Holographic interferometry with both beams traversing the object.

The purpose of this letter is to report an extension of holo­ graphic interferometry in which both waves used to record the hologram pass through the transparent object being studied. This technique has features which are unique in the sense that they cannot be duplicated by classical interferometry. Basically, the scheme provides for an interferometer of variable sensitivity with a strong bias toward detection of changes of phase in one direction. Figure 1 depicts the optical arrangement for recording the hologram. Two coherent plane waves traverse a phase object at slightly different angles. The resulting intensity pattern is recorded on film to form a hologram. No additional reference wave is used during this exposure. We will show below that if this hologram is reconstructed by one of the plane waves, the fringe pattern obtained by interfering this reconstructed wave with the other plane wave is indicative of the difference in optical path of the two waves which traversed the object. This tech­ nique is suggested by the method of holographic imaging through aberrating media developed by Goodman and co-workers.They demonstrated that by placing a coherent point source very close to an object and recording a hologram, aberrations caused by refracting media between the object and the hologram plane are minimized since both the object and reference waves are aberrated in approximately the same manner. We have ex­ ploited this concept by applying it to interferometry. Let the two waves striking the hologram in Fig. 1 be denoted by