The use of monochrome transparencies to retrieve color images using a white-light source was first reported by Ives1
as early as 1906. Later, techniques for generating true color images by reflection holographic2-3 and rainbow holographic processes4,5 were developed. Nevertheless, each of these techniques has its own advantages and drawbacks. Over a decade ago a technique, similar to Ives, was described by Mueller,6 which employed a tricolor grid screen for image encoding. Decoding was performed using three quasi-monochromatic sources to retrieve the color image. Since then similar work has been reported by Macovski,7 Grousson and Kinany,8 and Yu.9 In a more recent article we proposed a different method for retrieving color holographic images using a white-light optical processor.10
In this Letter we will demonstrate a very simple technique that utilizes coherent speckles to encode color images onto black-and-white film for later reconstruction by a white-light optical processing system. In principle, this technique could be the simpler of the two white-light processing methods used for producing color images. Although some authors such as Grover11 and others12 have made use of coherent speckles as information carriers, and Francon13 have described a similar processing technique, we are simply describing a color-image retrieval technique by applying a well-known speckle carrier encoding into a white-light decoding process.
For encoding, a diffuse color object illuminated by coherent light is imaged onto a photographic plate through a narrow slit by an aerial imaging lens as shown in Fig. 1. Let us assume that the recording was sequentially performed using only red
Fig. 1. Optical process for construction of a multiplexed specklegram.
Fig. 2. White-light optical processor for reconstructing a color image from an encoded specklegram.
Fig. 3. Color spatial filtering of the smeared Fourier spectra.
and green coherent illuminations with the slit oriented in one direction and 90°, respectively, thereby giving us a red and green encoded color image multiplexed onto one photographic film. Due to the different orientations of the slit aperture, the red encoded specklegram will have speckles elongated in one direction, while the green encoding has speckles elongated in the other direction (90° apart). Thus an encoded monochrome multiplex specklegram can be recorded. Decoding the color image we insert the multiplex specklegram in the input plane P1 of a white-light optical processor as shown in Fig. 2. Since the elongated speckles of each specklegram are orientated ~90° apart, the corresponding Fourier spectra would be distributed in confined directions perpendicular to these elongations in the spatial frequency plane P2 as shown in Fig. 3. That is, the spectrum of the red color image is spread in one direction, and the spectrum of the green color image is spread in the other direction. By color filtering each set of Fourier spectra with respective red and green color filters, as shown in Figs. 2 and 3, a full color image can be reproduced at the output image plane P3.
For experimental demonstration, Fig. 4 shows a black-and-white photograph of the multicolor image obtained by this specklegraphic technique. Although the resolution of the reconstructed color image suffered a severe drawback, the color reproduction is relatively faithful. With the use of a finer diffuser and appropriate slit size in the construction process, an optimum color-image reproduction may be obtained.
Fig. 4. Black-and-white photograph of the reconstructed color image of a field of tulips.
We wish to acknowledge the support of the U.S. Army TARADCOM contract DAAR 30-80-C-0110.
References 1. H. E. Ives, Br. J. Photogr. (3 Aug 1906), p. 609. 2. Y. N. Denisyuk, Sov. Phys. Dokl. 7, 543 (1962). 3. F. T. S. Yu, Introduction of Diffraction Information Processing
and Holography (MIT Press, Cambridge, 1973). 4. P. Hariharam, W. H. Steel, and Z. S. Hegedus, Opt. Lett. 1, 8
(1977). 5. H. Chen, A. Tai, and F. T. S. Yu, Appl. Opt. 17, 1490 (1978). 6. P. F. Mueller, Appl. Opt. 8, 2051 (1969). 7. A. Macovski, Appl. Opt. 11, 416 (1972). 8. R. Grousson and R. S. Kinany, J. Opt. 9, 333 (1978). 9. F. T. S. Yu, Appl. Opt. 19, 2457 (1980).
10. P. H. Ruterbusch and F. T. S. Yu, submitted to Opt. Laser Tech., to be published.
11. C. P. Grover, Opt. Commun. 6, 258 (1972). 12. C. P. Grover and M. May, J. Opt. Soc. Am. 63, 533 (1973). 13. M. Francon, review article, in Laser Speckle, J. C. Dainty, Ed.
