Patent Publication Number: US-2003234751-A1

Title: Image display apparatus having optical scanner

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001] This application claims the benefit of Korean Application No. 2002-34647, filed Jun. 20, 2002, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002] 1. Field of the Invention  
       [0003] The present invention relates to an image display apparatus, and more particularly, to a single-panel color image display apparatus using a single display device to produce an image according to image signals input thereto, and capable of achieving the same optical efficiency and resolution as a three-panel color image display apparatus using three display devices.  
       [0004] 2. Description of the Related Art  
       [0005]FIG. 1 shows an example of a conventional single-panel color image display apparatus, and FIG. 2 shows optical paths in the micro-lens array and liquid crystal display (LCD) of FIG. 1. Referring to FIGS. 1 and 2, the conventional single-panel color image display apparatus includes an illumination optical system, three dichroic mirrors  4 R,  4 G, and  4 B, tilted with respect to each other, a micro-lens array  10 , and an LCD  20 . The illumination optical system includes a lamp  1 , which is a white light source, a spherical mirror  2  disposed to surround one side of the lamp  1 , and a condenser lens  3  that collects a divergent beam emitted from the lamp  1  and a divergent beam reflected by the spherical mirror  2 , and converts the divergent beams into collimating beams.  
       [0006] The white light emitted by the illumination optical system is separated by the three dichroic mirrors  4 R,  4 G, and  4 B into three primary color beams: red (R), green (G), blue (B). The dichroic mirror  4 R reflects the red beam of white light coming from the illumination optical system and transmits the remaining color beams. The dichroic mirror  4 G reflects the green (G) beam among the color beams transmitted through the dichroic mirror  4 R and transmits the remaining color beam, i.e., the blue (B) beam. The dichroic mirror  4 B reflects the blue (B) beam.  
       [0007] The three dichroic mirrors  4 R,  4 G, and  4 B are inclined with respect to each other with an angle of θ, thus forming a fan shape. In other words, the dichroic mirrors  4 R and  4 B are tilted with respect to the dichroic mirror  4 G with angles of −θ and +θ, respectively. Here, + and − signs denote clockwise and counterclockwise directions.  
       [0008] Thus, main rays of the red (R) and blue (B) beams are incident onto the micro-lens array  10  at angles of −θ and +θ relative to a main ray of the green (G) beam. The micro-lens array  10  includes a plurality of cylindrical micro-lens units  10   a  arranged horizontally. The micro-lens array  10  converges the red (R), green (G), and blue (B) beams incident at different angles onto signal electrodes  21 R,  21 G, and  21 B of the LCD  20  in a stripe pattern.  
       [0009] The LCD  20  is constructed of a liquid crystal layer  23  sandwiched between two sheets of transparent glass substrates  24  and  25 . The liquid crystal layer  23  has a transparent conductive layer  22  and the signal electrodes  21 R,  21 G, and  21 B are formed on either side of the conductive layer  22  in a matrix structure.  
       [0010] In the conventional single-panel color image display apparatus configured as above, the white light coming from the illumination optical system is separated into three primary colors by the three dichroic mirrors  4 R,  4 G, and  4 B and converged onto the signal electrodes  21 R,  21 G, and  21 B of the LCD  20 , respectively. The signal electrodes  21 R,  21 G, and  21 B are arranged horizontally at regular intervals due to differences in angles at which the main rays of the red (R), green (G), and blue (B) beams are incident. The signal electrodes  21 R,  21 G, and  21 B are combined into a single image pixel, each serving as a sub-pixel.  
       [0011] As described above, each micro-lens unit  10   a  corresponds to three sub-pixels of the primary colors of red (R), green (G), and blue (B). The three sub-pixels are displayed as a single image pixel, on a screen  7  which allows a viewer to see a color image composed of a plurality of the image pixels.  
       [0012] However, the conventional single-panel color image display apparatus described above is configured so that the three sub-pixels constitute a single image pixel, thereby lowering a resolution of the LCD  20  to one-third. For example, the LCD  20  needs to have a physical resolution three times larger than it actually has in order to obtain the same resolution as a projection single-panel image display apparatus using a color wheel, as disclosed in U.S. Pat. Nos. 5,633,755 and 5,159,485.  
       [0013] If the physical resolution of the LCD  20  is increased three times as described above, the aperture ratio decreases to reduce optical efficiency. A reduced optical efficiency lowers yield and increases the manufacturing cost. Additionally, the size of the LCD  20  may be increased in order to increase its physical resolution three times. As the size of the LCD  20  increases, the size of the condenser lens  3 , a field lens  5 , or a projection lens  6  increases, thereby increasing the manufacturing cost.  
