Abstract:
Disclosed herein are systems and related methods for reducing speckle on display screen. More specifically, screen vibration is used to reduce speckle, and in accordance with the disclosed principles, the vibration may be achieved by using wave-based actuation (e.g., acoustic or electromagnetic waves) to vibrate the screen. In an exemplary embodiment, a speckle reducing system may comprise at least one actuating element located proximate to, but not in physical contact with, a display screen. In addition, the at least one actuating element may be configured to generate waves directed towards the display screen. When the waves impact the display screen, the waves impart vibration to the display screen.

Description:
PRIORITY CLAIM 
     This disclosure is a nonprovisional conversion of, and thus claims priority to, U.S. Provisional Patent Application No. 61/505,212, filed Jul. 7, 2011, and entitled “Speckle reduction using screen vibration techniques and apparatus,” which is hereby incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     Generally, this disclosure generally relates to cinematic screens, and more particularly relates to reduction of speckle in cinematic screens. 
     BACKGROUND 
     The use of coherent or partially coherent sources can have advantages in display or illumination over standard incoherent sources (lamps) in that they can achieve higher brightness, better reliability, and larger color gamut. However, with this partial coherence comes the issue of speckle. Speckle is due to interference of the light on the screen or target that causes variations in intensity that can be seen by the observer or an instrument. These typically high frequency intensity variations are very undesirable for display or imaging applications. 
     BRIEF SUMMARY 
     Screen vibration may be used to reduce speckle in display applications. The vibration may be achieved by using actuation (e.g., acoustic or electromagnetic waves) to vibrate the screen. This is particularly relevant for large theater screens (&gt;20 feet) but can also be used for home or business screens, possibly in conjunction with other speckle reduction methods. 
     In one aspect, disclosed herein are systems for reducing such speckle on a display screen. In an exemplary embodiment, such a system may comprise at least one actuating element located proximate to, but not in physical contact with, a display screen. In addition, in such embodiments, the at least one actuating element is configured to generate waves directed towards the display screen. When the waves impact the display screen, the waves impart vibration to the display screen. 
     In another aspect, methods of reducing speckle on a display screen are disclosed herein. In one exemplary embodiment, such a method may comprise positioning at least one actuating element proximate to, but not in physical contact with, a display screen. In addition, such exemplary methods may also include generating waves with the at least one actuating element, and directing the generated waves towards the display screen. When the waves impact the display screen, the waves impart vibration to the display screen. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments are illustrated by way of example in the accompanying figures, in which like reference numbers indicate similar parts, and in which: 
         FIG. 1  is a schematic diagram showing a top down view of a configuration for a theater that uses acoustic waves to actuate the screen, in accordance with the present disclosure; 
         FIG. 2  is a schematic diagram showing a top down view of a configuration for a theater that uses electromagnetic waves to actuate the screen, in accordance with the present disclosure; and 
         FIG. 3  is a schematic diagram of a screen having antenna elements incorporated therein, in accordance with the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     There are various known techniques for addressing these high frequency intensity variations. How to measure and characterize speckle is also well known. Speckle is measured by measuring the contrast of the light intensity. This is defined as the standard deviation over the mean of the intensity. For how to measure speckle, See Jacques Gollier,  Speckle Measurement Procedure , Conference Projector Summit 2010, Las Vegas Nev., May 7, 2010, herein incorporated by reference. 
     Some currently known techniques to reduce speckle include a first family of techniques to reduce speckle that involves using moving diffusers (one or many) to achieve changes to the phase locally to temporally average out some of the speckle over the observer&#39;s/detector&#39;s integration period. See, e.g., U.S. Pat. No. 5,313,479, entitled “Speckle-free display system using coherent light,” U.S. Pat. No. 4,035,068, “Speckle minimization in projection displays by reduced spatial coherence of the image light”, and U.S. Pat. No. 7,585,078, “Illumination system capable of eliminating laser speckle and projection system employing the same,” all of which are herein incorporated by reference. The diffusers can also be vibrating with an amplitude that is large enough to cover several diffractive elements to achieve some averaging as well. See U.S. Pat. No. 7,922,333, entitled “Projector, screen, projector system, and scintillation removing apparatus for removing scintillation on an image,” herein incorporated by reference. 
