Abstract:
A display device using a light modulator and having an improved numerical aperture (NA) of an after-edge lens system is disclosed. The display device includes an illumination lens, a diffractive light modulator, an NA improvement unit, a filter system and a projection system. The illumination lens converts light into linear parallel light, and outputs the linear parallel light. The diffractive light modulator produces diffracted light beams having a plurality of diffraction orders by modulating the linear parallel light incident from the illumination lens according to an external control signal. The NA improvement unit causes + and − diffracted light beams of the diffracted light beams to come close to each other. The filter system passes only some of the diffracted light beams having predetermined orders, therethrough. The projection system focuses the diffracted light beams onto an object and allows the focused diffracted light to scan the object.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates generally to a display device using a light modulator and, more particularly, to a display device using a light modulator and having an improved numerical aperture of an after-edge lens system, in which the numerical aperture of the lens system, which is used to focus diffracted light beams having + and − orders that are formed by the light modulator, is significantly reduced.  
         [0003]     2. Description of the Related Art  
         [0004]     With the development of micro technology, so-called Micro-Electro-Mechanical System (MEMS) devices and small-sized apparatuses into which MEMS devices are assembled are attracting attention.  
         [0005]     An MEMS device constitutes a microstructure on a substrate, such as a silicon substrate or glass substrate, and is a device that is formed by electrically and mechanically connecting a driving body for outputting mechanical driving force to a semiconductor integrated circuit for controlling the driving body. A basic feature of the MEMS device is that the driving body having a mechanical structure is placed in a portion of the MEMS device. The driving body is electrically operated using Coulomb&#39;s force generated between electrodes.  
         [0006]      FIGS. 1 and 2  show a representative construction of an optical MEMS device that uses the reflection or diffraction of light and is applied to an optical switch and an optical modulation element.  
         [0007]     An optical MEMS device  1  shown in  FIG. 1  includes a substrate  2 , a substrate-side electrode  3  formed on the substrate  2 , a crossbeam  6  provided with a driving-side electrode  4  that is disposed parallel to the substrate-side electrode  3 , and a support  7  configured to support one end of the crossbeam  6 . The crossbeam  6  and the substrate-side electrode  3  are electrically insulated from each other by an aperture  8  therebetween.  
         [0008]     The substrate  2  may be formed of a substrate in which an insulation film is formed on a semiconductor substrate such as a silicon (Si) or gallium arsenide (GaAs) substrate, or an insulation substrate such as a glass substrate. The substrate-side electrode  3  may be formed of a poly-crystal silicon film doped with an impurity, or a metallic film such as a Cr deposition film. The crossbeam  6  includes an insulation film  5  such as a silicon nitride film (SiN film), and a driving-side electrode  4  composed of, for example, an aluminum (AL) film that is formed on the insulation film  5  to have a film thickness of about 100 nm, and that is adapted to also function as a reflective film. The crossbeam  6  is mounted in a cantilever manner such that only one end thereof is supported by the support  7 .  
         [0009]     In the optical MEMS device  1 , the crossbeam  6  is displaced by electrostatic attraction or electrostatic repulsion that is generated between the crossbeam  6  and the substrate-side electrode  3  by voltage applied to the substrate-side electrode  3  and the driving-side electrode  4 . For example, the crossbeam  6  is displaced between an equilibrium state and a tilt state with respect to the substrate-side electrode  3 , as shown in the solid and dotted lines of  FIG. 1 .  
         [0010]     Another optical MEMS device  11  shown in  FIG. 2  includes a substrate  12 , a substrate-side electrode  13  formed on the substrate  12 , and a beam  14  formed across the substrate-side electrode  13  in the form of a bridge. The crossbeam  14  and the substrate-side electrode  13  are electrically insulated from each other by an aperture  10  that is positioned therebetween.  
         [0011]     The crossbeam  14  includes a bridge member  15  formed on the substrate  12  across the substrate-side electrode  13  in the form of a bridge and formed of, for example, an SiN film, and a driving-side electrode  16  formed on the bridge member  15  parallel to the substrate-side electrode  13 , adapted to serve as a reflective film, and formed of, for example, an Al film having a film thickness of about 100 nm. The substrate  12 , the substrate-side electrode  13  and the crossbeam  14  may have the same construction and material as described in conjunction with  FIG. 1 . The crossbeam  14  is mounted in a so-called cantilever manner such that only one end thereof is supported by the support  7 .  
