Image display apparatus

The present invention provides an image display apparatus comprising a light source, a coupling lens, an integrator and a light valve. If .theta..sub.LV1 is the largest value that can be taken by the angle formed by light irradiating said light valve and the optical axis as viewed along the short edges of said light valve, .theta..sub.LV2 is the largest value that can be taken by the angle formed by light irradiating said light valve and the optical axis as viewed along the long edges of said light valve, L.sub.LV1 is the length of the short edges of said light valve, L.sub.LV2 is the length of the long edge of said light valve and NA.sub.LED is the effective numerical aperture of said coupling lens, then, the surface area of the light emitting region of said light source is not greater than (L.sub.LV1.times..theta..sub.LV1 /NA.sub.LED).times.(L.sub.LV2.times..theta..sub.LV2 /NA.sub.LED). Such an image display apparatus is adapted to evenly and uniformly irradiate the light valve of the apparatus with rays of light emitted from the light source with an improved light irradiation efficiency.

TECHNICAL FIELD
 This invention relates to an image display apparatus for displaying images
 by modulating light from a light source by means of light valves.
 BACKGROUND ART
 FIG. 1 of the accompanying drawings schematically illustrate the
 configuration of a known projection type image display apparatus. The
 image display apparatus comprises a reflector plate 100 having a
 reflecting surface of paraboloid of revolution, a light source 101
 arranged at the focal point of the reflector plate 100, an integrator 102,
 a red color separating mirror 103R, a green color separating mirror 103G
 and a blue color separating mirror 103B.
 The image display apparatus additionally comprises a cuboidal color
 synthesizer prism 104 so arranged as to have one of its surface 104G be
 stricken by green color light reflected by the green color separating
 mirror 103G, a reflector mirror 105R so arranged as to cause red color
 light reflected by the red color separating mirror 103R to strike another
 surface 104R of the color synthesizer prism 104 perpendicular to the
 surface 104G, another reflector mirror 105B so arranged as to cause blue
 color light reflected by the blue color separating mirror 103B to strike
 still another surface 104B of the color synthesizer prism 104 parallel to
 the surface 104R, an optical path length regulating lens 106 arranged
 between the green color separating mirror 103G and the blue color
 separating mirror 103B and another optical path length regulating lens 107
 arranged between the blue color separating mirror 103B and the reflector
 mirror 105B.
 The image display apparatus comprises still additionally a light valve for
 red light 108R arranged between the reflector mirror 105R and the surface
 104R of the color synthesizer prism, a lens 109R arranged between the
 reflector mirror 105R and the light valve for red light 108R, a light
 valve for green light 108G arranged between the green color separating
 mirror 103G and the surface 104G of the color synthesizer prism, another
 lens 109G arranged between the green color separating mirror 103G and the
 light valve for green light 108G, a light valve for blue light 108B
 arranged between the reflector mirror 105B and the surface 104B of the
 color synthesizer prism and still another lens 109B arranged between the
 reflector mirror 105B and the light valve for blue 108B.
 The image display apparatus further comprises a projector lens 110 arranged
 opposite to the surface of the color synthesizer prism 104 parallel to the
 surface 104G.
 In a projection type image display apparatus having the above described
 configuration, a white lamp such as a xenon lamp or a metal halide lamp is
 typically used for the light source 101. Light emitted from the light
 source 101 is reflected by the reflector plate 100 and both ultraviolet
 rays and infrared rays are removed from reflected light by means of a cut
 filter (not shown) before reflected light is made to strike the red color
 separating mirror 103R. Of the light striking the red color separating
 mirror 103R, red color light is reflected by the red color separating
 mirror 103R and further by the reflector mirror 105R before it passes
 through the lens 109R and the light valve for red light 108R and enters
 the color synthesizer prism 104. Light other than red Of the light
 striking the green color separating mirror 103G, green color light is
 reflected by the green color separating mirror 103G and then enters the
 color synthesizer prism 104 by way of the lens 109G and the light valve
 for green 108G. Of the light striking the green color separating mirror
 103G, light other than green color light passes through the green color
 separating mirror 103G and strikes the blue color separating mirror 103B
 by way of the optical path length regulating lens 106.
 Of the light striking the blue color separating mirror 103B, blue color
 light is reflected by the blue color separating mirror 103B and, after
 passing through the optical path length regulating lens 107, it is further
 reflected by the reflector mirror 105B and enters the color synthesizer
 prism 104 by way of the lens 109B and the light valve for blue light 108B.
 All lights of different colors that enter the color synthesizer prism 104
 are synthetically combined together by the color synthesizer prism 104 and
 passes through the projector lens 110 before projected onto a transmission
 type or reflection type screen.
 Known projection type image display apparatus having the above described
 configuration is accompanied by the problem of a large spectrum width of
 light of each of the three primary colors separated by the respective
 color separating mirrors and a poor color purity. They are also
 accompanied by the problem of the difficulty with which the brightness of
 the entire quantity of light is regulated because a white lamp is used as
 light source and energized to show a predetermined level of brightness.
 Still additionally, they are accompanied by the problem of the extreme
 difficulty with which the brightness of each primary color is regulated
 because light of each primary color is separated by means of a color
 separating mirror.
