Abstract:
Disclosed is a method for reducing speckling in liquid crystal displays with coherent illumination. The method consists of providing a liquid-crystal display illuminated, e.g., with a laser light, in which the image is formed by passing the light through the light redirecting holographic elements arranged in a matrix pattern, then changing the direction of the beams emitted from the holographic elements by passing the emitted beams through the polarization-changing liquid crystal elements, and converting the image-carrying beams produced by the liquid crystal elements into a visible image by passing them to a viewer through a polarization analyzer.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to liquid-crystal displays (LCDs), in particular a method of reducing speckles in LCD&#39;s with coherent illumination, e.g., laser illumination. The invention provides the LCDs of the aforementioned type with improved image brightness, increased contrast, and reduced speckling in the image plane. 
       BACKGROUND OF THE INVENTION 
       [0002]    An active matrix liquid-crystal display (AMLCD) is a type of flat-panel display which currently is the overwhelming choice of notebook computer and smart-phone manufacturers because of its light weight, very good image quality, wide variety of colors, and fast response time. 
         [0003]    Most commonly, an AMLCD display is a multilayered structure that comprises a light-guide panel (LGP) that emits white light; a matrix of color light filters; two polarization layers; and a liquid-crystal matrix that contains a plurality of liquid-crystal cells and is combined with a matrix of thin-film transistors (TFT). 
         [0004]    Optical response to the light passing through each liquid crystal cell results from electrical drive of each cell of the TFT matrix. Each color liquid-crystal cell should contain at least three cells (subpixels) of liquid crystals. Each subpixel is covered by a specific color light filters, e.g., by a red, green, or blue filter. Each pixel may contain more than three subpixels, which can be organized into different geometrical patterns. Since the cells of the liquid crystal matrix are driven independently, at any given time some pixels may maintain an electrical state while others are in the stage of updating. 
         [0005]    Such a display provides much brighter, sharper images than a display of the same size with a passive matrix. Further improvement of the AMLCDs may be possible only with introduction of laser light sources. However, the AMLCDs based on laser light sources do not yet exist on the market. 
         [0006]    Advantages of lasers as light sources for active LCDs in comparison with conventional sources or now incoming LEDs are well known because laser illumination devices provide higher brightness, greater image contrast, wider variety of colors, smaller dimensions, and better performance efficiency. In spite of these advantages, laser illumination devices have not yet achieved widespread application because of a fundamental phenomenon that leads to microscopic image degradation, i.e., observation of a floating granular pattern in front of the image plane. This pattern is known as a “speckle” pattern caused by interference of light waves having different phases and amplitudes but the same frequency. The interaction of these waves produces a resultant wave. The amplitude and intensity of the resultant wave varies randomly. 
         [0007]    The phenomenon of speckle formation can be explained as follows. When the surface of an object is illuminated with coherent light, e.g., laser light, each point of the illuminated surface acts as a secondary point of light source that reflects (transmits) and scatters a spherical wave. However, since the illuminated surface, itself, has its own surface microstructure, these waves will have different phases and amplitudes. More specifically, in the majority of cases, the light-reflecting or light-permeable surface that constitutes an object of illumination has surface roughness that is comparable to the wavelength of the illumination light. The main contribution to scattering of light is assumed to be the small portions of a surface that are irregularly arranged and that possess light reflection or refraction properties. With an increase in the steepness of roughness and the size of the illuminated area, the number of light illuminating points is increased. Propagation of such reflected (transmitted) light to the point of observation leads to interference of diphased but coherent waves at that point. As a result, the viewer sees a granulated or speckled pattern. In other words, speckles comprise an interference picture of irregular wavefronts that is formed when coherent light falls onto a rough surface or a surface that contains micro nonuniformities. In general, a speckled picture is formed when coherent light propagates in the space and meets micro nonuniformities. 
         [0008]    Thus, the phenomenon of speckle formation essentially restricts the scope of application of laser illumination devices in fields such as active displays, microscopy, optical metrology, optical coherent tomography, etc. For example, speckles are considered to be a problem in laser-based display systems such as laser television. 
