Optical: systems and elements – Lens – Diffusing
Reexamination Certificate
2001-08-27
2003-07-15
Dang, Hung Xuan (Department: 2873)
Optical: systems and elements
Lens
Diffusing
C359S634000, C353S031000, C345S088000, C372S108000
Reexamination Certificate
active
06594090
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to projection display apparatus employing a laser as a light source. More particularly, the invention relates to laser projection display apparatus having means for reducing the appearance of coherence-induced artifacts and speckle in the display.
BACKGROUND OF THE INVENTION
Projection display systems for the display of video images are well-known in the prior art. Typically, these systems have taken the basic form of a white light source, most notably a xenon arc lamp, illuminating one or more light valves or spatial light modulators with appropriate color filtering to form the desired image, the image being projected onto a viewing screen.
Lasers have been known to be attractive alternative light sources to arc lamps for projection displays. One potential advantage is a wider color gamut featuring very saturated colors. Laser illumination offers the potential for simple, low-cost efficient optical systems, providing improved efficiency and higher contrast when paired with some spatial light modulators. One disadvantage of lasers for projection display has been the lack of a cost-effective laser source with sufficient power at visible wavelengths.
Spatial light modulators provide another component that enables laser display systems. Examples of two-dimensional spatial light modulators are reflective liquid crystal modulators such as the liquid-crystal-on-silicon (LCOS) modulators available from JVC, Three-Five, Aurora, and Philips, and micromirror arrays such as the Digital Light Processing (DLP) chips available from Texas Instruments. Advantages of two-dimensional modulators over one-dimensional array modulators and raster-scanned systems are the absence of scanning required, absence of streak artifacts due to nonuniformities in the modulator array, and immunity to laser noise at frequencies much greater than the frame refresh rate (≧120 Hz). A further advantage of two-dimensional spatial light modulators is the wide tolerance for reduction of the spatial coherence of the illuminating beam. Examples of one-dimensional or linear spatial light modulators are the Grating Light Valve (GLV) produced by Silicon Light Machines and conformal grating modulators; see U.S. Ser. No. 09/491,354, filed Jan. 26, 2000, by Kowarz, and U.S. Ser. No. 09/867,927 filed May 30, 2001, by Kowarz et al.
Although high power visible lasers offer new opportunities for the design of projection systems, including the possibilities of expanded color gamut and simplified optical designs, laser light is in other ways not optimum for use in image projection systems with spatial light modulators. In particular, lasers are very bright sources, which emit generally coherent light within a very small optical volume (etendue or lagrange). Etendue is the product of the focal spot area and the solid angle of the beam at the focus. Lagrange is the product of the focal spot radius and the numerical aperture. For example, a single mode green wavelength laser with a diffraction-limited beam has a lagrange of about 0.3 &mgr;m, which is about 15,000 times smaller than the lagrange for a conventional white light lamp source, such as an arc lamp. With such a small lagrange, lasers can be used very effectively in raster scanning systems, including those for flying spot printers and laser light shows, where a tightly controlled beam is desirable.
On the other hand, in an image projection system, in which an image-bearing medium such as a film or a spatial light modulator is imaged to a screen or a target plane, the high coherence and small lagrange of the laser is ultimately undesirable. In such an imaging system, the lagrange is determined by the linear size of the projected area (size of the spatial light modulator) multiplied by the numerical aperture of the collection lens. The related quantity, etendue, is calculated similarly. In many white light projection systems, the projection lens is quite fast (f/3 for example) to collect as much light as possible. Even so, the typical white light lamp source overfills both the light valve and the projection lens, and significant light is lost. For example, in a representative system using a common 0.9″ diagonal light valve and an f/3 projection lens, the optimum light source would have approximately a 2.0-mm lagrange to provide proper filling without overfill. However, a standard white light lamp, with a typical lagrange of 2-10 mm, is not sufficiently bright and will generally overfill this representative system.
In the case of a laser display system using image area projection, the opposite problem arises, the lasers being too bright. Furthermore, it is not desirable to illuminate the spatial light modulator with a coherent source, because of the potential for interference effects, such as fringes, which may overlay the displayed image. This is especially true of liquid crystal modulators, wherein the thin-film structure can result in fringes in the image due to nonuniformities in the film layers. Likewise, coherent illumination of the entire light valve may create an artificial beam apodization, which is dependent on the image content, arising from diffraction from pixels and groups of pixels. Diffraction artifacts can also arise from illuminating the grid electrode pattern of a liquid crystal panel, an X-cube with a center discontinuity, or any dust or imperfections on the optical elements with a highly coherent beam of light. Therefore, a reduction of the source brightness (or an increase in the source lagrange) is a necessity for such laser projection systems.
A defined reduction of the source brightness can also provide an important opportunity. The projection display optical system can be designed to optimize and balance the system requirements for resolution, system light efficiency, and system simplicity. By defining the system f-number on the basis of a criterion other than system light efficiency, the specifications on other system components such as the projection lens, color filters, and polarization optics can be eased, dramatically reducing system costs compared to lamp-based projection systems.
While laser sources can be optimized for use in projection display illumination and imaging systems, there is the consequent major disadvantage of speckle to be dealt with. Speckle arises due to the high degree of coherence (both spatial and temporal) inherent in most laser sources. Speckle produces a noise component in the image that appears as a granular structure, which both degrades the actual sharpness of the image and annoys the viewer. As such, the speckle problem, as well as the historical lack of appropriate laser sources, has inhibited the development of marketable laser-based display systems.
The prior art is rich in ways of attempting to reduce speckle. One common approach is to reduce the temporal coherence by broadening the linewidth of the laser light. Other approaches to reducing the temporal coherence are to split the illuminating wavefront into beamlets and delay them relative to each other by longer than the coherence time of the laser, see for example U.S. Pat. No. 5,224,200, issued Jun. 29, 1993 to Rasmussen et al. Dynamically varying the speckle pattern by vibrating or dynamically altering the screen is another way of reducing the visibility of the speckle pattern. See, for example, U.S. Pat. No. 5,272,473 issued Dec. 21, 1993 to Thompson et al. Another speckle reduction approach involves coupling the laser light into a multimode optical fiber and vibrating the fiber to cause mode-scrambling as described in U.S. Pat. No. 3,588,217, issued Jun. 28, 1971 to Mathisen.
Another family of despeckling solutions uses a diffusing element that is moved or vibrated within the projector system. Typically, this is done at an intermediate image plane, as disclosed in U.S. Pat. No. 4,035,068, issued Jul. 12, 1977 to Rawson. One disadvantage of this approach is that the diffusion must occur precisely at the image plane or a softening of the image will occur. Also, the projection lens is complicated by the requir
Kruschwitz Brian E.
Kurtz Andrew F.
Close Thomas H.
Dang Hung Xuan
Eastman Kodak Company
Spector David N.
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