 The flux of light coming out of a white lamp normally shows a circular
 cross section. On the other hand, light valves to be irradiated with light
 normally have a rectangularly parallelepipedic profile. Therefore, the
 flux of light striking a light valve is required to have a diameter
 greater than the diagonal of the light valve in order to evenly irradiate
 the light valve with light. Then, there arises a problem of a poor
 irradiation efficiency of light emitted from the light source of any
 conventional image display apparatus.
 As pointed out above, conventional projection type image display apparatus
 comprising a white lamp as light source are accompanied by a number of
 problems. In an attempt to avoid these problems, there has been proposed a
 projection type image display apparatus employing light emitting diodes of
 the three primary colors that are independent from each other as light
 source ensuring an enhanced level of color purity in order to replace the
 white lamp. Such a projection type image display apparatus shows an
 improved color reproduction ability over a wide spectral range due to the
 light source with high color purity such as that of light emitting diodes.
 FIG. 2 shows an XY chromatogram illustrating the color reproduction
 spectrum of a projection type image display apparatus using light emitting
 diodes as light source, that of an CRT (Cathode-Ray Tube) using a
 fluorescent body and that of an NTSC (National Television System
 Committee) type image display apparatus, which are indicated respectively
 by symbols A1, A2 and A3 in FIG. 2.
 As seen from FIG. 2, while the color reproduction spectrum of a
 conventional projection type image display apparatus using a white lamp as
 light source is narrower than that of a CRT using a fluorescent body, a
 projection type image display apparatus using light emitting diodes as
 light source with high color purity can provide a color reproduction
 spectrum wider than that of a CRT using a fluorescent body.
 However, light emitting diodes provide a surface light source and rays of
 light emitted from the surface light source include extra-axial rays that
 are located off the optical axis of the optical system comprising the
 light emitting diodes. Then, rays of light including extra-axial rays
 emitted from the light source cannot evenly and uniformly irradiate a
 light valve and typically show a poor light irradiation efficiency.
 DISCLOSURE OF THE INVENTION
 Accordingly, it is the object of the present invention to provide an image
 display apparatus comprising a light source with an enhanced level of
 color purity other than a white lamp and adapted to evenly and uniformly
 irradiate the light valves of the apparatus with rays of light emitted
 from the light source with an improved light irradiation efficiency.
 According to the invention, the above object is achieved by providing an
 image display apparatus characterized in that it comprises: a coupling
 lens for focusing light emitted from said light source: an integrator
 adapted to receive focused light coming from said coupling lens; and a
 substantially rectangularly parallelepipedic light valve adapted to be
 irradiated with light coming from said light source by way of said
 coupling lens and said integrator; and that said integrator is adapted to
 uniformize the distribution of intensity of light within the region of
 irradiation of light over said light valve; and the surface area of the
 light emitting region of said light source is not greater than
 (L.sub.LV1.times..theta..sub.LV1
 /NA.sub.LED).times.(L.sub.LV2.times..theta..sub.LV2 /NA.sub.LED), where
 .theta..sub.LV1 represents the largest value that can be taken by the
 angle formed by light irradiating said light valve and the optical axis as
 viewed along the short edges of said light valve, .theta..sub.LV2
 represents the largest value that can be taken by the angle formed by
 light irradiating said light valve and the optical axis as viewed along
 the long edges of said light valve, L.sub.LV1 represents the length of the
 short edges of said light valve, L.sub.LV2 represents the length of the
 long edge of said light valve and NA.sub.LED represents the effective
 numerical aperture of said coupling lens.
 Thus, an image display apparatus according to the invention can efficiently
 irradiate the light valves with light emitted from the light source by
 appropriately defining the surface area of the light emitting region.
 Preferably, in an image display apparatus according to the invention, said
 light source comprises a light emitting diode, a semiconductor laser
 device or an organic electroluminescence device.
 Preferably, in an image display apparatus according to the invention, the
 light emitting region of said light source has a contour similar to that
 of the light valves. The light valves will be irradiated efficiently with
 light emitted from the light source when the light emitting region of the
 light source has a contour similar to that of the light valves.
 Preferably, in an image display apparatus according to the invention, said
 light source includes a plurality of light emitting devices and the total
 surface area of the light emitting regions of the light emitting devices
 is not greater than (L.sub.LV1.times..theta..sub.LV1
 /NA.sub.LED).times.(L.sub.LV2.times..theta..sub.LV2 /NA.sub.LED). As the
 total surface area of the light emitting regions of the light emitting
 devices is made not greater than (L.sub.LV1.times..theta..sub.LV1
 /NA.sub.LED).times.(L.sub.LV2.times..theta..sub.LV2 /NA.sub.LED), the
 light valves will be irradiated efficiently with light emitted from each
 of the light emitting devices.
 If the light source includes a plurality of light emitting devices in an
 image display apparatus according to the invention, preferably the light
 emitting region of each of said light emitting devices has a contour
 similar to that of the light valves. The light valves will be irradiated
 efficiently with light emitted from the light source when the light
 emitting region of each of the light emitting devices of the light source
 has a contour similar to that of the light valves.