         [0012]     In this optical MEMS device  11 , the crossbeam  14  is displaced by electrostatic attraction or repulsion that is generated between the MEMS device and the substrate-side electrode  13  by voltage applied to the substrate-side electrode  13  and the driving-side electrode  16 . For example, the crossbeam  6  can be displaced between an equilibrium state and a concave state with respect to the substrate-side electrode  3 , as shown by the solid and dotted lines of  FIG. 2 .  
         [0013]     The optical MEMS devices  1  and  11  can be applied as an optical switch having a switch function, in which, when light is radiated onto the surface of each of the driving-side electrodes  4  and  16  also serving as optical reflective films, reflected light is detected in one direction based on the fact that the reflection direction of light varies depending upon the driving position of the crossbeam  6  or  14 .  
         [0014]     Furthermore, the optical MEMS devices  1  and  11  can be applied as an optical modulation element that modulates the intensity of light. In the case where the reflection of light is used, the intensity of light is modulated by vibrating the crossbeams  6  and  14  based on the amount of reflected light in one direction per unit time. The optical modulation element uses so-called time modulation.  
         [0015]     In the case where the diffraction of light is used, an optical modulation element is formed by parallelly arranging a plurality of crossbeams  6  with respect to common substrate-side electrodes  3  and  13 , and the height of driving-side electrodes also serving as optical reflective films is changed by the approach and separation of an alternate crossbeam  6  or  14  to and from the common substrate-side electrodes  3  and  13 . The intensity of light, which is reflected by the driving-side electrodes, is then modulated via diffraction. This optical modulation element employs so-called spatial modulation.  
         [0016]      FIG. 3A  and  FIG. 3B  shows the construction of a Grating Light Valve (GLV) device that was developed by Silicon Light Machines (SLM) Corporation as an optical intensity conversion device for a laser display, i.e., a light modulator.  
         [0017]     As shown in  FIG. 3A and 3B , in the GLV device  21 , a common substrate-side electrode  23  made of a high melting point metal, such as tungsten or titanium, and a nitride film thereof or a thin polysilicon film is formed on an insulation substrate  22  such as a glass substrate. A plurality of, in this example, six beams  24  ( 24   1 ,  24   2 ,  24   3 ,  24   4 ,  24   5  and  24   6 ) are formed parallel to each other across the substrate-side electrode  23  in the form of a bridge. The substrate-side electrodes  23  and the crossbeams  24  have the same construction as described in conjunction with  FIG. 2 . That is, a crossbeam  24  is fabricated by forming a driving-side electrode  26 , which also serves as a reflective film and is made of an Al film having a thickness of about 100 nm, on the surface of a bridge member  25  that is parallel to the substrate-side electrode  23  and is formed of a SiN film.  
         [0018]     A bridge member  25  and crossbeams  24  composed of the driving-side electrodes  26  and adapted to also serve as a reflective film constitute a part that is commonly called a ribbon.  
         [0019]     The Al film used as the material of the driving-side electrodes  26  of the crossbeams  24  is a desired material for optical elements because (1) it can be formed relatively easily, (2) the wavelength dispersion of reflectance in a visible light region is small, (3) a natural Al oxide film created on the surface of an Al film serves as a protection film to protect a reflective surface.  
         [0020]     Meanwhile, a SiN (silicon nitride) film constituting the bridge member  25  is a SiN film formed by a reduced pressure CVD method. The SiN film has physical properties, such as strength and a coefficient of elasticity, which are suitable for the mechanical driving of the bridge member  25 .  
         [0021]     If a small voltage is applied between the substrate-side electrode  23  and the driving-side electrodes  26  also serving as the reflective film, the crossbeams  24  approach the substrate-side electrode  23  due to the above-described electrostatic phenomenon. If the application of the voltage is stopped, the crossbeams  24  return to their original state.  
         [0022]     The GLV device  21  alternately changes the height of the driving-side electrode  26  also serving as the optical reflective film via the approach and separation operations of the crossbeams  24  with respect to the substrate-side electrodes  23  (i.e., the approach and separation operations of the crossbeams), and modulates the intensity of light, which is reflected from the driving-side electrodes  26  by diffraction (one optical spot is projected for all the six beams  24 ).  