         [0009]    Quantitatively the speckles are usually evaluated by the amount of speckle contrast. Speckle contrast is reduced by creating many independent speckle patterns that are averaged in the eye retina or in a detector. The speckle contrast reduction can be achieved by various methods such as changing an illumination angle, using different polarization states, using laser sources of close but still different wavelengths, using rotating diffusers that destroy the spatial coherence of the laser light, or moving or vibrating light transmitting/reflecting membranes that are placed in the optical path of the illumination light. 
         [0010]    Heretofore, laser-illuminated displays with pixilated image formation did not exist in the industry. However, the principle of design and operation of such displays are described in Pending U.S. patent application Ser. Nos. 13/317544 and 13/331261 filed on 10/21/2011 and on 12/20/2011, respectively, by the same applicants. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is an exploded three-dimensional view of a fragment of a liquid-crystal display of the invention with linearly reciprocating micromirrors. 
           [0012]      FIG. 2  is a sectional view of the liquid-crystal display in  FIG. 1  along a portion of the light waveguide shown in  FIG. 1 . 
           [0013]      FIG. 3  is a three-dimensional view of a fragment of a liquid-crystal display in accordance with another aspect of the invention with swinging mirrors. 
           [0014]      FIG. 4  is a sectional view of the liquid-crystal display in  FIG. 1  along a portion of the light waveguide shown in  FIG. 3 . 
       
    
    
     SUMMARY OF THE INVENTION 
       [0015]    The object of the invention is to provide a laser-illuminated liquid-crystal display, e.g., laser diodes or superluminescent-emitting diodes with reduced speckling, improved brightness, high image contrast, wide range of color reproduction, miniature dimensions, and high performance efficiency. 
         [0016]    Although the following description of the method relates only to laser-illuminated LCD&#39;s, it is equally applicable to LCD&#39;s illuminated with super-luminescent diodes and diode-pumped solid-state (DPSS) lasers. 
         [0017]    The LCD of the invention with reduced speckling for which the proposed method pertains comprises a multilayered structure consisting essentially of the following layers arranged sequentially in the upward direction of the drawing: 1) a micromirror matrix; 2) an LGP; 3) a liquid-crystal matrix; and 4) a polarization analyzer. 
         [0018]    The micromirror matrix comprises a thin plane matrix of fully reflective micromirror elements manufactured according to the microelectromechanical systems (MEMS) technique. Each micromirror matrix element comprises a nonmoveable element and a moveable micromirror. According to one aspect of the invention, the micromirrors perform reciprocating movements, and according to another aspect of the invention the micromirrors perform tilting oscillations. 
         [0019]    Each reflective micromirror element, and hence, each micromirror, has approximately the same diagonal dimension as the respective element of the liquid-crystal matrix. This diagonal dimension ranges from several ten of microns to several hundred microns. The micromirrors are fully reflective and have the same arrangement pattern as the respective liquid-crystal matrix elements of the liquid-crystal matrix. 
         [0020]    The light-guide panel (LGP) is located between the micromirror matrix and the liquid-crystal matrix. Construction of the LGP is described in detail in pending U.S. patent application Ser. No. 13/317,544 (Lightguide Panel for Display with Laser Backlight) filed on Oct. 21, 2011 by the same applicants. The light-guide panel contains a plurality of parallel waveguides that forms a net, which is described in more detail in the aforementioned patent application. The light waveguides, in turn, contain holographic elements arranged in a matrix pattern that coincides and is aligned with the matrix pattern of the aforementioned micromirrors. To form color images, individual light waveguides transmit light of different wavelengths, and the transmitted light is emitted by the holographic elements from the waveguides in two opposite directions, which are substantially perpendicular to the plane of the waveguides. The downward-directed beam emitted by each holographic element falls onto a respective micromirror, is reflected therefrom, passes through the respective matrix cell of the liquid-crystal matrix, and then enters the polarization analyzer. The liquid-crystal matrix changes polarization of the laser beams that pass through its matrix cells, or elements, in accordance with the voltage applied to the respective elements of the liquid-crystal matrix. The light that undergoes such local polarization now carries image information. In order to convert this image information into an image visible by a viewer, the aforementioned spatial polarization-modulated light is, in turn, subject to spatial intensity modulation, which is carried out by the aforementioned polarization analyzer through which the spatial intensity-modulated light reaches the viewer. 
