GRIN TYPE DIFFUSER BASED ON VOLUME
This is a continuation of application Ser. No. 5 07/591,801, filed Oct. 2, 1990, now abandoned.
BACKGROUND OF THE INVENTION lField of the Invention
This invention pertains to the diffusion of light. More 10 particularly, this invention relates to volume holographic scattering of light based on speckle theory.
2. Description of the Prior Art
Diffusers are used to scatter light so that an object in the path of the diffused light is illuminated from a num- 15 ber of directions. Typical state of the art diffusers are made from ground glass or photographic emulsion, and a typical diffuser might be the outside of a frosted light bulb or a ground glass plate or other rough surface placed near a light source. Ground or milk glass diffus- 20 ers scatter light uniformly through large angles and have been used to average the effects of noise as discussed in M. J. Labatt and A. S. Marathay, Image Speckle Patterns of Weak Diffusers, J. Opt. Soc. of Amer. 65, 769 (1975). 25
It is well known that widen coherent light is incident on a surface which is optically rough, such as the above ground glass diffuser, a random intensity pattern is formed. This intensity pattern is typically called speckle. Speckle is present anywhere in space beyond a 30 diffuser illuminated with coherent light as discussed in J. C. Dainty, Optica Acta, 17, 761 (1970). A number of authors including Dainty have studied the statistics of speckle created by an illuminated diffuse object. The pioneering analysis in this regard was made by L. I. 35 Goldfischer, Autocorrelation Function and Power Spectral Density of Laser-Produced Speckle Patterns, J. Opt. Soc. of Amer., 55, 247 (1965). In his study, Goldfischer described the general structure of light scattered by diffusers as well as the characteristic speckle pattern. 40 Goldfischer exposed photographic film directly to backscattered radiation from a diffuse surface illuminated by a coherent monochromatic source.
In S. Lowenthal and H. Arsenault, Image Formation for Coherent Diffuse Objects: Statistical Properties, J. 45 Opt. Soc. of Amer., 60, 1478 (1970), a related concern, the determination of the statistics of the image given by an optical system with a coherent diffuse object was studied. While speckles are related to a random structure of a diffuser illuminated by coherent light, the 50 spatial coherence of light is related to the random structure of the source radiation itself. W. H. Carter and E. Wolf, Coherence and Radiomerry with Quasi Homogeneous Planar sources, J. opt. Soc. of Amer., 67, 785 (1977) studied spatial coherence and quasi homogene- 55 ous sources generally. M. G. Miller, et al., SecondOrder Statistics of Laser-Speckle Patterns, J. Opt. Soc. of Amer., 65, 779 (1975) discussed the statistics of laser speckle patterns in a plane some distance from a coherently illuminated object. M. Kowalczyk, Spectral and 60 Imaging Properties of Uniform Diffusers, J. Opt. Soc. of Amer., 1, 192 (1984) sets out the theory relating to thin phase diffusers in coherent imaging systems.
A specific form of computer generated diffuser called a kinoform has also been studied. A kinoform is a com- 65 puter generated wavefront reconstruction device which, like a hologram, provides the display of a two dimensional image. In contrast to a hologram, however,
a kinoform yields a single diffraction order and reference beams and image separation calculations are unnecessary. A kinoform is purely phase because it is based on the assumption that only phase information in a scattered wavefront is required for time construction of the image of the scattering object. Kinoforms are discussed in L. B. Lesem et al., The Kinoform: A New Wavefront Reconstruction Device, IBM J. Res. Develop., March 1969 and H. J. Caulfield, Kinoform Diffusers (SPIE Proc. Vol 25, 111 (1971). Caulfield provides a general description of kinoform please diffusers and found that the angular spectrum of light scattered from a diffuser increases when the angular size of the aperture of time diffuser increases. Caulfield tested this by varying time distance between a diffuser mask comprising ground glass and a kinoform made of bleached silver halide. The textbook by J. W. Goodman, Statistical Optics, Wiley Interscience (1985) is a useful reference in studying the statistics of diffusers, speckles, and partial coherence of light as well as the spatial coherence of light, scattered by moving diffusers. Collier et al., Optical Holography, Academic Press (1971) is also helpful and like the other references cited is incorporated by reference herein.
The above diffusers, based on ground or milk glass, bleached silver halide plates, or surface type photoresist plates provide diffusion primarily only at their surface. This reduces the path length of the light affected by the diffuser thereby reducing the flexibility of the diffuser (the number of applications to which it may be put) and lowering losses due to reflection of the light in unwanted directions.
A diffuser which increases the optical path interaction length of the light in the diffuser would make it possible to design diffusers for many different types of applications that are currently not served well by state of the art diffusers and increase the efficiency of transmitted light through the diffuser. A diffuser which may be rotated would provide regulated spatial coherence useful in a number of applications.
SUMMARY OF THE INVENTION
A volume holographic diffuser which presents a large optical pathlength to light incident the diffuser and increases transmission efficiency is presented. Specifically, a volume holographic medium recorded with a speckle pattern yields a diffuser having gradually changing refractive indices perpendicular to the surface with respect to the rest of the volume. The diffuser provides light beam shaping capability by recording speckles of a certain shape in the diffuser which scatter collimated light into a controlled pattern with smooth brightness variation. The controlled diffusion achieves structure-less illumination patterns having uniform brightness with no lot spots or visual glare.
The diffuser achieves controlled directionality (angular distribution) of the diffused beam by insuring that the collimated light beam incident the diffuser is scattered into a wider set of directions, generally increasing the entropy or disorder of the light beam through volume scattering. Through volume scattering, the diffuser takes advantage of the cumulative effects of passage of the light through an extended gradually scattering medium. Instead of scattering the light using sharp edges or boundaries found in ground glass or photoresist surface diffusers as well as the silver grains of bleached silver halide, the volume holographic diffuser of the present invention scatters light from the smoothly vary