         [0023]     The dynamic characteristics of the crossbeams that are driven using electrostatic attraction and repulsion are mostly determined by the material properties of a SiN film formed by the CVD method. The Al film usually serves as a mirror.  
         [0024]      FIG. 4  is a sectional view illustrating a depression-type diffractive light modulator using a piezoelectric material, which was developed by Samsung Electro-Mechanics.  
         [0025]     Referring to  FIG. 4 , the depression-type thin film piezoelectric light modulator developed by Samsung Electro-Mechanics includes a silicon substrate  40  and a plurality of elements  42   a  to  42   n.    
         [0026]     In this case, the elements  42   a  to  42   n  have uniform widths, are alternately arranged, and form the depression-type thin film piezoelectric light modulator. Alternatively, the elements  42   a  to  42   n  may be alternately arranged to have different widths and may form the depression-type thin film piezoelectric light modulator. Meanwhile, the elements  42   a  to  42   n  may be spaced apart from one another by regular intervals (each of the intervals is substantially identical to the width of the elements), in which case a micromirror layer formed on the entire top surface of the silicon substrate  40  diffracts incident light by reflecting the light.  
         [0027]     The silicon substrate  40  has a depressed portion to provide an air gap to the elements  42   a  to  42   n . An insulation layer  41  is deposited on the top surface of the silicon substrate  40 . The ends of the elements  42   a  to  42   n  are attached to both ends of the silicon substrate  40  beside the depressed portion.  
         [0028]     The elements  42   a  (although only the element  42   a  is described herein, the remaining elements  42   b  to  42   n  have the same construction and operation) has a rod shape. The element  42   a  includes a bottom support  43   a , the bottom surfaces of both ends of which are attached to both ends of the silicon substrate  40  beside the depressed portion of the silicon substrate  40  so that the center portion of the element  42   a  can be spaced apart from the depressed portion of the silicon substrate  40 , and the center portion of which is located above the depressed portion of the silicon substrate  40  and can move perpendicularly.  
         [0029]     The element  42   a  further includes a bottom electrode layer  44   a  formed on the left side of the bottom support  43   a  and adapted to provide piezoelectric voltage, a piezoelectric material layer  45   a  formed on the bottom electrode layer  44   a  and adapted to contract and expand and, thus, generate perpendicular driving force when voltage is applied to both ends thereof, and a top electrode layer  46   a  formed on the piezoelectric material layer  45   a  and adapted to provide piezoelectric voltage to the piezoelectric material layer  45   a.    
         [0030]     The element  42   a  further includes a bottom electrode layer  44   a ′ formed on the right side of the bottom support  43   a  and adapted to provide piezoelectric voltage, a piezoelectric material layer  45   a ′ formed on the bottom electrode layer  44   a ′ and adapted to contract and expand and, thus, generate perpendicular driving force when voltage is applied to both ends thereof, and a top electrode layer  46   a ′ formed on the piezoelectric material layer  45   a ′ and adapted to provide piezoelectric voltage to the piezoelectric material layer  45   a′.    
         [0031]     Korean Pat. Appl. No. 2004-74875 discloses a projection-type light modulator in detail, in addition to the depression-type light modulator described above.  
         [0032]      FIG. 5  illustrates an example of an optical apparatus, which employs a GLV device, that is, an optical modulation device, using a MEMS device, or the piezoelectric diffractive light modulator made by Samsung Electro-Mechanics. In this example, a case where the optical apparatus is applied to a laser display is described.  
         [0033]     A laser display  51  related to the example is used as a projector for a large screen, more particularly, a digital image projector, or as an image projection device for a computer.  
         [0034]     As shown in  FIG. 5 , the laser display  51  includes a laser light source  52 , a mirror  54  disposed opposite the laser light source  52 , an illumination optical system (lens group)  56  and a GLV device or a piezoelectric diffractive light modulator  58  that serves as an optical modulation element.  
         [0035]     The laser display  51  further includes a mirror  60  for reflecting laser light the optical intensity of which is modulated by the GLV device or piezoelectric diffractive light modulator  58 , a projection lens  62 , a filter  64 , a diffuser  66 , a mirror  68 , a galvano scanner  70 , a projection optical system (lens group)  72  and a screen  74 .  
