Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N9/00—Details of colour television systems
  • H04N9/12—Picture reproducers
  • H04N9/31—Projection devices for colour picture display, e.g. using electronic spatial light modulators [ESLM]
  • H04N9/3141—Constructional details thereof
  • H04N9/315—Modulator illumination systems
  • H04N9/3167—Modulator illumination systems for polarizing the light beam
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N5/00—Details of television systems
  • H04N5/74—Projection arrangements for image reproduction, e.g. using eidophor
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N5/00—Details of television systems
  • H04N5/74—Projection arrangements for image reproduction, e.g. using eidophor
  • H04N5/7416—Projection arrangements for image reproduction, e.g. using eidophor involving the use of a spatial light modulator, e.g. a light valve, controlled by a video signal
  • H04N5/7441—Projection arrangements for image reproduction, e.g. using eidophor involving the use of a spatial light modulator, e.g. a light valve, controlled by a video signal the modulator being an array of liquid crystal cells

Definitions

• the present invention relates to image projection systems. More specifically, the invention relates to projection systems that use liquid crystal imaging panels for generating the image.
• LCD liquid crystal display
• Some LCD panels operate in a reflective mode, in which incident illumination light is separated from reflected image light by using a polarizing beamsplitter in front of the reflective LCD panel. In such configurations, the illumination light is passed to the LCD panel via the polarization beamsplitter. The illumination light incident at the LCD panel is, therefore, polarized.
• the LCD panel operates by selectively adjusting the polarization modulation of the many pixels of the panel. Those pixels associated with dark areas of the image do not alter the polarization state of the light whereas those pixels associated with bright areas of the image do alter the polarization state of the light.
• the illumination light is reflected to the LCD panel by the polarization beamsplitter as reflected light, that light which has polarization that has been changed to the polarization state that is orthogonal to the polarization state of the incident light is transmitted through the polarization beamsplitter.
• the polarization beamsplitter only the light corresponding to pixels that actively modulate the incident light is transmitted through the polarization beamsplitter to the projector's lens system, while light reflected by pixels that correspond to dark areas of the image, i.e. pixels that are not actively modulating the light, is rejected by the polarization
• the beamsplitter can be used to separate the polarization modulated image light from the unmodulated light, which results in an image beam that can be projected.
• the contrast ratio which, qualitatively, is a measure of how bright the bright pixels are compared to the dark pixels.
• the liquid crystal manifests a residual birefringence even in the off (non-modulating) state. This residual birefringence increases the amount of light in the dark state, resulting in a reduction in the image contrast ratio.
• TN twisted nematic
• STN super twisted nematic
• VAN vertically aligned nematic
• a birefringent retardation plate with the same, but opposite retardation may be used to compensate for this residual birefringence
• a quarter- wave retardation plate inserted between the polarizing beamsplitter and the LCD panel, can also be used to compensate for the residual birefringence.
• a quarter- wave retardation plate can also be used to compensate for birefringence in components of the polarizing beamsplitter.
• a compensating retardation plate is not suitable for compensating birefringence in components of the polarizing beamsplitter, and so the use of a quarter- wave retarding plate is more desirable.
• the quarter wave retardation plate is oriented so that the slow or fast axis is rotated away from being parallel to the polarization plane of the illumination light by a few degrees.
• One disadvantage of this compensation technique is the very careful alignment required to achieve the optimum orientation of the quarter wave retarder. This alignment step increases the costs of producing a projection system.
• One embodiment of the invention is directed to a projection system that includes a first image-forming device and a first polarizing beamsplitter. Illumination light passes via the first polarizing beamsplitter to the first image-forming device. A first retardation element is disposed between the first image-forming device and the first polarizing beamsplitter. A bias controller is attached to the first image-forming device and applies a bias to pixels in the dark state so as to substantially maximize contrast in image light that has passed through the first polarizing beamsplitter from the first image-forming device.
• Another embodiment of the invention is directed to a method of operating a projection system.
• the method includes illuminating an image-forming device with illumination light that has passed through a polarizing beamsplitter and through a retarding element disposed between the image-forming device and the polarizing beamsplitter. At least some of the illumination light is reflected as image light. The image light is substantially separated from non-image light using the polarizing beamsplitter.
• a compensating bias signal is applied to pixels of the image-forming device so as to substantially minimize dark state brightness of the image light.
• the device includes a first sealed unit that has a first polarizing beamsplitter separated from a first image-forming device.