         [0021]    However, as mentioned above, micromirrors perform reciprocating linear or tilting movements. Therefore, the image shifts relative to the preceding image at each moment. In other words, the LCD of the invention produces a plurality of sequential images having in each moment its own speckle-distribution pattern. Thus, at each moment, the viewer sees the image in different micropositions, which are perceptible by the human eye as a quasistationary pattern. Each current image carries its one speckle. As a result, the speckle pattern seen by the viewer is smoothened. So that the final image is seen by the human eye to be immoveable, the micromirror elements should move at a frequency not less than 25 to 30 Hz. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    For better understanding the method of the invention it is necessary first to consider the structure of an LCD of the type applicable for reduction of speckles by the method of the invention.  FIG. 1  is an exploded three-dimensional view of a fragment of a liquid-crystal display of the aforementioned type. 
         [0023]    The LCD  20  shown in  FIG. 1  has a multilayered structure and consists essentially of the following layers arranged sequentially in the upward direction of the drawing: 1) a micromirror matrix  22 ; 2) a lightguide panel (hereinafter referred to as an LGP)  24 ; 3) a liquid-crystal matrix  26 ; and 4) a polarization analyzer  28 . 
         [0024]    The following describes each layer of the LCD of the invention in more detail. 
         [0025]    The micromirror matrix  22  comprises a thin plane matrix of fully reflective micromirror elements  22   a,    22   b  . . .  22   n  manufactured according to the MEMS (microelectromechanical systems) technique. Each micromirror element of the matrix comprises a nonmoveable element, such as elements  22   a ′,  22   b ′ . . .  22   n ′, and a moveable micromirror, such as micromirrors  22   a ″,  22   b ″ . . .  22   n ″. The nonmoveable elements are supported by a substrate S ( FIG. 2 ). As shown in  FIG. 1  by arrows A 1 , A 2  . . . An, the micromirrors perform reciprocating movements in the direction perpendicular to the plane of the display layers  22 ,  24 ,  26 , and  28 . Each micromirror element comprises a microcapacitor, the upper plate of which is the moveable micromirror. When a voltage is applied between the nonmoveable element and the moveable micromirror, the latter begins to move under the effect of the electrostatic force. The voltages applied to each micromirror of the matrix  22  are generated by a driver D 1  which is shown in  FIG. 2 . For the simplicity of the consideration, it is assumed that in the illustrated liquid crystal display  20  the micromirrors perform linear movements, although these movements may have a more complicated nature. 
         [0026]    The preferable amplitude of the reciprocating movements executed by the moveable micromirrors  22   a ″,  22   b ″ . . .  22   n ″ should not exceed ¼ wavelength of the light reflected from the respective micromirrors. Each reflective micromirror element  22   a,    22   b  . . .  22   n , and, hence, each micromirror, has approximately the same diagonal dimension as the respective element of the liquid-crystal matrix  26 . This dimension ranges from several tens of microns to several hundred microns.  FIG. 1  shows that the fully reflective micromirror elements  22   a,    22   b  . . .  22   n  have the same arrangement pattern as the respective liquid-crystal matrix elements  26   a,    26   b  . . .  26   n  of the liquid-crystal matrix  26 , i.e., these elements are vertically aligned. 
         [0027]    Also shown in  FIG. 1 , the LGP  24  is located between the micromirror matrix  22  and the liquid-crystal matrix  26 . Construction of the LGP  24  is described in detail in pending U.S. patent application Ser. No. 13/317,544 (Lightguide Panel for Display with Laser Backlight) filed Oct. 21, 2011 by the applicants. As described in the aforementioned patent application, laser light is introduced into the LGP  24  from laser light sources. Regarding the illustrated embodiment shown in  FIGS. 1 , the red, green, and blue lights (RGB) IR 1 , IG 1 , and IB 1  are emitted from the laser sources LR 1 , LG 1 , and LB 1 . These lights propagate through the waveguides  24 R 1 ,  24 G 1 , and  24 B 1 , etc. The LGP contains a plurality of such tripled waveguides, which, as described in U.S. patent application Ser. No. 13/317,544, may have different color combinations and arrangements. 