         [0036]     In the conventional laser display  51 , laser light radiated from the laser light source  52  is incident on the GLV device or piezoelectric diffractive light modulator  58  through the mirror  54  from the illumination optical system  56 .  
         [0037]     Further, the laser light is spatially modulated by being diffracted by the GLV device or piezoelectric diffractive light modulator  58 , reflected by the mirror  60 , and then separated by the projection lens  62  on a diffraction order basis. Thereafter, only signal components are extracted from the laser light by the filter  64 .  
         [0038]     Thereafter, the laser spectrum of the image signal is reduced by the diffuser  66 , and spread over the space by the galvano scanner  68  synchronized with the image signal through the mirror  68 , and is then projected by the projection optical system  70  onto the screen  72 .  
         [0039]     According to the prior art, if the distance between the diffraction gratings of the diffractive light modulator is shortened, the diffraction angle increase. As a result, the Numerical Aperture (NA) of the lens system located behind the projection lens increases.  
         [0040]      FIG. 6A  shows an example of a prior art optical system having a high diffraction angle. If the diffraction angle θ is large, the NA of the projection lens increases.  
         [0041]      FIG. 6B  is a view illustrating another example of a prior art optical system having a high diffraction angle. If the incidence angles of illumination beams are different but the diffraction angle θ is large, the NA of the projection lens increases, which is the same as in the embodiment of  FIG. 6A . As described above, when the NA of the lens system, such as a projection lens, located behind the diffractive light modulator increases, there are many limitations in designing the laser display. Further, if the NA is large, there great difficulty in designing a lens because F/# is low.  
         [0042]     Moreover, light progressing toward the center of the after-edge lens system, such as the projection lens, forms a radical axis optical system, which improves the performance of the lens. However, the structures of  FIGS. 6A and 6B  are disadvantageous in that the central portion of the after-edge lens system is not used but the peripheral portion of the after-edge lens system is used, so that it is difficult to expect good performance.  
       SUMMARY OF THE INVENTION  
       [0043]     Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a display device using a light modulator, in which the NA of a lens system, which is used to focus diffracted light beams having + and − orders that are formed by light modulators, is significantly reduced.  
         [0044]     In order to accomplish the above object, the present invention provides a display device using a light modulator and having an improved numerical aperture (NA) of an after-edge lens system, including an illumination lens for converting light output from a light source into linear parallel light, and outputting the linear parallel light; a diffractive light modulator for producing diffracted light beams having a plurality of diffraction orders by modulating the linear parallel light incident from the illumination lens according to an external control signal; an NA improvement unit for causing + and − diffracted light beams of the diffracted light beams to come close to each other; a filter system for passing only some of the diffracted light beams having predetermined orders, therethrough; and a projection system for focusing the diffracted light beams onto an object and allowing the focused diffracted light to scan the object. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0045]     The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:  
         [0046]      FIGS. 1 and 2  are views illustrating the representative constructions of optical MEMS devices that use the reflection or diffraction of light and are applied to an optical switch and an optical modulation element;  
         [0047]      FIG. 3A and 3B  shows the construction of a GLV device that was developed by SLM Corporation as an optical intensity conversion device for a laser display, i.e., a light modulator;  
         [0048]      FIG. 4  is a sectional view showing a depression-type diffractive light modulator using a piezoelectric material, which was developed by Samsung Electro-Mechanics;  
         [0049]      FIG. 5  is a diagram showing an example of an optical apparatus, which employs a GLV device, that is, an optical modulation device, using a MEMS device, or the piezoelectric diffractive light modulator made by Samsung Electro-Mechanics;  
         [0050]      FIG. 6A  is a view illustrating an example of a conventional optical system having a high diffraction angle, and  FIG. 6B  is a view showing another example of the conventional optical system having a high diffraction angle;  
         [0051]      FIG. 7  is a view showing the construction of a display device using a light modulator and having an improved NA of an after-edge lens system according to an embodiment of the present invention;  
         [0052]      FIG. 8A-8C  is a view showing the path of light passed through an illumination lens of  FIG. 7 ;  
         [0053]      FIG. 9  is a view showing an embodiment of the diffractive light modulator of  FIG. 7 ;  
         [0054]      FIGS. 10A and 10B  are views showing the diffraction angles of diffracted light beams generated by the diffractive light modulator;  
         [0055]      FIG. 11  is a view illustrating an improvement in the NA and compensation for the difference between optical paths;  
         [0056]      FIG. 12  is a view showing the path of light passed through the projection lens of  FIG. 7 ; and  
         [0057]      FIG. 13  is a front view showing the spatial filter of  FIG. 7 . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0058]     The present invention is described in detail in connection with preferred embodiments with reference to FIGS.  7  to  13  below.  