• a first seal connects between the polarizing beamsplitter and the image-forming device to enclose a sealed volume enclosed by the seal, the first polarizing beamsplitter and the first image-forming device.
• a retarding element is disposed within the sealed volume and is attached to one of the first polarizing
• FIG. 1 schematically illustrates an embodiment of a projection system that compensates for residual birefringence in the image-forming devices according to principles of the present invention
• FIGs. 2 A and 2B schematically illustrate the effects of residual birefringence in an image-forming device
• FIGs. 3A-3C schematically illustrate the effects of residual birefringence with a compensating retarding element
• FIGs. 4 A and 4B schematically illustrate the effects of residual birefringence with a compensating retarding element and a bias applied to the image-forming device, according to principles of the present invention
• FIGs. 5 A and 5B schematically illustrate embodiments of sealed imaging units according to principles of the present invention.
• FIG. 6 shows a graph of dark state brightness as a function of applied bias, for various orientation angles of the retarding element.
• the invention may be used in many different types of projection system.
• One exemplary embodiment of a multi-panel projection system 100 that may incorporate the invention described below is schematically illustrated in FIG. 1.
• the projection system 100 is a three-panel projection system, having a light source 102 that generates a light beam 104, containing light in three different color bands.
• the light beam 104 is split by color splitting elements 106 for example, dichroic mirrors, into first, second and third beams 104a, 104b and 104c containing light of different colors.
• the beams 104a, 104b and 104c may be, for example, red, green and blue in color respectively.
• Beam steering elements 108 for example mirrors or prisms, may be used to steer any of the beams 104, 104a, 104b and 104c.
• the beams 104a, 104b and 104c are directed to respective image forming devices 110a, 110b and 110c which may be, for example, LCD-based reflective image-forming panels, such as liquid crystal on silicon (LCoS) panels.
• the light beams 104a, 104b and 104c are coupled to and from the respective image-forming devices 110a, 110b and 110c via respective polarizing beamsplitters (PBSs) 112a, 112b and 112c.
• PBSs polarizing beamsplitters
• the illumination light beams 104a, 104b and 104c are reflected by the PBSs 112a, 112b and 112c to the image-forming devices 110a, 110b and 110c and the resulting image light beams 114a, 114b and 114c are transmitted through the PBSs 112a, 112b and 112c.
• the illumination light may be transmitted through the PBSs to the image-forming devices, while the image light is reflected by the PBSs.
• Retardation elements I l ia, 111b, 11 Ic for example quarter- wave retardation elements, are positioned between the image-forming devices 110a, 110b, 110c, and their respective PBSs 112a, 112b, 112c.
• the retardation elements I l ia, 111b, 11 Ic may be used for compensating for residual birefringence in the image forming devices 110a, 110b, 110c and, as is explained in greater detail below, for compensating birefringence in the PBSs 112a, 112b, 112c.
• the retardation elements I l ia, 111b, 111c may be used for compensating for skew ray effects as well as residual birefringence in the image forming devices.
• the color combiner unit 116 combines image light beams 114a, 114b and 114c of different colors, for example using one or more dichroic elements.
• the illustrated exemplary embodiment shows an x-cube color combiner, but other types of combiner may be used.
• the three image beams 114a, 114b and 114c are combined in the color combiner unit 116 to produce a single, colored image beam 118 that may be directed by a projection lens system 120 to a screen (not shown).
• a controller 130 is connected to the three image forming devices 110a, 110b, 110c.
• the controller 130 applies control signals to the image forming devices 110a, 110b, 110c that controls the image formed by each image forming device. Also, for reasons that are described further below, the controller 130 applies respective bias signals to each of the image forming devices 110a, 110b, 110c for maximizing the contrast in the respective image light beams 114a, 114b, 114c.
• the controller may be coupled to a video source, such as a computer or television tuner, to receive a video signal. The video signal is processed to produce the control signals that are directed to each of the image forming devices HOa, HOb, HOc.
• projection systems may use one or more PBSs.
• a projection system may use one or two image-forming devices, with respective PBSs, as is described in greater detail in U.S. Patent Applications Serial Nos. 10/439449 and' 10/914,596, incorporated herein by reference.
• the maximum number of image- forming devices is not limited to three, and projection systems may use more or fewer than three image-forming devices.
