The invention relates to a method for eliminating speckles in the distant field of a projection surface illuminated by a laser for example in connection with a laser image projection on an image screen and devices for its performance.
When using lasers for the illumination or image display on a projection surface, interferences occur due to the roughness of the surface in the space in front of the projection surface due to transit time differences of the waves that emanate from different portions of the projection surface. These interferences lead to a strong additional intensity modulation of the brightness distribution of the image screen in the eye of the viewer. The intensity modulation covers the image as a fine granulation pattern forming an additional noise proportion in the image which reduces the discernible resolution by a factor of up to 10 and leads, in multi-color laser images, to a color falsification by an incomplete color summation, see in this connection J. M. Artigas, A. Felipe and M. J. Buades, xe2x80x9cContrast Sensitivity of the Visual System in Speckle Imageryxe2x80x9d, Journal of the Optical Society of America, A, Vol. 11, No. 9, page 2345, (1994).
In recent years the technical preconditions for the realization of a laser projector with scanning laser beams has been substantially improved as disclosed, for example in an article by C. Deter xe2x80x9cLaser-Display-Technology-Where Do We Stand?xe2x80x9d in the publication xe2x80x9cPhysikalische Blxc3xa4tterxe2x80x9d, Vol. 52, 1996, No. 11, page 1129. Today substantially more efficient and more cost effective diode excited solid state lasers or fiber lasers can be employed. In the future even laser diodes with an electric/optical efficiency of 10 to 30% for monochromatic image projection in the colors red, green and blue (RGB) may be used. See in this connection for example W. E. Glenn and G. J. Dixon xe2x80x9cBright Future Projected for Lasers in Electronic Cinemasxe2x80x9d, Laser Focus World, November, 1993, page 73.
As far as we know there are no methods known at this time for the reduction or even elimination of laser speckles in laser beam projection systems which do not simultaneously diminish the image quality to such an extent that the advantages of the laser radiation disappears or that its use hardly brings any technical or economic advantages.
Basically, there are different methods for reducing laser speckles. Particularly in the laser and electron beam techniques different methods for the reduction of speckles are known. The following shall be mentioned here merely as examples: moving diffusing discs or phase plates, statistically controlled liquid crystal cells, glass fiber bundles with different transit times, moving glass fibers, widening of the line width of the laser and so forth.
The methods function only in the near field of the projected image pixel on the projection screen. For the viewer viewing the far field the transition differences are so small that the speckles do not disappear. The methods for destroying the localized coherence lead to a very strong beam degradation. As a result, an eventually reprocessed beam retains only a small intensity. The known methods for the destruction of the time coherence generally only result in a very limited increase of the emission band width and the resulting coherence length is still larger than the largest distance differences from the edges of a projected pixel to the viewer. The use of the shortest laser impulses (under 1 ps) for the reduction of the coherence length of the laser and thus the diminishing of the interference capability of the light is burdened with an enormous technical effort and expense.
It is the object of the invention to provide, especially for laser beam projections, a device which substantially eliminates or reduces the speckles perceived by the viewer while hardly or not changing the beam configuration and the beam density.
The above object has been achieved by a method for at least partly eliminating speckles in a laser image that is projected onto an image screen (37) having a screen surface area, by scanning said image screen by an illumination laser beam (34), comprising the following steps:
(a) projecting said illumination laser beam (34) onto said image screen (37),
(b) dividing said illumination laser beam (34) on its way to said image screen (37) into a plurality of partial beams (36),
(c) illuminating with each partial beam (36) a partial screen area that is smaller than said screen surface area illuminated by said illumination laser beam (34),
(d) generating different speckle patterns by respectively moving said partial beams (36) within an image element (32) to be projected onto said image screen (37), and
(e) heterodyning said different speckle patterns on one another in such a way that said different speckle patterns average
(f) each other out of said laser image.
