Holographic interferometry is used to study fluid flow phenomena. Gradients in the fluid can be caused by temperature variations, concentration variations, or a combination or the two. Holography with one wavelength cannot distinguish between the two types of variations. Holography with two wavelengths can be used to determine gradients caused by each of the two types of variations. This is done by comparing the two images that are obtained from the two wavelengths. The difference in fringe spacing from the images can be manipulated mathematically to determine whether the gradients were caused by concentration or temperature variations. At present, the two images are obtained by reconstructing the hologram with the first wavelength and taking a photograph, then reconstructing with the second wavelength to obtain a second photograph. Both images are recorded and overlapped simultaneously on the holographic film. Since the difference in fringe spacing in some cases can be very small, great care must be exercised in obtaining and recording the individual photographs.
U.S. Pat. No. 3,860,346 discloses the use of adjustable mirrors in real-time holographic interferometry to change the position of the reference beam relative to the object beam.
U.S. Pat. No. 3,802,758 discloses a holographic system wherein a dual hologram holder is used to adjust the position of one hologram relative to the other. U.S. Pat. No. 3,758,187 discloses a holographic system wherein a rotating polygon mirror is used to adjust the position of a reference beam. U.S. Pat. No. 4,597,630 discloses the use of a dove prism to split an object beam into two beams in a holography system. U.S. Pat. No. 4,428,675 discloses a double-exposure system.
However, the prior art does not disclose the expedient of a prism system to modify the positions of a pair of object beams in a dual-beam holographic interferometry system. This causes the images on the hologram during reconstruction. Then during reconstruction the two images will be displaced enough so that one large format photograph may be taken that will contain both images.