     A second family of techniques to reduce speckle involves using moving mirrors or phase modulators to achieve the temporal averaging. See, e.g., U.S. Published Patent Application No. 2011/0102748, entitled “Optical system and method,” and U.S. Published Patent Application No. 2010/0053476, entitled “Systems and methods for despeckling a laser light source,” U.S. Pat. No. 4,155,630, entitled “Speckle elimination by random spatial phase modulation,” and U.S. Pat. No. 7,489,714, entitled “Speckle reduction laser and laser display apparatus having the same,” all herein incorporated by reference in their entireties. Generally, a disadvantage of this family of techniques includes the use of expensive moving parts or phase modulators. 
     A third family of techniques to reduce speckle involves using a large core, long, very high numerical aperture (NA) multimode fiber to “decoher” a laser beam. See, e.g., U.S. Published Patent Application No. 2009/0168025, entitled “Decohered laser light production system,” (herein incorporated by reference), which discusses the use of a 12 mm diameter core fiber with an NA of 0.65. This large fiber may provide some reduction in speckle but deleteriously destroys the brightness of the system since the etendue is so very large. Similarly, using a very long multimode fiber can have some benefits. For instance, see U.S. Published Patent Application No. 2010/0079848, “Speckle reduction in display systems that employ coherent light sources” but reduces the power with absorption. However, multimode fiber speckle issues and solutions seems to be well known in the field. See, e.g., Joseph Goodman,  Speckle Phenomena in Optics , Ch. 7 (Roberts and Company 2006). All references cited in this paragraph are herein incorporated by reference. 
     A fourth family of techniques has been proposed that involve dividing the beam up into parts, and then forcing each part to have different path lengths or changes of polarization before recombining the beams. Examples using fiber bundles or splitter/combiners or lenslet arrays include: U.S. Published Patent Application No. 2005/0008290 “Static method for laser speckle reduction and apparatus for reducing speckle;” U.S. Pat. No. 4,360,372, “Fiber optic element for reducing speckle noise;” U.S. Pat. No. 6,895,149, entitled “Apparatus for beam homogenization and speckle reduction;” U.S. Pat. No. 7,379,651, entitled “Method and apparatus for reducing laser speckle;” U.S. Pat. No. 7,527,384, entitled “Illumination system to eliminate laser speckle and projection system employing the same;” U.S. Pat. No. 7,719,738, entitled “Method and apparatus for reducing laser speckle.” U.S. Pat. No. 6,594,090, entitled “Laser projection display system,” which uses a lenslet integrator in conjunction with a moving diffuser to reduce speckle, states that the integrator makes the diffuser more effective. Some published applications disclose the use of a moving lenslet array instead of a diffuser to reduce speckle. See, e.g., U.S. Published Patent Application No. 2010/0296065, entitled “Out-of-plane motion of speckle reduction element,” and U.S. Published Patent Application No. 2010/0296064, entitled “Projection with lenslet arrangement on speck reduction element.” These teachings use expensive fiber bundles or lens arrays or many fiber coupler/splitters to achieve some reduction in speckle. All references cited in this paragraph are herein incorporated by reference. 
     Another family of solutions exist utilizing sources with larger spectral bandwidths. This can be achieved by chirping the drive current, using several lasers of different wavelengths or other means. 
     Moving the screen is also a potential solution to the undesirable problem of speckle. In chapter six of the book “Speckle Phenomena in Optics”, supra, the linear shift rate of the screen in x or y or screen rotation (these motions are the plane of the screen which is roughly normal to the projection) is calculated in order to average out some of the speckle during the observer&#39;s/detector&#39;s time integration period. By moving it, the light hits different parts of the screen which then changes the speckle pattern. If this is done fast relative to the detector&#39;s integration period (for example, the eye is roughly 20 Hz) then the detector will see an average of several speckle patterns, which results in a lower speckle contrast. In U.S. Pat. No. 5,272,473, entitled “Reduced-speckle display system,” the use of a transducer attached directly to the screen to mechanically generate surface acoustic waves to minimize speckle is taught. U.S. Pat. No. 6,122,023, entitled “Non-speckle liquid crystal projection display” teaches the use of a highly scattering liquid crystal as a screen, and then electrically changing the liquid crystal states to alleviate speckle. Other teachings have used scattering liquids or diffuser cells as screens to improve speckle. See, e.g., U.S. Pat. No. 6,844,970, entitled “Projection television set, screens, and method;” U.S. Pat. No. 7,199,933, “Image projection screen with reduced speckle noise;” U.S. Pat. No. 7,244,028, “Laser illuminated projection displays”, U.S. Pat. No. 7,342,719, “Projection screen with reduced speckle,” and U.S. Published Patent Application No. 2010/0118397, “Reduced laser speckle projection screen.” All references cited in this paragraph are herein incorporated by reference. 