 Preferably, in an image display apparatus according to the invention, the
 display area of each of the light valves for actually displaying an image
 has short edges with a length equal to or smaller than the length
 L.sub.LV1 of the short edges of said light valves and long edges with a
 length equal to or smaller than the length L.sub.LV2 of the long edges of
 said light valves.
 More specifically, it is preferable that the length L.sub.LV1 of the short
 edges and the length L.sub.LV2 of the long edges of said light valves are
 respectively greater than the length of the short edges and the length of
 the long edges of said display area of each of the light valves by 2 to
 50%.
 Preferably, in an image display apparatus according to the invention, said
 light source comprises a light emitting diode or a semiconductor laser
 device adapted to emit red light. If said light source of an image display
 apparatus according to the invention comprises a light emitting diode or a
 semiconductor laser device adapted to emit red light, it is preferably
 made of a material containing at least an element selected from a group of
 elements including B, Al, Ga, In and Tl and at least an element selected
 from a group of elements including N, P, As and Sb.
 Alternatively or additionally, in an image display apparatus according to
 the invention, said light source may comprise a light emitting diode or a
 semiconductor laser device adapted to emit green light. If said light
 source of an image display apparatus according to the invention comprises
 a light emitting diode or a semiconductor laser device adapted to emit
 green light, it is preferably made of a material containing at least an
 element selected from a group of elements including B, Al, Ga, In and Tl
 and at least an element selected from a group of elements including N, P,
 As and Sb or a material containing at least an element selected from a
 group of elements including Be, Mg, Zn and Cd and at least an element
 selected from a group of elements including O, S, Se and Te.
 Still alternatively or additionally, in an image display apparatus
 according to the invention, said light source may comprise a light
 emitting diode or a semiconductor laser device adapted to emit blue light.
 If said light source of an image display apparatus according to the
 invention comprises a light emitting diode or a semiconductor laser device
 adapted to emit blue light, it is preferably made of a material containing
 at least an element selected from a group of elements including B, Al, Ga,
 In and Tl and at least an element selected from a group of elements
 including N, P, As and Sb or a material containing at least an element
 selected from a group of elements including Be, Mg, Zn and Cd and at least
 an element selected from a group of elements including O, S, Se and Te.

BEST MODES FOR CARRYING OUT THE INVENTION
 Now, the present invention will be described by referring to the
 accompanying drawings that illustrate preferred embodiments of the
 invention.
 FIG. 3 is a schematic illustration of an embodiment of image display
 apparatus according to the invention, showing its schematic configuration.
 This embodiment of image display apparatus according to the invention
 comprises a cuboidal dichroic mirror 1, an lighting/optical system for
 green light 2G arranged vis-a-vis a surface 1G of the dichroic mirror 1,
 an lighting/optical system for red light 2R arranged vis-a-vis a surface
 2R of the dichroic mirror 1 perpendicular to the surface 1G and a
 lighting/optical system for blue light 2B arranged vis-a-vis a surface 1B
 of the dichroic mirror 1 parallel to the surface 1R.
 This embodiment of image display apparatus according to the invention
 additionally comprises a projector lens 3 arranged vis-a-vis the surface
 of the dichroic mirror 1 that is parallel to the surface 1G. The projector
 lens 3 is adapted to project light for the image produced by color
 synthesis of rays of light emitted from the lighting/optical systems 2G,
 2R, 2B by means of the dichroic mirror 1 onto a transmission type or
 reflection type screen (not shown).
 FIG. 4 is a schematic illustration of part the lighting/optical system 2
 (as representative of 2G, 2R, 2B) that can be used for this embodiment of
 image display apparatus according to the invention.
 The lighting/optical system 2 comprises a light emitting diode 4 operating
 as light source, a coupling lens 5, a first fly eye lens 6, a second fly
 eye lens 7, a first condenser lens 8, a second condenser lens 9 and a
 light valve 10 arranged in the above mentioned order, the light valve 10
 being arranged vis-a-vis the corresponding surface of the dichroic mirror
 1. The fly eye lenses 6, 7 operate as integrator for uniformizing the
 distribution of intensity of light emitted from the light emitting diode 4
 and irradiating the light valve 10 within the region of irradiation of
 light over the light valve 10 and are arranged between the light emitting
 diode 4 and the light valve 10.
 Rays of light emitted from the light emitting diode 4 is substantially
 collimated by the coupling lens 5. Since the light emitting diode 4
 provides a surface light source, rays of light emitted from the light
 emitting diode 4 include extra-axial rays that are located off the optical
 axis of the optical system comprising the light emitting diode. After
 passing through the coupling lens 5, light enter the first fly eye lens 6.
 The first fly eye lens 6 and the second fly eye lens 7 are arranged as two
 separate rows showing a conjugate positional relationship to provide a
 telecentric optical system. Thus, the fly eye lenses 6, 7 uniformize the
 angular distribution of the angles between the extra-axial rays of light
 and the optical axis. After passing through the second fly eye lens 7,
 light enters the first condenser lens.
 In this embodiment, the second condenser lens 9 is so arranged as to show a
 conjugate positional relationship with the first condenser lens 8 and
 provide a telecentric optical system. Rays of light are then converged by
 the condenser lenses 8, 9 before entering the light valve 10.