       SUMMARY OF THE INVENTION  
       [0014] Accordingly, it is an aspect of the present invention to provide a projection single-panel color image display apparatus having a compact structure, low price, high brightness, high resolution, and high optical efficiency as compared with a three-panel color image display apparatus.  
       [0015] Additional aspects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.  
       [0016] The foregoing and/or other objects of the present invention are achieved by providing an image display apparatus, including an illumination optical system to emit a beam; a spectrometer that separates the beam emitted from the illumination optical system by reflecting and diffracting the beam at different diffraction angles according to a wavelength of the beam while rocking back and forth at a predetermined angle with respect to a rotational axis; a relay lens system that converts the beam separated by the spectrometer into parallel beams; an image optical system that produces an image by reflecting the parallel beams incident from the relay lens system; a screen; and a projection optical system that projects the beams reflected by the image optical system onto the screen.  
       [0017] Here, the spectrometer may be an optical scanner that reflects and diffracts the beam coming from the illumination optical system according to a wavelength of the beam. Alternatively, the spectrometer may include a plurality of optical scanners arranged in an array and sequentially driven by a vertical synchronization signal.  
       [0018] The optical scanner may have on its surface a hologram layer that separates the beam coming from the illumination optical system by reflecting and diffracting the beam according to its wavelength. The hologram layer may be a reflective type hologram layer.  
       [0019] The optical scanner may have on its surface a diffraction grating layer that separates the beam incident from the illumination optical system by reflecting and diffracting the beam according to its wavelength. The illumination optical system may be driven so as not to emit any beam during a return time when the spectrometer is returned to its original position. The time it takes for the spectrometer to return to its original position may be set as approximately one-tenth ({fraction (1/10)}) of a period (T).  
       [0020] Here, the illumination optical system may include a plurality of light sources to emit a plurality of color beams including red, green, and blue beams; a color filter disposed in front of the plurality of light sources to make the plurality of color beams propagate along a common axis in order to produce white light; and a collimator lens that converges the white light onto the spectrometer.  
       [0021] Each of the plurality of light sources may be a light-emitting diode (LED), laser diode (LD), or an arc lamp.  
       [0022] The relay lens system may include a collimator lens that makes beams reflected from the spectrometer parallel to an optical axis; a fly lens that makes the beams passing through the collimator lens correspond on a one-to-one basis to the image optical system; and a field lens that converts the beam passing through the fly lens into parallel beams. The fly lens may have two lenslets provided for each optical scanner. Here, the image optical system includes: a panel that modulates the beam passing through the relay lens system and produces an image; and an optical path splitting unit disposed between optical paths of the relay lens system and the panel in order to direct the beam coming from the relay lens system to the panel while directing the beam reflected off the panel to the projection optical system.  
       [0023] The image display apparatus according to the embodiment of the present invention is configured to have a spectrometer, to reflect and diffract the beams according to beam wavelengths between the illumination optical system and the relay lens system. The white light is separated by the spectrometer into different color beams and the resultant beams are mapped on a one-to-one correspondence to a single panel, thereby providing a full-size color image. The embodiment of the present invention makes it possible to offer a low priced, high brightness, and high resolution projection single-plate color image display apparatus. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0024] These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the preferred embodiment, taken in conjunction with the accompanying drawings of which:  
     [0025]FIG. 1 shows an example of a conventional single-plate color image display apparatus;  
     [0026]FIG. 2 shows optical paths in the micro-lens array and liquid crystal display (LCD) of FIG. 1;  
     [0027]FIG. 3 shows the configuration of an image display apparatus according to an embodiment of the present invention;  
     [0028]FIG. 4 is a perspective view of an optical scanner used in the image display apparatus of FIG. 3, according to the embodiment of the present invention of FIG. 3;  
     [0029]FIG. 5A is a graph illustrating a first method of driving an optical scanner, according to the embodiment of the present invention of FIG. 3;  
     [0030]FIG. 5B shows vertical scanning performed by the first method of driving an optical scanner, according to the embodiment of the present invention of FIG. 3;  
     [0031]FIG. 6A is a graph illustrating a second method of driving an optical scanner, according to the embodiment of the present invention of FIG. 3;  
     [0032]FIG. 6B shows vertical scanning performed by the second method of driving an optical scanner, according to the embodiment of the present invention of FIG. 3; and  
     [0033]FIG. 7 shows the configuration of an image display apparatus according to another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0034] Reference will now be made in detail to the present preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout.  
