Multiple exposure holography of time varying three-dimensional fields.

Most applications of holography have been limited to three-di­ mensional scenes that are either stationary or are stopped by an extremely short time exposure of laser light. Such a hologram gives no direct information about the actual time variations oc­ curring in the scene. Leith et al. extended holography to time motion study by rotation of the hologram for different exposures of the scene. This results in a number of time-separated holo­ grams that can be viewed individually and is one type of exten­ sion of motion picture photography to motion picture holography. Many technical photography applications exist in which motion can be studied more efficiently by the use of multiple exposure of a single plate. The analogous extension to holography also exists in which time-separated images of a particular three-dimensional scene can be superimposed one upon the other by a single multiply exposed hologram. Techniques for accomplishing this have been developed at Arnold Engineering Development Center for flow field visualization and diagnostics. Standard in-line holography of particle fields in motion was performed with a Q-spoiled ruby laser using Kodak SO-243 film. Multiple exposure was accomplished by using a passive Q-switch with a cryptocyanine dye solution adjusted for multiple pulsing. I t is well known that for a given dye concentration in a passive Q-switch, if the excitation energy applied to the flash lamp is in­ creased above that necessary for Q-switching, the Q-switch can continue to open and close as long as the lamp can pump the ruby above its threshold after a pulse. For the system vised, as many as ten pulses could easily be produced. Each pulse is ap­ proximately 15 nsec wide, allowing a stop-action hologram to be constructed. The time separation of the pulses could be con­ trolled reasonably well and ranged from 50 μsec to approximately 250 μsec apart. In the doubly pulsed mode, careful control of conditions resulted in reproducible equal-energy pulses separated by approximately 200 μsec, the time separation also being repro­ ducible. I t is possible, in a straightforward manner, to accom­ plish multiple pulsing by active Q-switching and with much better controllability and reproducibility; however, active Q-switches are not as suitable for holography and are much more expensive. Even so, such a system would probably be desirable in an opera­ tional instrument because of control and ease of operation. The moving object field in one case consisted of a pair of Plexiglas plates separated by 5 cm and mounted in a holder. On one plate, a cross was formed by pressing 100-μ diam indentures into the Plexiglas; on the other plate an X was similarly formed. The plates were removed through the apparatus, and the laser was fired when the plates passed through the correct position. The reconstructed image was magnified by a convex lens and imaged into the field of a camera. An optical dc filter, placed at the focal point of the lens, removes the unmodulated dc portion of the light that usually passes on to the reconstruction plane. This increases image contrast and removes the troublesome secondary