 The embodiment of image display apparatus according to the invention and
 illustrated in FIG. 3 comprises a total of three light valves including a
 light valve for red light 10R, a light valve for green light 10G and a
 light valve for blue light 10R. Rays of light emitted onto the light
 valves 10R, 10G, 10B are spatially modulated by the respective light
 valves 10R, 10G, 10B and, after passing through the respective light
 valves 10R, 10G, 10B, the modulated rays of red light, green light and
 blue light enter the dichroic mirror 1, where they are subjected to color
 synthesis. Then, the synthesized light is projected onto the screen by way
 of the projector lens 3.
 Since the embodiment of image display apparatus according to the invention
 and illustrated in FIG. 3 comprises light emitting diodes 4 as light
 sources that are excellent in terms of color purity, it provides an
 enhanced color reproduction ability over a wide spectral range.
 Additionally, since the light emitting diodes 4 operate as independent
 light sources for red light, green light and blue light, the brightness of
 light can be regulated independently for the three primary colors and the
 overall quantity of light can also be regulated without difficulty in the
 embodiment of image display apparatus.
 Now, a geometrical arrangement of the lenses and the light emitting diodes
 4 in an image display apparatus according to the invention good for
 uniformizing the intensity of light emitted from the light emitting diodes
 4 and improving the irradiation efficiency of light of the light valve 10
 will be discussed by referring to FIG. 4. Note that, in the following
 description, affix "1" accompanying a symbol indicates that the symbol
 relates to the direction of the short edges of the light valve 10, whereas
 affix "2" accompanying a symbol indicates that the symbol relates to the
 direction of the long edges of the light valve 10.
 Firstly, the geometrical relationship of one of the light emitting diode 4
 and each of the related lenses will be discussed in terms of the direction
 of the short edges of the light valve 10 by referring to FIG. 5 showing
 part the lighting/optical system 2 in a simplified fashion.
 If the effective numerical aperture of the coupling lens 5 is NA.sub.LED,
 the light coupling efficiency .eta..sub.LED of the light emitting diode 4
 is expressed by formula (1) below.
EQU .eta..sub.LED =NA.sub.LED.sup.2 (1)
 If the length of the light emitting 4 is r.sub.1 and the effective focal
 length of the coupling lens 5 is f.sub.LED, the largest value that can be
 taken by the angle formed by extra-axial rays of light irradiating said
 light valve and the optical axis of the optical system is expressed by
 formula (2) below.
EQU .theta..sub.LED1 =r.sub.1 /(2.times.f.sub.LED) (2)
 The exit pupil diameter D.sub.LED1 for light transmitted through the
 coupling lens 5 is expressed by formula (3) below.
EQU D.sub.LED1 =2.times.NA.sub.LED.times.f.sub.LED (3)
 Thus, formula (4) below can be derived from the formulas (2) and (3) above.
EQU D.sub.LED1 =(NA.sub.LED.times.r.sub.1)/.theta..sub.LED1 (4)
 On the other hand, if the number of element lenses of the fly eye lenses 6,
 7 is N.sub.1, the effective focal length of the fly eye lenses 6, 7 is
 f.sub.EYE and the effective numerical aperture of the fly eye lenses 6, 7
 is NA.sub.EYE1, the exit pupil diameter D.sub.LED1 is expressed by formula
 (5) below.
EQU D.sub.LED1 =2.times.N.sub.1.times.f.sub.EYE.times.NA.sub.EYE1 (5)
 Furthermore, if the effective focal length, of the condenser lenses 8, 9 is
 f.sub.c and the largest value that can be taken by the angle formed by
 rays of light irradiating the light valve 10 and the optical axis of the
 optical system is .theta..sub.LV1, the exit pupil diameter D.sub.LED1 is
 expressed by formula (6) below.
 D.sub.LED1 =2.times.f.sub.c.times..theta..sub.LV1 (6)
 Then, if the length of the light valve 10 is L.sub.LV1, the relationship
 among the length L.sub.LV1 of the light valve 10, the effective numerical
 aperture NA.sub.EYE1 of the fly eye lenses 6, 7 and the effective focal
 length fc of the condenser lenses 8, 9 is expressed by formula (7) below.
EQU 2.times.f.sub.c.times.NA.sub.EYE1 =L.sub.LV1 (7)
 Thus, formula (8) below can be derived from the formulas (6) and (7) above.
EQU NA.sub.EYE1 =(L.sub.LV1.times..theta..sub.LV1)/.sub.DLED1 (8)
 Likewise, formula (9) below can be derived from the formulas (4) and (8)
 above.
EQU NA.sub.EYE1
 =.theta..sub.LED1.times.{(L.sub.LV1.times..theta..sub.LV1)/(NA.sub.
 LED.times.r.sub.1)} (9)
 Finally, formula (10) below can be derived from the formula (9) above.
EQU .theta..sub.LED1 /NA.sub.EYE1 =(r.sub.1 /L.sub.LV1).times.(NA.sub.LED
 /.theta..sub.LV1) (10)
 The role of the second fly eye lens 7 is important when considering the
 conditions under which the light valve 10 is irradiated with light.
