DIRECT MEASUREMENTS USING HOLOGRAPHY Nils Abramson [1] described a hologram as “a window with a memory,” and pointed out that all the measurements that can be made through an ordinary window can also be made through a holographic plate of the same size. Provided that (a) the glass plate is of high optical quality, (b) there is no distortion of the emulsion during or after processing, and (c) the reconstruction geometry is identical with the exposing geometry, the precision of measurement that is theoretically possible is equal to the diraction-limited resolution, which depends only on the wavelength of the laser and the dimensions of the holographic plate. For measurement purposes, the hologram is usually reversed in its holder in the (collimated) reconstruction beam, to produce a real image that is suitable for direct measurement. Tozer and Webster [2] showed that a holographic image formed by a 1 m sq plate could have a resolution of better than 1 μm. (For an 8 × 10 in. plate, the resolution would be somewhat less, around 6-10 μm.)

Kilpatrick and Watson [3] discussed the problems of underwater holography in the context of structural examinations of oil rigs, in particular the aberrations resulting from the need to shoot the hologram underwater but display it in air. ey showed that if the dimensions of the holographic camera faceplate and the air cell in front of the faceplate were correctly chosen, a change from the 694 nm of a ruby pulse exposing laser to the 514 nm of the continuous wave (CW) argon viewing laser would provide almost exact compensation.