     [0035] Referring to FIG. 3, a single-panel color image display apparatus according to a first embodiment of the present invention includes: an illumination optical system  31  that emits beams; and a spectrometer  33  that separates the beams by reflecting and diffracting beams coming from the illumination optical system  31  at different diffraction angles according to beams&#39; wavelength, while periodically rotating back and forth for specific angles with respect to a rotational axis. The apparatus also includes a relay lens system  35  that converts the beams separated by the spectrometer  33  into parallel beams; an image optical system  37  that performs modulation on the incident beams from the relay lens system  35  and produces an image; and a projection optical system  39  that projects the beams reflected off the image optical system  37  onto a screen (not shown).  
     [0036] The illumination optical system  31  includes a plurality of light sources  31 - 1  to emit color beams including red, green, and blue, a color filter  31 - 2  positioned in front of the plurality of light sources  31 - 1  and designed so that the color beams propagate along a common axis and are combined to produce white light, and a converging lens  31 - 3  that converges the white light onto the spectrometer  33 . Here, the plurality of light sources  31 - 1  are three individual light-emitting devices  32   a ,  32   b , and  32   c  such as light emitting diodes (LEDs) or laser diodes (LDs).  
     [0037] The color filter  31 - 2  changes optical paths of two color beams among the red, green, and blue beams incident parallel to each other along different optical paths to an optical path of the remaining beam, and produces white light so that the three beams propagate along the same optical path. A dichroic X, cross prism, or Total International Reflection (TIR) prism serving the same purpose as the color filter  31 - 2  may instead be used as a color separation/combination system in place of the color filter  31 - 2 .  
     [0038] The converging lens  31 - 3  converges the white light coming from the color filter  31 - 2  onto the spectrometer  33 . The spectrometer  33  includes an optical scanner  34  having on its surface a hologram layer or diffraction grating layer to reflect and diffract the beam incident from the converging lens  31 - 3  according to the beam&#39;s wavelength. The optical scanner  34  diffracts and reflects the beam while periodically rotating back and forth at predetermined angles as a result of driving signals and separates the beam according to its wavelength.  
     [0039]FIG. 4 shows the optical scanner  34  used in the color image display apparatus according to the first embodiment of the present invention. Referring to FIG. 4, the optical scanner  34  may include a substrate  45 , a stage  41   a  disposed above the substrate  45 , a pair of rotational axes  42  connected to either side of the stage  41   a , driving comb electrodes  43  arranged in parallel on the bottom of the stage  41   a , and fixed comb electrodes  44  arranged alternately in parallel with the driving comb electrodes  43  on the substrate  45 . A hologram layer  41  is placed on the stage  41   a . In this case, a diffraction grating layer having a large number of grooves or slits cut in the surface may be formed in place of the hologram layer  41 .  
     [0040] The stage  41   a  rocks back and forth at a predetermined angle φ relative to the pair of rotational axes  42  provided on either side of the stage. Angle φ represents the maximum slope angle due to an electrostatic attractive force (also called a “Coulomb force”) between the driving comb electrodes  43  and the fixed comb electrodes  44 . That is, if an attractive force is exerted between the driving comb electrodes  43  and the fixed comb electrodes  44  disposed on the left side with respect to the pair of rotational axes  42 , the stage  41   a  is tilted to the left. In contrast, if an attractive force is exerted between the driving comb electrodes  43  and the fixed comb electrodes  44  disposed on the right side with respect to the rotational axes  42 , the stage  41   a  is tilted to the right. To repeatedly induce a rocking motion, a voltage is repeatedly applied between the driving comb electrodes  43  and the fixed comb electrodes  44  disposed on the left and right sides to alternately produce the electrostatic attractive force. Here, the stage  41   a  may be driven at a maximum slope angle φ of approximately between 8 and 10 degrees, while the optical scanner  34  may scan at a vertical scanning speed of 60 Hz.  
     [0041]FIG. 5A is a graph illustrating an example of a vertical scanning signal applied across the optical scanner  34 , and FIG. 5B shows a vertical scanning method performed by the optical scanner  34  according to the vertical scanning signal illustrated in FIG. 5A. Referring to FIGS. 5A and 5B, a vertical scanning signal is applied across the optical scanner  34  for a period T, and the optical scanner  34  rocks back and forth between 0 and 9 degrees. First, when a vertical scanning signal is applied for a period of {fraction (9/10)} T along a solid line, as shown in FIG. 5A, the stage  41   a  is tilted from left (0 degrees) to right (9 degrees) and a beam is scanned from top to bottom along a solid line onto a panel  37 - 1 , as shown in FIG. 5B. Then, when a vertical scanning signal is applied for a return time of {fraction (1/10)} T along a dotted line, as shown in FIG. 5A, the stage  41   a  returns from right (9 degrees) to left (0 degrees) along a dotted line as shown in FIG. 5B. During this return time, no beam is applied through the illumination optical system  31 . By repeatedly rocking from left to right, the optical scanner  34  produces an image.  