 Referring to FIG. 6, of the rays of light passing through component lens
 6a of the first fly eye lens 7, those entering corresponding component
 lens 7a of the second fly eye lens 7 are then made to irradiate the light
 valve 10. However, of the rays of light passing through component lens 6a
 of the first fly eye lens 6, those entering component lens 7b of the
 second fly eye lens 7 located adjacent to the component lens 7a are not
 made to irradiate the light valve 10. FIG. 7 shows the relationship
 between the ratio of the angle of inclination .theta..sub.LED1 of
 extra-axial rays to the effective numerical aperture NA.sub.EYE1 of the
 fly eye lenses 6, 7 and the efficiency of irradiation of light of the
 light valve 10. As seen from FIG. 7, the efficiency of irradiation of
 light of the light valve 10 declines when the ratio of .theta..sub.LED1
 /NA.sub.EYE1 exceeds 1.
 Therefore, it is desirable that light irradiating the light valve 10
 satisfies the requirement of formula (11) below.
EQU .theta..sub.LED1 /NA.sub.EYE1.ltoreq.1 (11)
 In other words, all rays of light emitted from the light emitting diode 4
 strike the light valve 10 when the requirement of .theta..sub.LED1
 /NA.sub.EYE1.ltoreq.1 is met. On the other hand, however, some rays of
 light emitted from the light emitting diode 4 do not strike the light
 valve 10 to reduce the efficiency of irradiation of light when
 .theta..sub.LED1 /NA.sub.EYE1 &gt;1.
 It will be appreciated that formula (12) below can be derived from the
 formulas (10) and (11) above.
EQU r.sub.1.ltoreq.L.sub.LV1.times.(.theta..sub.LV1 /NA.sub.LED) (12)
 The formula (12) above is in fact a versional expression of
 Lagrange-Helmholtz's formula for the relationship between the size of an
 object and that of an image thereof. Thus, the efficiency of irradiation
 of light of the light valve 10 can be optimized in the direction along the
 short edges of the light valve 10 by selecting appropriate values that
 satisfy the requirement of formula (12) for the parameters of the
 lighting/optical system 2.
 While the efficiency of irradiation of light of the light valve 10 is
 described above from the viewpoint of the direction of the short edges of
 the light valve 10, it will be appreciated that a similar statement
 applies to the efficiency of light irradiation of the light valve 10 in
 terms of the direction of the long edges of the light valve 10. Therefore,
 it is desirable that light irradiating the light valve 10 satisfies the
 requirement of formula (13) below in order to realized an enhanced
 efficiency of light irradiation of the light valve 10 along the direction
 of the long edges of the light valve 10.
EQU r.sub.2.ltoreq.L.sub.LV2.times.(.theta..sub.LV2 /NA.sub.LED) (13)
 Thus, the efficiency of irradiation of light of the light valve 10 can be
 optimized in the direction along the long edges of the light valve 10 by
 selecting appropriate values that satisfy the requirement of formula (13)
 for the parameters of the lighting/optical system 2.
 As will be clearly understood from the above description, for light emitted
 from each of the monochromatic light emitting diodes 4 to efficiently
 irradiate the light valve 10, the total surface area of the light emitting
 region of the light emitting diode is preferably not greater than
 (L.sub.LV1.times..theta..sub.LV1
 /NA.sub.LED).times.(L.sub.LV2.times..theta..sub.LV2 /NA.sub.LED).
 For example, in the case of a transmission type light valve, the largest
 values that can be take by the angles .theta..sub.LV1 and .theta..sub.LV2
 formed by light irradiating the light valve 10 and the optical axis of the
 optical system are subjected to limitations attributable various factors
 of the display apparatus including the contrast of the liquid crystal and
 the angle of view of the projector lens in terms of both the direction of
 the short edges and that of the long edges of the light valve 10. In the
 case of a reflection type light valve, again, .theta..sub.LV1 and
 .theta..sub.LV2 are also subjected to limitations such as the incident
 angle dependency of the polarizing prism of the apparatus in terms of both
 the direction of the short edges and that of the long edges of the light
 valve 10. Thus, more often than not, the requirement of .theta..sub.LV1
 =.theta..sub.LV2 will have to be met. Additionally, since the effective
 numerical aperture NA.sub.LED normally takes a same value for both the
 direction of the short edges and that of the long edges of the light valve
 10, the ratio of r.sub.1 /r.sub.2 will agree with the ratio of L.sub.LV1
 /L.sub.LV2.
 Thus, the light emitting region of the light emitting diode 4 has a contour
 similar to that of the light valve 10. When the light emitting region of
 the light emitting diode 4 has a contour similar to that of the light
 valve 10, rays of light that can be wasted will be minimized to
 consequently improve the efficiency of irradiation of light emitted from
 the light emitting diode 4.
 Apart from the light emitting region of the light emitting diode 4, all the
 component lenses of the fly eye lenses 6, 7 preferably show a contour
 similar to that of the light valve 10 for the same reason. The efficiency
 of irradiation of light emitted from the light emitting diode will be
 improved when the component lenses of the fly eye lenses 6, 7 have a
 contour similar to that of the light valve 10.