• different types of light sources may be used, including white light sources, such as high-pressure mercury lamps, and colored light sources, such as light emitting diodes. The intention with the illustrated embodiment is not to limit how the illumination light reaching the image forming devices is generated, nor to limit how the light is processed before reaching the image forming devices.
• liquid crystal image-forming devices such as the vertically aligned nematic (VAN) mode LCD devices, have of the order of 5 nm of residual retardation in the dark state. This can be compensated with a retarder that provides at most 5 nm
• a quarter-wave retarder for compensating the residual birefringence.
• a quarter-wave retarder is effective at compensating for stress birefringence in the glass prism of a polarizing beamsplitter, e.g. see U.S. Patent Application Serial No. 11/088,153, incorporated by reference, and, in the case of the PBS being a MacNeille PBS, for compensating for skew ray birefringence.
• FIGs. 2A and 2B illustrate a situation without a retardation element between the polarizing beamsplitter 202 and the image-forming device 204.
• light 206 propagating in the z-direction from the polarizing beamsplitter 202 is assumed to linearly polarized, i.e.
• the light 210 is no longer linearly polarized, but contains a combination of polarized components, i.e. the light contains a combination of x-polarized and y-polarized components.
• the light is shown as being elliptically polarized, with the y- polarized component being detrimental to the contrast ratio of the projection system. It will be appreciated that in practical systems, it is difficult to obtain completely linearly polarized light propagating out of the polarizing beamsplitter. This description, however, is directed to that light that is polarized parallel to the x-direction.
• the light 306 propagates from the polarizing beamsplitter 302, through the retarding element 308, to the image-forming device 304.
• the light 306 is linearly polarized and then passes through the retarder element 308 to become elliptically polarized, at position B.
• the light 306 is shown having clockwise elliptical polarization.
• the reflected light 310 shown in FIG. 3B, is also elliptically polarized, but is now elliptically polarized in the counter-clockwise direction.
• the retarding element 308 provides exactly enough retardation to compensate for the residual retardation in the image-forming device 304, then the light 312 that has passed through the retarding element 308 is linearly polarized before reaching the polarizing beamsplitter 302.
• the orientation condition for exact compensation of the residual birefringence is that the retarding element is oriented to an angle, ⁇ , that is equal to ⁇ C, the angle that provides exact compensation.
• the value of ⁇ is critical to achieving high contrast: orienting the retarding element to an angle other than ⁇ C can result in a significant drop in contrast.
• the contrast can drop by 30% or more when the value of ⁇ is about ⁇ 0.5° from the value of ⁇ C.
• FIGs 4 A and 4B An exemplary arrangement for carrying out the invention is schematically illustrated in FIGs 4 A and 4B.
• light 406 propagates towards the image- forming device 404 from the polarizing beamsplitter 402.
• the light 406 is assumed to be substantially linearly polarized on exiting from the polarizing beamsplitter 402.
• the light 406 passes through the retarding element 408 and becomes elliptically polarized light 410.
• the retarding element 408 is oriented to an angle ⁇ that is not equal to ⁇ C.
• a bias controller 412 applies a bias to the image-forming device 404.
• the light 414 reflected from the image-forming device 404 has a polarization state that can be adjusted by adjusting the bias to the image-forming device.
• the polarization state is adjusted so that the reflected light 416 passing out of the retarding element 408 to the polarizing beamsplitter 402 has a substantially maximized contrast ratio, which corresponds to the light 416 having a polarization state that is substantially linear. This condition occurs when the bias voltage, Vb, applied by the controller 412 is substantially equal to Vc, the value required for maximum contrast.
• this compensation technique is to consider that the light experiences three independent sources of birefringence during the round trip between the polarizing beamsplitter and the image-forming device, namely the birefringence from the retarding element, be, the residual birefringence, br and the bias birefringence, bb.
• the residual birefringence, br and the bias birefringence, bb both increase the retardance seen by the incident polarized light ray. Maximum contrast occurs when the orientation of be is such that overall retardation is minimized. Since there is always some stray birefringence in the image-forming device, 404, another way of understanding this compensation technique is to consider that a
• compensator 408 may be oriented with its optic axis at a non-zero angle, ⁇ C, to the input polarization such that the amount of retardation the polarized light ray will experience in its round trip through the compensator 408 and image forming device 404 is minimized.
• ⁇ C non-zero angle
• the compensator 408 is rotated to an angle ⁇ > ⁇ C, the light ray 416 becomes overcompensated and the light ray 416 traveling back to the PBS 402 contains a larger y- polarization component than is desired.