     [0042] However, the rocking motion performed by the optical scanner  34  does not scan the beam onto the panel  37 - 1  during the return time. Thus, it is highly desirable to adopt a driving method in which the beam is also scanned during the return time.  
     [0043]FIG. 6A is a graph illustrating another example of a vertical scanning signal applied across the optical scanner  34 , and FIG. 6B shows a vertical scanning method performed by the optical scanner  34  according to the vertical scanning signal illustrated in FIG. 6A. Referring to FIGS. 6A and 6B, first, when a vertical scanning signal is applied across the optical scanner  34  for a period of {fraction (1/2)} T along an ascending solid line, as shown in FIG. 6A, the stage  41   a  is tilted from left (0 degrees) to right (9 degrees) and a beam is scanned from top to bottom of the panel  37 - 1  as shown in FIG. 6B. Then, when a vertical scanning signal is applied during a return time of {fraction (1/2)} T along a solid line in the graph, as shown in FIG. 6A, the stage  41   a  is tilted from right (9 degrees) to left (0 degrees) and a beam is scanned from the bottom to the top of the panel  37 - 1 , as shown in FIG. 6B. Thus, this approach scans the beam during a return time as opposed to the method illustrated in FIG. 5A.  
     [0044] In the color image display apparatus according to the first embodiment of the present invention, white light is separated into red, green, and blue beams and horizontally scanned onto the panel  37 - 1  by diffracting the white light at a different diffraction angle (Δθ) at the hologram layer  41  formed on the stage  41   a . This is done according to the light wavelength, while driving the optical scanner  34  according to the methods shown in FIGS. 5A and 6A. For example, the white light may be diffracted at diffraction angles having a difference of about 5 degrees for each wavelength.  
     [0045] The hologram layer  41  is formed as a reflective type hologram in which a hologram recorded as a result of interference of a reference beam and an object beam is reproduced by illuminating it with a reconstruction beam having the same incidence angle as the reference beam which is diffracted or reflected from the hologram. A diffraction grating layer having a large number of grooves or slits cut in a metal layer such as aluminum (Al) or lead (Pb) may be attached onto the stage  41   a  in place of the hologram layer  41 . However, examples of a spectrometer stacked on the stage  41   a  are not limited to this example and may include any device that can separate a color beam according to the wavelength of each color beam.  
     [0046] That is, the image display apparatus, according to the first embodiment of the present invention, having a spectrometer on top of the optical scanner  34  can improve resolution and optical efficiency only with the panel  37 - 1  used as an optical modulator.  
     [0047] A beam emitted from the illumination optical system  31  is diffracted, reflected, and separated into different optical paths according to its wavelength by the spectrometer  33 . Then, the separated beams are incident onto the relay lens system  35 .  
     [0048] Referring back to FIG. 3, the relay lens system  35  includes a collimator lens  35 - 1  that makes optical paths of the beams reflected from the optical scanner  34  at different angles parallel to an optical axis  38 , first and second fly lenses  35 - 2  and  35 - 3  that make each color beam passing through the collimator lens  35 - 1  correspond on a one-to-one basis over the entire surface of the panel  37 - 1 , and first and second field lenses  35 - 4  and  35 - 5  that convert the beams passing through the first and second fly lenses  35 - 2  and  35 - 3  into parallel beams and propagate the parallel beams to the panel  37 - 1 . Here, the two lenslets  30   a  and  30   b  are respectively arranged on the surfaces of the first and second fly lenses  35 - 2  and  35 - 3  and two lenslets per scanner are prepared.  
     [0049] The collimator lens  35 - 1 , located a focal length away from the center of the optical scanner  34 , makes each color beam incident at a predetermined angle parallel with respect to the optical axis  38 . The beams sequentially passing through the collimator lens  35 - 1 , the first and second focusing lenses  35 - 2  and  35 - 3 , and the first and second field lenses  35 - 4  and  35 - 5  are incident onto the image optical system  37 , which in turn performs optical modulation on the beams to produce an image.  
     [0050] As shown in FIG. 3, the image optical system  37  includes the panel  37 - 1 , which modulates the beams passing through the relay lens system  35  according to an image signal and produces an image, and the optical path splitting unit  37 - 2 , disposed between optical paths of the relay lens system  35  and the panel  37 - 1 , to direct the beam coming from the relay lens system  35  to the panel  37 - 1  while directing the beam reflected off the panel  37 - 1  to the projection optical system  39 . Here, a DMD device and a TIR prism may be used for the panel  37 - 1  and the optical path splitting unit  37 - 2 , respectively. Alternatively, a liquid crystal on silicon (LCOS) and a polarization beam splitter (PBS) may be used for the same purpose.  