 The lengths L.sub.LV1, L.sub.LV2 defined above as those of the short and
 long edges of the above light valve 10 may alternatively be defined to be
 those of the short and long edges of the display area where images are
 actually displayed and hence the area that is to be illuminated. However,
 the illumination of light of the light valve can become uneven due to a
 number of factors including various aberrations. Then, if the lengths
 L.sub.LV1, L.sub.LV2 are defined as those of the short and long edges of
 the display were for the design of the lighting/optical system 2,
 unilluminated dark zones can appear along the periphery of the display
 area of the light valve 10. Additionally, margins such as an alignment
 margin and a dimensional margin will have to be taken into consideration
 particularly for the manufacturing process. All in all, the region of the
 light valve 10 to be irradiated with light is preferably made slightly
 larger than the display area for actually displaying images.
 Thus, in the above described embodiment of image display apparatus
 according to the invention, it is desirable that the display area of the
 light valve 10 has short edges not longer than L.sub.LV1 and long edges
 not longer than L.sub.LV2. Then, as a result, the area of the light valve
 10 to be irradiated with light will be dimensionally slightly greater than
 the display area of the light valve 10 where images are actually
 displayed. By selecting values for the dimensions of the area to be
 irradiated with light of the light valve 10 slightly greater than those of
 the dimensions of the display area, the entire surface of the display area
 will be evenly and uniformly irradiated with light if errors arise due to
 aberrations of the optical system and misalignment of some of the
 components in the manufacturing process.
 When selecting values for the dimensions of the area to be irradiated with
 light of the light valve 10 slightly greater than those of the dimensions
 of the display area, it is preferable that the length L.sub.LV1 of the
 short edges and the length L.sub.LV2 of the long edges of the area to be
 irradiated with light of the light valves are respectively greater than
 the length of the short edges and the length of the long edges of the
 display area of each of the light valves by 2 to 50%, taking aberrations
 of the optical system and margins such as an alignment margin and a
 dimensional margin in the manufacturing process into consideration. Then,
 the entire surface of the display area will be evenly and uniformly
 irradiated with light if errors arise due to aberrations of the optical
 system and misalignment of some of the components in the manufacturing
 process.
 FIGS. 8 and 9 schematically illustrate a light emitting diode 4 that can be
 used for an image display apparatus according to the invention, showing
 its configuration. FIG. 8 is a schematic plan view of the light emitting
 diode 4 and FIG. 9 is a schematic cross sectional view of the light
 emitting diode 4. The light emitting diode 4 comprises a metal substrate
 11, a reflector plate 12, an adhesive layer 13, alight emitting material
 layer 14, a metal electrode 15, a transparent electrode 16 and a lead pin
 17.
 The metal substrate 11 is typically disk-shaped and has a circular recess
 11a at the center thereof. The top surface of the metal substrate 11 and
 the bottom surface of the recess 11a are linked by a slope 11b and the
 reflector plate 12 is formed on the slope 11b.
 The adhesive layer 13, the light emitting material layer 14, the network of
 the metal electrode 15 and the transparent electrode 16 typically made of
 ITO are laid sequentially on the bottom surface of the recess 11a in the
 above mentioned order.
 By using a network for the metal electrode 15, an even and uniform current
 distribution can be ensured over the entire surface of the transparent
 electrode 16. Alternatively, the transparent electrode 16 may be arranged
 directly on the light emitting material layer 14. Then, the network of
 metal electrode 15 will be arranged on the transparent electrode 16. The
 metal electrode 15 is connected to the lead pin 17 located adjacent to the
 metal substrate 11 by way of a lead wire 17a. The lead pin 17 is
 electrically isolated from the metal substrate 11 by an insulation layer
 18.
 The light emitting diode 4 operating as light source of the image display
 apparatus is typically made of a compound semiconductor as will be
 described below.
 If the light source of an image display apparatus according to the
 invention comprises a light emitting diode adapted to emit red light, it
 is preferably made of a compound semiconductor selected from a group of
 compounds of the GaP type including GaAlAs, GaAsP and AlGaPAs, a group of
 GaAs type compounds and that of AlAs type compounds.
 If, on the other hand, the light source of an image display apparatus
 according to the invention comprises a light emitting diode adapted to
 emit green light, it is preferably made of a compound semiconductor
 selected from a group of compounds of the GaN type including InGaN and
 AlInGaN and a group of ZnSe type compounds.
 If, finally, the light source of an image display apparatus according to
 the invention comprises a light emitting diode adapted to emit blue light,
 it is preferably made of a compound semiconductor selected from a group of
 compounds of the GaN type including InGaN and AlInGaN, a group of ZnSeN
 type compounds and that of SiC type compounds.
 The light emitting region of the light emitting diode 4 is substantially
 rectangularly parallelepipedic and has short edges with a length of r1 and
 long edges with a length of r2. The light emitting region of the light
 emitting diode 4 preferably has a contour similar to that of the light
 valve 10. If the light emitting region of the light emitting diode 4 has a
 contour similar to that of the light valve 10, rays of light that can be
 wasted will be minimized to consequently improve the efficiency of
 irradiation of light emitted from the light emitting diode 4.
 Referring now to FIG. 10, for the purpose of the present invention, the
 light emitting diode 4 may be covered by a highly transparent resin
 material 19 having a large refractive index and a spherical lens 20 and a
 coupling lens 21 may be arranged in front of it. The numerical aperture of
 the lenses can by raised when the light emitting diode 4 is covered by a
 highly transparent resin material 19 having a large refractive index and a
 spherical lens 2 and a coupling lens 21 are arranged in front of it.