         [0059]      FIG. 7  is a view illustrating the construction of a display device using a light modulator and having an improved NA of an after-edge lens system according to an embodiment of the present invention.  
         [0060]     Referring to  FIG. 7 , the display device using the light modulator and having the improved NA of the after-edge lens system according to the embodiment of the present invention includes a light source  700 , an illumination lens  710 , a diffractive light modulator  720 , an optical path compensator, a filter system  740 , a projection system  750  and a screen  760 .  
         [0061]     A light source fabricated using a semiconductor, such as a Light Emitting Diode (LED) or Laser Diode (LD), may be used as the light source  700 .  
         [0062]     A cross section of the light sources  700  is shown in “A” of  FIG. 8A-8C . Referring to “A” of  FIG. 8A-8C , the cross section of the light source  700  is circular, and the intensity profile of the light beam has Gaussian distribution as shown in “B” of  FIG. 8A-8C .  
         [0063]     The illumination lens  710  converts incident light into linear parallel light having an elliptical cross section. The illumination lens  710  includes a cylinder lens  711  and a collimator lens  712 .  
         [0064]     That is, the illumination lens  710  converts a light beam, which is radiated from the light source  700 , into linear light coplanar with optical paths, and then focuses it on a diffractive light modulator  720 , which will be described later.  
         [0065]     In this case, the cylinder lens  711  converts the parallel light, which is radiated from the light source  700 , into linear light shown in “C” of  FIG. 8A-8C , and then allow the linear light to be incident on the diffractive light modulator  720  through the collimator lens  712 .  
         [0066]     In this case, the collimator lens  712  converts spherical light, which is radiated from the light source  700  through the cylinder lens  711 , into parallel light, and then allows it to be incident on the diffractive light modulator  720 .  
         [0067]     The collimator lens  712  includes a concave lens  712   a  and a convex lens  712   b , as shown in  FIG. 8A-8C .  
         [0068]     The concave lens  712   a  perpendicularly spreads linear light incident from the cylinder lens  711 , as shown in “D” of  FIG. 8A-8C , and then allows it to be incident on the convex lens  712   b . The convex lens  712   b  converts the light beam incident from the concave lens  712   a  into parallel light, as shown in “E” of  FIG. 8A , and then outputs the parallel light.  FIG. 8A  is a perspective view illustrating an optical system including a light source, a cylinder lens and a collimator lens,  FIG. 8B  is a plan view of  FIG. 8A ,  FIG. 8C  is a side sectional view of  FIG. 8A .  
         [0069]     The diffractive light modulator  720  diffracts incident light to output diffracted light having a plurality of diffraction orders. The filter system  740  passes some of diffracted light beams having desired orders through the projection system  750 .  
         [0070]     An example of the diffractive light modulator  720  is shown in  FIG. 9 .  
         [0071]     Referring to  FIG. 9 , the diffractive light modulator according to the embodiment of the present invention includes a silicon substrate  901 , an insulation layer  902 , a lower micro mirror  903 , and a plurality of elements  910   a  to  910   n . Although, in the present embodiment, the insulation layer and the lower micro mirror are separately constructed, the insulation layer itself can function as the lower micro mirror if it has a light-reflecting characteristic.  
         [0072]     The silicon substrate  901  is provided with a depressed portion to provide air spaces to the elements  910   a  to  910   n . The insulation layer  902  is formed on the silicon substrate  901 . The lower micro mirror  903  is deposited on the insulation layer  902  above the depressed portion of the silicon substrate  901 . The bottoms of the elements  910   a  to  910   n  are attached to both sides of the insulation layer  902  beside the depressed portion of the silicon substrate  901 . The silicon substrate  901  can be fabricated of a single material such as Si, Al 2 O 3 , ZrO 2 , quartz or SiO 2 . The upper and lower layers (divided by dotted lines in the drawing) of the silicon substrate  901  can be fabricated of heterogeneous materials.  
