters, mirrors, mechano- or electo-optical modulators etc. from the same laser. Alternatively, a two frequency (Zeerman) laser or two different lasers may be used. Derivative signals may be obtained in arrangements utilizing an array of three or six detectors, coupled to a plurality of phase meters, enabling surface strain characteristics to be investigated, Figs. 4, 5 (not shown).