 In an image display apparatus according to the invention, the light source
 may include a plurality of light emitting devices.
 FIG. 11 is a schematic illustration of an embodiment of image display
 apparatus according to the invention and comprising a plurality of light
 emitting diodes for the light sources. The embodiment of image display
 apparatus of FIG. 11 comprises a cuboidal dichroic mirror 30, an
 lighting/optical system for green light 31G arranged vis-a-vis a surface
 30G of the dichroic mirror 30, an lighting/optical system for red light
 31R arranged vis-a-vis a surface 30R of the dichroic mirror 30
 perpendicular to the surface 30G and a lighting/optical system for blue
 light 31B arranged vis-a-vis a surface 30B of the dichroic mirror 30
 parallel to the surface 30R.
 This embodiment of image display apparatus according to the invention
 additionally comprises a projector lens 37 arranged vis-a-vis the surface
 of the dichroic mirror 30 that is parallel to the surface 30G. The
 projector lens 37 is adapted to project light for the image produced by
 color synthesis of rays of light emitted from the lighting/optical systems
 31G, 31R, 31B by means of the dichroic mirror 30 onto a transmission type
 or reflection type screen (not shown).
 Each of the illustrated lighting/optical systems 31 (as representative of
 31G, 31R, 31B) comprises a light emitting diode 32 operating as light
 source, a plurality of coupling lenses 33, a fly eye lens 34, a condenser
 lens 35 and a light valve 36 arranged in the above mentioned order, the
 light valve 35 being arranged vis-a-vis the corresponding surface of the
 dichroic mirror 30.
 While the image display apparatus of FIG. 11 has a configuration same as
 that of the image display apparatus of FIG. 3, a plurality of light
 emitting diodes 32 are used for each of the light sources. FIG. 12 is a
 view of one of the light sources of the image display apparatus as viewed
 from the light emitting side thereof. As shown in FIG. 12, the light
 source comprises a total of four light emitting diodes 32 arranged in two
 paired rows. Thus, according to the invention, a plurality of light
 emitting diodes 32 are arranged two-dimensionally and a same number of
 coupling lenses 33 are arranged vis-a-vis the respective light emitting
 diodes 32. Any number of light emitting diodes may be used in the
 lighting/optical system for the purpose of the invention. The intensity of
 light emitted from the light source is the sum of the intensity of light
 emitted from each of the light emitting diodes 32.
 In an image display apparatus according to the invention and comprising a
 plurality of light emitting diodes for each of the light sources,
 preferably, the total surface area of the light emitting regions of the
 light emitting diodes of each primary color is not greater than
 (L.sub.LV1.times..theta..sub.LV1
 /NA.sub.LED).times.(L.sub.LV2.times..theta..sub.LV2 /NA.sub.LED). Still
 preferably, the light emitting region of each of said light emitting
 diodes has a contour similar to that of the light valves. Since the
 contour of the surface area of each of the light valves irradiated with
 light mainly depends on the contour of the light emitting region of each
 individual light emitting diode, the mode or arrangement of the
 two-dimensionally arranged light emitting diodes is not important. For
 example, a total of four light emitting diodes may be arranged in a single
 row.
 The present invention is equally applicable to both an image display
 apparatus comprising transmission type light valves and an image display
 apparatus comprising reflection type light valves.
 FIG. 13 is a schematic illustration of still another embodiment of image
 display apparatus according to the invention and comprising a single plate
 transmission type light valve so that the process of color synthesis takes
 place in front of the integrator of the lighting/optical systems.
 This embodiment of image display apparatus comprises a cuboidal dichroic
 mirror 40, a green light emitting diode unit 41G arranged vis-a-vis a
 surface 40G of the dichroic mirror 40, a red light emitting diode unit 41R
 arranged vis-a-vis a surface 40R of the dichroic mirror 40 perpendicular
 to the surface 40G and a blue light emitting diode unit 41B arranged
 vis-a-vis a surface 40B of the dichroic mirror 40 parallel to the surface
 40R. Each of the light emitting diode units 41R, 41G, 41B comprises a
 light source having a plurality of light emitting diodes and a same number
 of coupling lenses arranged in front of the respective light emitting
 diodes.
 This embodiment of image display apparatus according to the invention
 additionally comprises an integrator 42 arranged vis-a-vis the surface of
 the dichroic mirror 40 parallel to the surface 40G, a condenser lens 43, a
 light valve 44 and a projector lens 45.
 With the embodiment of image display apparatus illustrated in FIG. 13, red
 light, green light and light emitting emitted respectively from the light
 emitting diode units 41R, 41G, 41B enter the dichroic mirror 40, where
 they are subjected to a process of color synthesis. Color-synthesized
 light is then irradiated onto the light valve 44 by way of the integrator
 42 and the condenser lens 43. Light that enters the light valve 44 is
 spatially modulated by the latter before it passes through the light valve
 44 and projected onto a screen by way of the projector lens 45.
 FIG. 14 is a schematic illustration of still another embodiment of image
 display apparatus according to the invention and comprising single plate
 reflection type light valves so that the process of color synthesis takes
 place in front of the integrator of the lighting/optical systems.
