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Multicolor Images with Volume Photopolymer Holograms E. T. Kurtzner and K. A. Haines Both authors were with the Holotron Corporation when this work was done; E. T. Kurtzner is now with Engineering Department, E. I. du Pont de Nemours & Co., Wilmington, Delaware 19898; K. A. Haines is now with Electrical Engi- neering Department, University of Canterbury, Christ Church, New Zealand. Received 12 April 1971. Volume holograms are capable of reconstructing multicolor images from a single noncoherent point source. As early as 1965, Hartmann, Upatnieks, and others reported on various tech- niques 1-3 which were reflection or back-beam configurations (i.e., the reference beam is brought in from the side of the emulsion opposite to the object beam). In practice these holograms suffer from low diffraction efficiencies. However, they have excellent wavelength sensitivity and can reconstruct images which are essentially the same color as the objects. Transmission holo- grams (non-back-beam) require much, thicker emulsions to achieve good color sensitivity, 4,5 as demonstrated by Friesem and Walker in their work with photochromic materials. 6 Ultimately either type of hologram must be quite thick in order to have good resolution in the image. We are reporting here on the construction of transmission vol- ume dielectric holograms in photopolymer materials. 7 These holograms have the capability of diffracting 100% of the incident light into the image. 6 If the recording material is thick enough, complex three-dimensional images with good resolution may be reconstructed in white light. What makes the photopolymer particularly attractive is that it is self-developing and may be used to construct holograms in essentially real time. The un- exposed material is easily handled, having the consistency of Neoprene, and is practically transparent. In practice, we attempt to balance the sensitizer concentration with the photopolymer thickness after the polymer has set up and before it is used. In this way the full thickness of the material may be used at the expense of sensitivity. This results in a material having a sen- sitivity of about 30 mJ/cm 2 , which is a factor of 100 slower than 649F. After exposure, most of the residual dye is bleached in ordinary sunlight to give transparent (transmission of 95%) holograms, which are about as brittle and hard as lucite. No discernible shrinkage was observed during the hologram construc- tion process. Figure 1 shows a holographic image reconstructed in white light of a hologram constructed with the green line of an argon laser. When this image was reconstructed with the argon line, the diffraction efficiency was 45%, a figure which is routinely achieved. The polymer thickness of this particular hologram is approximately 600 μm. Theoretically this results in an image whose wavelength spread is about 30 Å. For an image which is 10 cm from the hologram, the resolution should be about 0.4 mm. The measured resolution in the image of Fig. 1 is less than 0.5 mm, indicating that the full depth of the photopolymer is being utilized. We have found that the best way to construct multicolor holo- grams is to remove any prisms or filters from the laser source to allow the radiation containing all of the frequencies normally occurring in the argon laser to illuminate the object and to im- pinge on the hologram plane as the reference beam. There are approximately eight lines in the argon laser and this forms eight holographic interference patterns which are incoherent with each other. Constructing a hologram in this manner, and reilluminat- ing it with the unfiltered multicolor laser beam, gives multicolor images which retain about 25% of the incident energy. Figure 2 is a black and white photograph of a holographic image contain- ing a multicolored assortment of sugar cubes which was recon- structed in white light from a tungsten high intensity lamp. Good color rendition was retained in the image. Fig. 1. White light reconstruction of a single-color image. Fig. 2. White light reconstruction of a multicolor image. 2194 APPLIED OPTICS / Vol. 10, No. 9 / September 1971