According to the invention there is also provided an apparatus for the performance of the present method, said apparatus comprising an image screen (37), a laser source for generating said illumination laser beam (34), a beam splitting element (35) positioned between said laser source and said image screen (37) for dividing said illumination laser beam (34) into a plurality of partial beams (36) and for illuminating with each partial beam (36) a partial screen area that is smaller than a surface area of said image screen (37) illuminated by said illumination laser beam (34), scanning means for moving said partial beams (36) within a respective image element (32) on said image screen to form different speckle patterns, and heterodyning means for superimposing said different speckle patterns on one another in such a way that said different speckle patterns average each other out of said laser image.
The invention is based on the experimental fact that the speckle modulation decreases in the eye of the viewer when the viewer moves his or her head rapidly, namely when the eye averages the speckle images in time and space. Similarly, motion of the projection screen causes the disappearance of the speckle pattern. The motion of the projection screen is performable and shall be described in detail in the following. However, moving larger screens is cumbersome and expensive.
Thus, according to the invention the motion of the projection screen is replaced by the motion of the projection beam. For this purpose it is not necessary that the image screen or the projected pixel are moved relative to each other, nor is it necessary that the screen or the pixel are moving relative to the viewer. Rather, the projected pixel shall be divided and the subdivided areas shall move within the pixel.
This object is achieved according to the invention with the aid of a phase hologram having a high diffraction efficiency in combination with the scattering characteristic of the projection screen. The laser beam is suitably divided into different partial beams by the hologram inserted into the beam path, whereby each of the individual partial beams illuminates a smaller surface area than the original laser beam. Each partial beam produces on the projection screen a different speckled pattern. The instantaneous surface roughness or topography thereby enters significantly into the speckle formation.
By the movement of the partial beams over the pixel surface of the image speckle to be produced, for example by the rotation of the hologram or by installation of a scanner (beam deflector) in the beam path for scanning the image or any other suitable device for the beam movement one obtains a different, moving speckle field for each partial beam. The viewer now sees simultaneously the heterodyning of all the speckle images produced by the moving partial beams. Due to the integration process in the eye, the speckle contrast is thereby averaged in space and time. For the eye of the viewer the speckles have disappeared.
When a phase hologram is used for the present purpose, the known principles and manufacturing methods may be used for producing the phase hologram. Important for this purpose is the maintaining of a good beam collimation and a large diffraction efficiency. This can, for example be achieved by a multiphase hologram. The hologram may, for example be arranged in front of the image scanning system or downstream of the image scanner in an intermediate image plane of the projection lens or in another suitable position.
The motion of the partial beams can be achieved in that, for example the phase hologram is rotated about its optical axis or in that the beams are moved on the screen by the scanner itself. However, other known image rotation principles can be employed as well. The rotational speed of the hologram is to be adapted to the scanner system with regard to the following factors: integration time of the eye, image frequency, number of the projected pixels per image, geometry of the viewer room and so forth.
As has been shown experimentally, it is possible to use simultaneously the rotation and translatory motion of the partial beams by respectively moving the scanner.
The only requirement to be met by the image screen is, that the screen is not constructed as an optical mirror per se. This is not a limitation for the invention because after reflection at a mirror either no speckles or only minute speckles occur and a mirror is excluded anyway because in such a case the projection geometry would be satisfied only for one point in space and thus viewing the image would be impossible, more or less.
In order to achieve the splitting of a laser beam by a phase hologram into partial beams the diffracting structures are arranged in grid form in two dimensions, either regularly or irregularly. This grid form structure is equipped with a further structure for the forming of the produced partial beams (collimation, focussing). This further structure may look like or similar to a Fresnel zone plate.
This basic structure can be inscribed individually for each partial beam as an array in a hologram or it can be distributed for all partial beams over the entire hologram surface. The structure can, however, be calculated through direct Fourier transformation or other respective formalisms.
An individual hologram written in accordance with the above instructions can be used directly. However, it may be advantageous to copy such a hologram structure and to use it in an arrangement as an array. In this manner a redundance of the beam forming element is achieved and also a substantial independence of the system from the used beam cross-section and from the intensity distribution on the hologram plate. Additionally, a precise centering of the hologram relative to the beam axis is no longer necessary.