     The present disclosure teaches the use of screen vibration to reduce speckle in display applications. The vibration may be achieved by using wave-based actuation (e.g., acoustic or electromagnetic waves) to vibrate the screen. This is particularly relevant for large theater screens (&gt;20 feet) but can also be used for home or business screens, possibly in conjunction with other speckle reduction methods. The advantages of the disclosed techniques include vibration of the screen using existing resources or inexpensive additional actuating elements (e.g., speakers) to reduce speckle. In addition, such a vibration technique does not use mechanical transducers that can wear out or mechanical attachments to the screen which can fail as well. In addition, it should be possible to achieve a more uniform vibration of the screen using the disclosed techniques. 
     To effectively average out speckle, the velocity of the screen should be larger than one half of an optical wavelength of light divided by the integration time of the eye (typically 10-50 ms). This can be achieved by using frequencies or frequency induced in the screens that are above 20 Hz. The desired vibration amplitude of the screen is ideally about one half of an optical wavelength but can be as small as 50 nm and still achieve some useful effect. Since the motion is what matters, screens can be vibrated at lower frequencies such as 5-25 Hz and still achieve speckle reduction with larger amplitudes of screen motion. This low frequency excitation causing speckle reduction has been experimentally verified. Additionally, newer screen materials can be used very effectively with this technique. 
       FIG. 1  is a schematic diagram showing a typical configuration for a theater  100  which has speakers  120  behind the screen  110  (Speaker set  1 ) and speakers  130  in front of the screen  110  (e.g., on sides and even the back wall of the theater) as represented by speaker set  2 . The speaker(s)  120  behind the screen  110  can be used to vibrate the screen  110 , in accordance with the disclosed principles. Most theater screens absorb (or reflect) almost all energy above 15 kHz. However, it is possible to use an acoustic frequency nearly above or below what can be heard by humans to shake the screen  110 . Humans typically hear about 35-16,000 Hz, with younger children typically being able to hear maybe up to 18,000 Hz or even 20,000 Hz. The tone or multiple tones can be produced by the standard audio speaker(s) and added to the speaker&#39;s amplifier, or separate speaker(s) and amplifiers could be used specifically for this purpose. The screen will also have a resonance frequency or frequencies, and by exciting the screen at these frequencies, or multiple or sub-harmonics of these frequencies, better vibration can be achieved. Preferred ranges of excitation frequencies lay in the range 10-100 Hz. Alternatively the speakers in front of the screen (set  2 ) can be used to vibrate the screen or a combination of set  1  and set  2  can be used. In an advantageous embodiment, the preferred set of speaker to use may be set  120  which are physically near the screen  110 . Exemplary speakers for use with the disclosed principles include those commonly referred to as sub-woofers, which work well in the preferred range of frequencies discussed above. Multiple tones can be used at the same time to excite the screen  110  as well as single tones. Depending on the size of the screen  110  and how far the vibrations effectively travel in the screen  110  before being attenuated, multiple speakers may be used to excite the entire screen  110  or most of the screen  110 . The speakers can be spatially arranged across the back of the theater (e.g., in embodiments where the speakers are behind the screen  110 ) to excite the screen more uniformly. 
       FIG. 2  is a schematic diagram showing a top down view of a configuration for a theater  200  that uses electromagnetic waves to actuate a screen  210 . Thus, in accordance with this embodiment of the disclosed principles, electromagnetic waves rather than acoustic excitation can be used for screen vibration. In such embodiments, antenna(s) may be used to absorb the excitation and convert it to vibration of the screen. The antennas can be located behind or attached to the back of the screen and thus not show on the front of the screen, such as the antennas  220  of Antenna Set  1  illustrated in  FIG. 2 . Radiating antennas  230  can also be in front of the screen, such as the antennas  230  of Antenna Set  2  illustrated in  FIG. 2 . Moreover, radiating antennas may be located both behind and in front of the screen  210 , in accordance with the disclosed principles. 
     The frequencies that can easily be used range from 10 MHz to approximately 6 GHz. The antennas can be omnidirectional or directional. On the screen  210  itself, arrays of antennas  240  can be used absorb the radiation and convert that to vibration of the entire screen.  FIG. 3  shows an example of an array of half wave antenna elements  240  that can be vibrated with the electromagnetic radiation. The antenna elements  240  can be spaced randomly, or at ¼, ½, or whole wavelengths, or multiples of the radiation wavelength. Examples of various types of antennas that may be employed with the disclosed principles include horn, dipole, monopole, loop, waveguide, micro-mechanical, micro-strip, and many other types of antenna used in microwave communications or in microwave ovens. For examples, see the book,  Modern Antenna Handbook , Constantine A. Balanis (Wiley 2008). 
