White light holographic portraits (still or animated) Nicole Aebischer and Bernard Carquille

Université de Franche-Comté, Faculté des Sciences et des Techniques, Laboratoire de Physique Generate et Opti-que, associé au CNRS, 25030 Besançon cedex, France. Received 29 August 1978. 0003-6935/78/1201-3698$0.50/0. © 1978 Optical Society of America. The technique presented in this Letter deals with real size

3-D portraits. The reconstructed image of the face can be observed partly in front of the holographic plate (20 cm X 25 cm), partly behind it when illuminated in white light coming from an ordinary projector. The average color is that of natural skin. The illuminating angle is about 45°, the ob­server facing the plate (Fig. 1). Horizontal as well as vertical parallax effects can be quite large according to the field angle determined by experimental conditions. The 3-D image is observed from a so-called secondary hologram illuminated in reflection with white light. This hologram is recorded from the reconstructed image obtained from a primary hologram, the latter being recorded in the usual way. Further devel­opment is animation: it is performed by using a primary hologram consisting in a number of juxtaposed parallel bands, each of them representing the object in a particular position. Reconstruction from the secondary hologram shows contin­uous movement when the observer moves orthogonally with respect to the main orientation of the bands. This is cine-holography.

An essential difference from Benton holograms1 is that there is no rainbow effect, which is most important for portrait holography. The latter is also linked to the loss of vertical parallax. Moreover, it is more convenient to illuminate a portrait by reflection (which is our case) than by transmission. A comparison can also be made with Denisyuk holograms2: these require one step only, but the object ought to be set behind the plate. It cannot be adapted to 3-D motion pictures either. In the case of animated scenes, multiplex techniques lead to results comparable with ours. However, the over-all process is much more complicated than what is described now.

The first experimental step is the recording of a primary hologram by means of a highly coherent ruby laser [Fig. 2(a)].

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One has to work in the vicinity of the zero path difference (between reference and information beams). The second step consists in recording a hologram of the pseudoscopic image reconstructed from the primary one [Fig. 2(b)]. It is impor­tant to note the role of the large lens L′: it permits the re­construction of a pseudoscopic image of the same size as that of the object, otherwise a distortion would be introduced due to the magnification ratio.3 (One works in converging light; the focusing point should be at the same distance from the hologram as the reference source from the plate at first re­cording.) Practically, the aerial pseudoscopic image is ob­tained by illuminating the primary hologram with krypton laser red light, its emulsion facing the secondary one. There is no need to have the emulsion of the second holographic plate on one side or the other. (However, we chose the reference side in order to use the glass support as a protective cover when displaying the final image.)

Fig. 1. Observation of the holographic portrait from a reflection hologram.

Fig. 2. Schematic diagram of (a) recording the primary hologram and (b) reconstruction of the pseudoscopic image and recording the secondary hologram (A, object; H1, primary hologram; D, diffuser; B.S., beam splitter; L, divergent lens; L′, converging lens; H2, secon­

dary hologram). Fig. 3. A few successive images from a sequence of the animated

portrait, as the observer moves from top downward.

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Chromatic discrimination characterizing such a type of hologram needs further processing. Due to shrinkage of the emulsion,4 the image would be of a greenish tone when illu­minated in white light; natural skin tone is obtained by dying the plate in a solution of triethanolamine (2-5%). As said before, animated portraits result from a primary hologram made of a series of bands. One uses a horizontal slot (15-20 mm) displaced step by step, each band corresponding to one elementary hologram. The second part of the process is the same as before. For example, movements of the face are re­corded and reconstructed as shown (Fig. 3). Up to now, the only restriction in displaying movement is the optimization of the distance for reconstruction of animated scenes (which is not the case for still portraits).

Various sequences for very short movies have been achieved (hand playing the piano, couple dancing, etc.). A modest moving system displaying colored 3-D animated objects was built at the Laboratoire d'Optique de Besancon a few months ago (French patent).

The authors thank J.-C. Viénot, Director of the LOBE, for his interest and helpful criticisms.

References 1. S. A. Benton, J. Opt. Soc. Am. 59, 1545A (1969). 2. Yu. N. Denisyuk and V. I. Sikhamov, Opt. Spectrosc. 25, 166

(1968). 3. J. C. Viénot, P. Smigielski, and H. Royer, Holographie Optique

(Dunod, Paris, 1971), pp. 34-36. 4. L. H. Lin and C. V. Bianco, Appl. Opt. 6, 1255 (1967).

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