• the bias applied by the image-forming device 404 through application of the bias voltage Vc, can largely compensate for this added retardation. In this way the projection system may be adjusted to best contrast through an electronic adjustment rather than a mechanical one.
• the core of a projection system can be aligned using the following steps:
• the quarter- wave retarder is attached to either the image-forming device or the polarizing beamsplitter. It may be desired to attach the quarter- wave retarder using a substantially index matching optical adhesive. This reduces the number of reflecting surfaces, which results in lower losses and reduced image ghosting.
• the image-forming devices and polarizing beamsplitters are arranged around the color combiner.
• each image-forming device (three positions, x, y, z, three angles, pitch, roll and yaw) are adjusted so that each is in focus, oriented properly in the horizontal and vertical directions and centered in the optical system and with the other image-forming devices iv)
• the dark state bias voltage, Vb, of each image-forming device is adjusted for the lowest light output from the color channel controlled by that image-forming device.
• this technique also permits a configuration that reduces the amount of dust or other unwanted particles settling on optical surfaces, particularly the image-forming device.
• Dust on the image-forming device can strongly affect both the dark and bright states of the image-forming device. In the bright state, the dust shows up as a more-or-less focused spot of the complementary color to the color channel of the image- forming device. For example, for an image-forming device in the green channel of a three- channel projection system, the dust particle produces a magenta spot. In the dark state, the dust appears as a spot of the same color as the color channel.
• the dust particle produces a green spot. It has been found to be complicated and often expensive to develop attachment methods for the image-forming devices and compensators that produce a dust- tight seal enclosing the faces of the polarizing beamsplitter, the image-forming device and both sides of a free-standing compensating retarder. By attaching the compensating retarder to either the image-forming device to the polarizing beamsplitter, the problem is significantly simplified: there is now only one gap to seal, rather than two, and the reduced number of attachment points facilitates simple sealing structures. These two possibilities are schematically illustrated in FIGs. 5A and 5B.
• a seal 508 is formed between the polarizing beamsplitter 502 and the image-forming device 504 for excluding dust from the optical surfaces.
• the retarding element 506 is attached to the polarizing beamsplitter 502 and in FIG. 5B the retarding element 506 is attached to the image-forming device 504.
• the compensating technique described above in which a bias is applied to the image-forming device 504 permits the attachment of the retarding element 506 to either the beamsplitter 502 or the image-forming device 504 to be made simply, without high precision orientation of the retarding element 506 being needed to achieve an orientation angle of exactly Oc.
• the retarding element 506 need not be attached to either the polarizing beamsplitter 502 or the image-forming device 504, but may be positioned between the polarizing beamsplitter 502 or the image-forming device 504 and supported using some other support structure.
• a graph shown the brightness of the dark state is shown in FIG. 6 for various orientations of the retarder, as a function of the bias voltage applied to the image-forming device.
• angles described above are relative to the best angle for orientation of the retarder, i.e. that orientation that produces the least bright dark state when the image- forming device is unbiased.
• this orientation corresponds to the fast or slow axis of the retarder being set at an angle to 0.247° ⁇ 0.017° (one standard deviation) relative to the polarization axis of the incident light.
• the fast or slow axis of the retarder may be set at an angle whose value differs from the value of ⁇ c by less than 1°, less than 0.5° or less than 0.25°.

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• Engineering & Computer Science (AREA)
• Multimedia (AREA)
• Signal Processing (AREA)
• Transforming Electric Information Into Light Information (AREA)
• Projection Apparatus (AREA)
• Liquid Crystal (AREA)

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• 2005
  • 2005-12-08 US US11/297,544 patent/US7542194B2/en not_active Expired - Fee Related
• 2006
  • 2006-12-01 WO PCT/US2006/046040 patent/WO2007067431A1/en not_active Ceased
  • 2006-12-01 KR KR1020087015876A patent/KR20080075211A/ko not_active Ceased
  • 2006-12-01 EP EP06838805A patent/EP1980100A4/en not_active Withdrawn
  • 2006-12-01 JP JP2008544392A patent/JP5084742B2/ja not_active Expired - Fee Related
  • 2006-12-01 CN CNB2006800460051A patent/CN100571354C/zh not_active Expired - Fee Related
  • 2006-12-07 TW TW095145724A patent/TW200727052A/zh unknown

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