     The screen used in accordance with the disclosed principles can be adapted from a conventional movie screen or a polarization preserving screen for showing three-dimensional content. An exemplary polarization preserving screen is shown in commonly-owned U.S. Pat. No. 7,898,734, entitled “Polarization preserving front projection screen,” herein incorporated by reference. Conventional transmission projection screens typically include a polymer substrate (usually poly-vinyl-chloride (PVC)) roll stock that is perforated for acoustic transmission and then seamed together to the desired screen size. These screens are typically 0.2-0.6 mm thick, heavily plasticized, and embossed with a matte texture. 
     To produce a polarization preserving screen, the same screen may then be sprayed with a polarization preserving coating. The conventional coating may include a type of metal flake (for example, ball-milled aluminum powder) encased in a polymer binder. Significant optical performance improvements can be realized by utilizing a metalized embossed surface, as described in U.S. Published Patent Application No. 2009/0297797. Moreover, in accordance with the disclosed principles, such metal flakes or similar elements coated on the screen may be employed to absorb electromagnetic waves, rather than attaching antenna elements to the screen as described above, and thereby provide the vibration of the screen. 
     For proper fidelity, a more rigid substrate, such as polyester or polycarbonate, could be used for the screen. A hybrid approach may use the embossed surface to texture metal flake as described in commonly-owned U.S. patent application Ser. No. 12/977,026, entitled “Polarization preserving projection screen with engineered pigment and method for making same,” or to physically chop the metalized substrate as described in commonly-owned U.S. patent application Ser. No. 12/976,986, entitled “Polarization preserving projection screen with engineered particle and method for making same” (both herein incorporated by reference), either of which could then be utilized to replace the metal flake in a conventional screen system. This could be applied on either a conventional (PVC) or more rigid substrate. Rear-projection polarization preserving screens typically include a diffusely scattering transparent polymer substrate, an embossed transparent substrate, or a combination of the two. These more rigid screen materials may be employed in preferred embodiments, as they transmit vibrations more effectively over a large area. This means that fewer speakers or other movement source(s) may be used to vibrate the entire screen. 
     In addition, vibration of the screen has been experimentally shown to reduce speckle for standard projection lamps, so the principles disclosed herein may also be applied with lamp based projection. Particularly, some of the polarization preserving screens exhibit noticeable speckle with standard lamp based projection. Additionally, the principles disclosed herein may be utilized for both rear and front projection displays. It is also useful for both 2D (two dimensional) and 3D (three dimensional) displays. 
     As may be used herein, the terms “substantially” and “approximately” provide an industry-accepted tolerance for its corresponding term and/or relativity between items. Such an industry-accepted tolerance ranges from less than one percent to ten percent and corresponds to, but is not limited to, component values, angles, et cetera. Such relativity between items ranges between less than one percent to ten percent. 
     While various embodiments in accordance with the principles disclosed herein have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of this disclosure should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with any claims and their equivalents issuing from this disclosure. Furthermore, the above advantages and features are provided in described embodiments, but shall not limit the application of such issued claims to processes and structures accomplishing any or all of the above advantages. 
     Additionally, the section headings herein are provided for consistency with the suggestions under 37 CFR 1.77 or otherwise to provide organizational cues. These headings shall not limit or characterize the embodiment(s) set out in any claims that may issue from this disclosure. Specifically and by way of example, although the headings refer to a “Technical Field,” the claims should not be limited by the language chosen under this heading to describe the so-called field. Further, a description of a technology in the “Background” is not to be construed as an admission that certain technology is prior art to any embodiment(s) in this disclosure. Neither is the “Summary” to be considered as a characterization of the embodiment(s) set forth in issued claims. Furthermore, any reference in this disclosure to “invention” in the singular should not be used to argue that there is only a single point of novelty in this disclosure. Multiple embodiments may be set forth according to the limitations of the multiple claims issuing from this disclosure, and such claims accordingly define the embodiment(s), and their equivalents, that are protected thereby. In all instances, the scope of such claims shall be considered on their own merits in light of this disclosure, but should not be constrained by the headings set forth herein.