         [0073]     The lower micro mirror  903  is deposited above the silicon substrate  901 , and diffracts incident light by reflecting it. The lower micro mirror  903  can be fabricated of a metallic material such as Al, Pt, Cr or Ag.  
         [0074]     The element  910   a  (although only the element  910   a  is described herein, the remaining elements have the same construction and operation) has a ribbon shape. The element  910   a  includes a lower support  911   a , both sides of the bottom of which are attached to both sides of the insulation layer  902  beside the depressed portion of the silicon substrate  901 , so that the central portion of the lower support  911   a  is spaced apart from the depressed portion of the silicon substrate  901 .  
         [0075]     Piezoelectric layers  920   a  and  920   a ′ are formed on both sides of the lower support  911   a . Driving force is provided to the element  910   a  by the contraction and expansion of the piezoelectric layers  920   a  and  920   a′.    
         [0076]     The lower support  911   a  may be fabricated of Si oxide such as SiO 2 , Si nitride such as Si 3 N 4 , a ceramic substrate such as Si, ZrO 2  and Al 2 O 3 , and Si carbide. However, the lower support  911   a  may be omitted when necessary.  
         [0077]     Each of the piezoelectric layers  920   a  and  920   a ′ includes lower electrode layers  921   a  and  921   a ′ configured to provide a piezoelectric voltage, piezoelectric material layers  922   a  and  922   a ′ formed on the lower electrode layers  921   a  and  921   a ′ and configured to contract and expand and generate vertical driving force when voltages are applied to both surfaces thereof, and upper electrode layers  923   a  and  923   a ′ formed on the piezoelectric material layers  922   a  and  922   a ′ and configured to provide a piezoelectric voltage to the piezoelectric material layers  922   a  and  922   a ′. When voltage is applied to the upper electrode layers  923   a  and  923   a ′ and the lower electrode layers  921   a  and  921   a ′, the piezoelectric material layers  922   a  and  922   a ′ contract and expand, thus causing vertical movement of the lower support  911   a.    
         [0078]     The electrodes  921   a ,  921   a ′,  923   a  and  923   a ′ may be fabricated of a material such as Pt, Ta/Pt, Ni, Au, Al or RuO 2 , and may be deposited by sputtering or evaporation to have a thickness within a range of 0.01 to 3 μm.  
         [0079]     Meanwhile, an upper micro mirror  930  is deposited on the center portion of the top of the lower support  911   a , and includes a plurality of open holes  931   a   1  to  931   a   3 . In this case, the open holes  931   a   1  to  931   a   3  preferably have a rectangular shape, but may have any closed curve shape such as a circle or an ellipse. When the lower support  911   a  is fabricated of a light-reflective material, the upper micro mirror  930  is not necessary. In this case, the lower support  911   a  may function as the upper micro mirror.  
         [0080]     The open holes  931   a   1  to  931   a   3  pass light incident on the element  910   a  therethrough, and allow the light to be incident on the portion of the lower micro mirror  903  corresponding to the portion where the open holes  931   a   1  to  931   a   3  are formed, so that the lower micro mirror  903  and the upper micro mirror  930  can form a pixel.  
         [0081]     That is, for example, the portion “A” of the upper micro mirror  930  where the open holes  931   a   1  to  931   a   3  are formed, and the portion “B” of the lower micro mirror  903  can form a single pixel.  
         [0082]     In this case, the incident light, which passes through the portion where the open holes  931   a   1  to  931   a   3  of the upper micro mirror  930  are formed, can be incident on the corresponding portion of the lower micro mirror  903 . When the distance between the upper micro mirror  930  and the lower micro mirror  903  is an odd multiple of λ/4, maximally diffracted light is produced. In addition, an open hole-type diffractive light modulator applicable to the present invention is disclosed in Korean Pat. Appl. No. 2004-030199.  
         [0083]     Meanwhile, the diffractive light modulator  720  forms diffracted light by diffracting linear light incident from the illumination lens  710 , and cause the diffracted light to be incident on the filter system  740 .  
         [0084]     In this case, +1-order diffracted light and −1-order diffracted light, which are formed when the linear light incident from the illumination lens  710  is perpendicularly incident on the diffractive light modulator  720 , are shown in  FIGS. 10A and 10B .  