         [0028]    The waveguides  24 R 1 ,  24 G 1 , and  24 B 1 , etc., contain illuminating holographic elements. For simplicity,  FIG. 1  shows only the holographic elements  24   a,    24   b,  . . .  24   n  that are formed on the waveguide  24 R 1 , which is used for propagating red light. The red laser light beams shown by arrows  24   a ′,  24   b ′ . . .  24   n ′ are directed from these holographic elements downward, i.e., in the direction opposite to the viewer of the display image (not shown). The upward-directed arrows  24   a ″,  24   b ″ . . .  24   n ″ designate laser light beams that are reflected from the micromirror elements  22   a,    22   b  . . .  22   n  and that pass through the same respective holographic elements to the matrix elements  26   a,    26   b  . . .  26   n  of the liquid-crystal matrix  26 . 
         [0029]    The liquid-crystal matrix  26  changes polarization of the laser beams that pass through its matrix elements in accordance with the voltage applied to the respective elements of the liquid-crystal matrix. The light that undergoes such a local polarization now carries image information. In order to convert this image information into an image visible by the viewer, the aforementioned spatial polarization-modulated light is, in turn, subject to a spatial intensity modulation which is done by the polarization analyzer  28  through which the spatial intensity-modulated light reaches the viewer. 
         [0030]    The foregoing description relates only to the processing of laser light that is received from the laser source LR 1  and propagates through one laser light waveguide, i.e., to light of a predetermined wavelength, e.g., red light. As mentioned above, the liquid-crystal display of the type to which the present invention pertains may contain a plurality of laser light waveguides that transmit lights of different wavelengths. For example, these may be RGB light waveguides. In this case, the different color waveguides are arranged in the LCD in a predetermined sequence, e.g., in the form of parallel and color-alternating strips. Also, the light waveguides  24 G 1 ,  24 B 1  . . . of other wavelengths will have structures similar to the waveguide  24 R 1  and will differ from the light waveguide  24 R 1  in that they have different holographic element parameters. 
         [0031]    Reduction of speckling in the final picture seen by the viewer is a result of averaging a plurality of images, viewed by the viewer, that are formed by lights reflected by the moving micromirrors  22   a ″,  22   b ″ . . .  22   n ″ at different positions of the latter. In order that the final image is seen by the human eye to be immoveable, the micromirror elements should move at a frequency not less than 25 to 30 Hz. 
         [0032]      FIG. 2  is a sectional view of the LCD  20  in  FIG. 1  through a portion of the light waveguide  24 R 1  shown in  FIG. 1 . The LCD  20  is shown in an assembled state. The components that are identical to those shown in  FIG. 1  are designated in  FIG. 2  by the same reference numerals. Besides those components that are shown in  FIG. 1 ,  FIG. 2  shows additional components  24 R 1   a,    24 R 1   b  that comprise auxiliary micromirrors formed on the surface of the upper cladding  24 R 1 ′ of the light waveguide  24 R 1 . The auxiliary micromirrors have their reflective surfaces facing the holographic elements  24   a,    24   b,  . . .  24   n  and are aligned with them. 
         [0033]    As described in the aforementioned patent application, the holographic elements  24   a ,  24   b  . . .  24   n  emit light in opposite directions, shown by arrows  24   a ′ and  24   a ″, and possess light-focusing properties. The aforementioned micromirrors  24 R 1   a,    24 R 1   b  . . . reflect the beams  24   a ″,  24   b ″ . . . that did not pass directly to the viewer and that sent them back to the moving micromirrors  22   a ″,  22   b ″ . . . As a result, all of the light emitted by the holographic elements is efficiently used. In  FIG. 2 , arrows C 1  and C 2  show the direction of light toward the viewer. 
         [0034]    Since the micromirrors are moving, at each moment the viewer sees the image in different micropositions, which are perceptible by the human eye as a quasistationary pattern. Each image carries its one speckles. As a result, the speckle pattern seen by the viewer is smoothened. 