     [0051] The projection optical system  39  has a projection lens  36  that illuminates a screen (not shown) with the beams reflected from the panel  37 - 1  and emitted through the optical path splitting unit  37 - 2  and reconstructs the image produced by the panel  37 - 1 .  
     [0052] A process for reconstructing an image will now be described. A beam emitted from the illumination optical system  31  is reflected, diffracted at different diffraction angles, and separated into different color beams according to its wavelength by the spectrometer  33 . The separated color beams are converted into a parallel beam after passing through the relay lens system  35  and incident onto the image optical system  37 . The image optical system  37  modulates each incident color beam and directs the resultant beam to the projection optical system  39 , and the projection optical system  39  projects the produced image onto the screen and enlarges the image.  
     [0053]FIG. 7 shows the configuration of a color image display apparatus according to a second embodiment of the present invention. Referring to FIG. 7, the color image display apparatus includes an illumination optical system  51  to emit white light, a spectrometer  53  having a plurality of optical scanners  54   a - 54   e  arranged in an array so as to form an area over which the white light is emitted, a relay lens system  55  that converts each color beam reflected, diffracted and separated by the spectrometer  53  into a parallel beam, an image optical system  57  that modulates the beam passing through the relay lens system  55  and produces an image, and a projection optical system  59  that projects the beam emitted from the image optical system  57  onto a screen (not shown) and reconstructs the image.  
     [0054] The illumination optical system  51  includes an arc lamp  51 - 1  as a lamp type light source to emit white light. The arc lamp  51 - 1  is configured to have a light-emitting source  52   a  at the center surrounded by an arc-shaped reflector  52   b  so that a beam emitted from the light-emitting source  52   a  may be directed as parallel beams or reflected by the reflector  52   b  to produce parallel beams. A predetermined optical element  51 - 2  may be further provided in front of the arc lamp  51 - 1  to convert the incident beam into uniform parallel beams. A focusing lens array, a converging lens, etc., may be used as the predetermined optical element  51 - 2 .  
     [0055] The spectrometer  53  separates the beams according to their wavelength while repeatedly rocking back and forth at a predetermined angle by sequentially driving the plurality of optical scanners  54   a - 54   e  according to an applied vertical synchronization signal. The optical scanners  54   a - 54   e  are the same as the optical scanner  34  in the color image display apparatus according to the first embodiment of the present invention in terms of configuration and driving method using a vertical scanning signal, and thus a detailed description thereof will be omitted.  
     [0056] A difference lies in the manner of driving the optical scanners. That is, in the color image display apparatus according to the second embodiment, vertical scanning signals driving the optical scanners  54   a - 54   e  are applied sequentially.  
     [0057] The relay lens system  55  includes a collimating lens  55 - 1 , first and second fly lenses  55 - 2  and  55 - 3 , and first and second field lenses  55 - 4  and  55 - 5 . These lenses have the same function as those in the relay lens system  35  according to the first embodiment of the present invention. A difference is that ten lenslets are respectively arranged on the first and second focusing lenses  55 - 4  or  55 - 5  since two outlets are provided for each of the optical scanners  54   a - 54   e . That is, the number of the lenslets  50   a  is twice as much as the number of the optical scanners  54   a - 54   e . Thus, as the number of the optical scanners  54  increases, the number of the lenslets  50   a  may increase twice as much as the number of the scanners  54 .  
     [0058] The image optical system  57  includes a panel  57 - 1  that modulates the incident beam and produces an image and an optical path splitting unit  57 - 2  that separates the incident beam from the exit beam. The projection lens system  59  includes a projection lens  56 . The panel  57 - 1 , optical path splitting unit  57 - 2 , and the projection lens  56  have the same functions as described in the color image display apparatus according to the first embodiment of the present invention.  
     [0059] As described above, the single-panel color image display apparatus according to the embodiments of the present invention is constructed to have a beam separation layer such as a hologram layer to separate a beam on the surface of an optical scanner that repeatedly rocks back and forth at a predetermined angle. The hologram layer acts to reflect and diffract the white light at a different diffraction angle according to each wavelength so that each resultant beam corresponds on a one-to-one basis to each horizontal scanning line of a panel, thus providing a high brightness and high resolution image using a single panel. Thus, the image display apparatus according to the embodiment of the present invention has the advantage of low manufacturing cost and compact structure.  
     [0060] Although a few preferred embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.