         [0085]      FIG. 10A  shows that, when incident light is perpendicularly incident, the +1-order diffracted light and the −1-order diffracted light are formed in both directions. An angle θ that is formed with respect to the incident light is proportional to the wavelength. That is, the longer the wavelength, the larger the angle θ.  
         [0086]      FIG. 10B  shows +1-order linear diffracted light and −1-order linear diffracted light, which are formed when linear parallel light is incident on the diffractive light modulator, in three dimensions.  
         [0087]     Meanwhile, the filter system  740  includes a pair of NA improvement mirrors  741   a  and  741   b , a projection lens  742  and a spatial filter  743 .  
         [0088]     In this case, the NA improvement mirrors  741   a  and  741   b  have are independent of each other and have different reflection angles, and reflect incident diffracted light having corresponding diffraction orders. That is, the NA improvement mirror  741   a  reflects +1-order diffracted light, and the NA improvement mirror  741   b  reflects −1-order diffracted light. In this case, the NA improvement mirror  741   a  and  741   b  have different reflection angles, so that, if the +1-order diffracted light and the −1-order diffracted light can be converged as shown in  FIG. 11 , the NA of the projection lens  742  can be improved and, thus, a lens having a low NA can be used. From  FIG. 11 , it can be understood that the NA improvement mirrors  741   a  and  741   b  are independent of each other and have different reflection angles. The reflection angle of the NA improvement mirror  741   a  is greater than that of the NA improvement mirror  741   b.    
         [0089]     Furthermore, from  FIG. 11 , it can be understood that optical paths “a” and “b” along which the diffracted light formed by the diffractive light modulator  720  progresses toward the NA improvement mirrors  741   a  and  741   b  are the same.  
         [0090]     However, it can be understood that the +1-order diffracted light reflected from the NA improvement mirror  741   a  and the −1-order diffracted light reflected from the NA improvement mirror  741   b  have a difference “c” in their optical paths. That is, it can be understood that there occurs a path difference c of S′−S″ in the drawing. This difference between the optical paths may not influence the application of the display device using the light modulator and having the improved NA, which can be solved by locating an optical path compensator on the optical path of the −1-order diffracted light.  
         [0091]     In this case, the optical path compensator can be made of a material whose refractive index is not 1. Glass through which light can be transmitted may be used as the material of the optical path compensator.  
         [0092]     If a compensation medium is used as the optical path compensator, the length of a compensated optical path can be usually determined by the following Equation 1 when the refractive index of the compensation medium is N and the thickness of the compensation medium is t. 
 
Δ=( N− 1)* t   (1) 
 
         [0093]     The projection lens separates incident diffracted light beams on an order basis, and then converges the light beams. The spatial filter  743  has spatially separated slits, and, therefore, can transmit only diffracted light beams having desired orders.  
         [0094]     In this case, the projection lens  742  focuses the light beams output from the NA improvement mirrors  741   a  and  741   b , as shown in  FIG. 12 . The +1-order diffracted light is focused on a location above a location on the 0-order diffracted light is focused, and the −1-order diffracted light is focused on a location below the location on which the 0-order diffracted light is focused. If the slits of the spatial filter  743  are located near the focal points, only diffracted light having desired orders can be transmitted through the spatial filter  743 . As shown in  FIG. 13  showing the front view of the spatial filter  743 , the location of the focal point of the +1-order diffracted light and the location of the focal point of the −1-order diffracted light are different. Thus, the +1-order diffracted light and the −1-order diffracted light can be separated using the spatial filter  743 .  
         [0095]     The projection system  750  projects incident diffracted light onto the screen  760 . That is, the projection system  750  serves to focus diffracted beams having predetermined diffraction coefficients, which are incident through the spatial filter  743 , onto the screen  760 , thus forming a spot. In detail, the projection system  750  includes a projection lens  751  and a galvano mirror  752 .  
         [0096]     The projection lens  751  serves to focus the +1-order diffracted light and the −1-order diffracted light. The galvano mirror  752  serves to allow the beam to scan the screen  760 .  
         [0097]     As described above, in accordance with the present invention, an increase in the NA of a lens is not required even when a diffraction angle increases, so that the present invention is advantageous in that an optical system can be easily designed.  
         [0098]     Further, in accordance with the present invention, + order diffracted light and − order diffracted light can be converged, so that a radical axis optical system can be designed, thus improving the performance of a lens.  
         [0099]     Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.