         [0035]    An analytical description of speckle reduction is illustrated in the following example. The two micromirrors shown in  FIG. 2  are assumed to be so close to each other that the viewer does not see them as separate points but rather perceives them as parts of a continuous light field. This is a condition for observation on the screen of an image instead of separate pixels. Thus, the light intensity perceived from the two aforementioned micromirrors can be written as follows: 
         [0000]        I =|(√ I   1 ) e   iΦ   1 +(√ I   2 ) e   −iΦ   2 | 2 =|√ I   1 +(√ I   2 ) e   i(Φ     2     −Φ     1     ) | 2 ,  (1)
 
         [0000]    where I 1  and I 2  are light intensities produced by two mirrors independently, and Φ 1  and Φ 2  are the phases of two optical fields. Assuming that I 1 =I 2 =I 0 , (1) can be rewritten as: 
         [0000]        I =2 I   0 (1+cos (Φ 2 −Φ 1 ))  (2),
 
         [0000]    where the term “cos (Φ 2 −Φ 1 )” represents speckle formation: depending on the relative phase between two optical signals, the perceived intensity varies from zero to 4I 0 . As the relative phase depends on many different factors, such as relative positions of the viewer and the two pixels, presence of microdefects or irregularities in pixel positions, surface roughness, etc., the viewer sees strong intensity variation throughout the screen, which is called a speckle pattern. 
         [0036]    If micromirror elements shift vertically in the direction of arrows A 1  and A 2 , respectively, the interference term cos (Φ 2 −Φ 1 ) in (2) takes the form (3): 
         [0000]      cos (Φ 2 −Φ 1   +A   2 ·4 π/λ−A   1 ·4π/λ)  (3),
 
         [0000]    where λ is the light wavelength. Thus, if the mirrors move independently of one another at relatively high frequency with the amplitudes larger than ¼ of the wavelength, the viewer perceives the interference term as its time-averaged value, which is equal to zero due to the symmetry of the cosine function. As a result, the perceived intensity becomes equal to the algebraic sum of the two original pixel intensities, i.e., 2I, and speckles disappear due to the absence of the interference term. This simplified model illustrates the disclosed method applied for a particular pair of micromirrors. Taking into account that more micromirrors are used in the same manner, it is possible to explain the disclosed speckle reduction method in general. 
         [0037]      FIGS. 3 and 4  are views similar to those shown in  FIGS. 1 and 2  but illustrate an LCD  120  according to another aspect of the invention. The components of the LCD  120  that are similar to those shown in  FIGS. 1 and 2  are designated by the same reference numerals but with the addition of 100. For example, the fully reflective micromirror elements of the LCD  120  are designated by reference numerals  122   a,    122   b  . . .  122   n;  the respective liquid-crystal matrix elements are shown by reference numerals  126   a,    126   b  . . .  126   n,  etc. However, since the major parts of the LCD  120  are identical to those of the LCD  20 , not all of them are designated in the drawings of the second modification. The lasers and their laser beams are designated as in  FIGS. 1 and 2  but with an addition of a prime, i.e., LR 1 ′, LG 1 ′, LB 1 ′, IR 1 ′, IG 1 ′, and IB 1 ′. 
         [0038]    The main difference of the LCD  120  in  FIGS. 3 and 4  from the LCD in  FIGS. 1 and 2  is that the micromirrors  122   a ″,  122   b ″. . .  122   n ″ perform tilting reciprocations instead of linear reciprocations. Tilting reciprocations are shown in  FIGS. 3 and 4  by arrows B 1 , B 2  . . . Bn. 
         [0039]    Tilting is carried out in directions perpendicular to axes X1-X1, Y1-Y1 . . . Z1-Z1, as shown in  FIG. 3 . 
         [0040]    The laser beams  125   a,    125   b  . . .  125   n  reflected from the micromirrors  122   a ″,  122   b ″ . . .  122   n ″ moving in the angular direction of tilting shift along the axis X1′-X1′ that extends along the waveguide  124 R 1 . For the viewer, such movement of the image produces the same speckle smoothening effect as in the case of the first modification. 
         [0041]    The shifting of each individual beam in the transverse direction of the cell of the liquid-crystal matrix preferably should not exceed the boundaries of this cell. 
         [0042]    Although the method of the invention has been shown and described with reference to specific embodiments, these embodiments should not be construed as limiting the areas of application of the invention, and any changes and modifications are possible provided these changes and modifications do not depart from the scope of the attached patent claims. For example, the micromirrors may perform combined linear and tilting reciprocations. The movements are not necessarily periodic and may be of a random nature. The pixels and wavegudies may have different constructions and layouts.