 This embodiment of image display apparatus comprises a cuboidal dichroic
 mirror 50, a green light emitting diode unit 51G arranged vis-a-vis a
 surface 50G of the dichroic mirror 50, a red light emitting diode unit 41R
 arranged vis-a-vis a surface 5OR of the dichroic mirror 50 perpendicular
 to the surface 50G and a blue light emitting diode unit 51B arranged
 vis-a-vis a surface 50B of the dichroic mirror 50 parallel to the surface
 50R. Each of the light emitting diode units 51R, 51G, 51B comprises a
 light source having a plurality of light emitting diodes and a same number
 of coupling lenses arranged in front of the respective light emitting
 diodes.
 This embodiment of image display apparatus according to the invention
 additionally comprises an integrator 52 arranged vis-a-vis the surface of
 the dichroic mirror 50 parallel to the surface 50G, a condenser lens 53, a
 polarizing beam splitter 54, a light valve 55 arranged on the light path
 of light reflected by the polarizing beam splitter 54 and a projector lens
 56 arranged by turn on the light path of light reflected by the light
 valve 55.
 With the embodiment of image display apparatus illustrated in FIG. 14, red
 light, green light and light emitting emitted respectively from the light
 emitting diode units 51R, 51G, 51B enter the dichroic mirror 50, where
 they are subjected to a process of color synthesis. Color-synthesized
 light then enters the polarizing beam splitter 54 by way of the integrator
 52 and the condenser lens 53.
 Of the light that enters the polarizing beam splitter 54, only the fraction
 thereof, or S-polarized light, that is polarized in the direction
 perpendicular to the plane of incidence is reflected by the plane of
 reflection of the polarizing beam splitter 54 and enters the light valve
 55. The reflection type light valve 55 modifies the state of polarization
 of incident light for each of the pixels according to the image to be
 displayed and reflects it toward the polarizing beam splitter 54. Of the
 light reflected by the light valve 55, the fraction thereof, or
 P-polarized light, that is polarized in the direction parallel to the
 plane of incidence passes the polarizing beam splitter 54 and enters the
 projector lens 56, which projects it onto the screen.
 With either of the image display apparatus illustrated in FIGS. 13 and 14,
 for light emitted from each of the monochromatic light emitting diodes 4
 to efficiently irradiate the light valve 10, the total surface area of the
 light emitting region of the light emitting diode is preferably not
 greater than (L.sub.LV1.times..theta..sub.LV1
 /NA.sub.LED).times.(L.sub.LV2.times..theta..sub.LV2 /NA.sub.LED).
 The present invention is also applicable to an image display apparatus
 adapted to polarization mode conversion.
 FIG. 15 is a schematic illustration of still another embodiment of image
 display apparatus according to the invention that comprises three
 transmission type light valves and adapted for polarization mode
 conversion.
 The embodiment of image display apparatus of FIG. 15 comprises a cuboidal
 dichroic mirror 60, an lighting/optical system for green light 61G
 arranged vis-a-vis a surface 60G of the dichroic mirror 60, an
 lighting/optical system for red light 61R arranged vis-a-vis a surface 60R
 of the dichroic mirror 60 perpendicular to the surface 30G, a
 lighting/optical system for blue light 61B arranged vis-a-vis a surface
 60B of the dichroic mirror 60 parallel to the surface 60R and a projector
 lens 67 arranged visa-vis the surface of the dichroic mirror 30 that is
 parallel to the surface 60G.
 Each of the illustrated lighting/optical systems 61 (as representative of
 61G, 61R, 61B) comprises a light emitting diode unit 62, a polarization
 mode conversion means 63, an integrator 64, a condenser lens 65 and a
 light valve 66 arranged in the above mentioned order, the light valve 66
 being disposed vis-a-vis the corresponding surface of the dichroic mirror
 60. The light emitting diode unit 62 includes a plurality of light
 emitting diodes operating as light source and a same number of coupling
 lenses, each being arranged in front of the corresponding light emitting
 diode.
 With the embodiment of image display apparatus illustrated in FIG. 15,
 light emitted from each of the light emitting diode unit 62 enters the
 polarization mode conversion means 63. The polarization mode conversion
 means typically has a polarizing beam splitter 63a and a half wavelength
 plate 63b as shown in FIG. 16.
 Light that hits the polarization mode conversion 63 firstly enters the
 polarizing beam splitter 63a. Of the light that enters the polarizing beam
 splitter 63a, only the fraction thereof, or S-polarized light, that is
 polarized in the direction perpendicular to the plane of incidence is
 reflected by the plane of reflection of the polarizing beam splitter 63a
 and enters the half wavelength plate 63b, which rotates the plane of
 polarization of incident light. On the other hand, of the light that
 enters the polarizing beam splitter 63a, the fraction thereof, or
 P-polarized light, that is polarized in the direction parallel to the
 plane of incidence is transmitted through the polarizing beam splitter 63a
 to move further forward. Light subjected to polarization mode conversion
 by the polarization mode conversion means 63 is then made to irradiate the
 light valve 66 by way of the integrator 64 and the condenser lens 65 as
 shown in FIG. 15.
 This embodiment of image display apparatus comprises a total of three light
 valves, including a light valve for red light 66R, a light valve for green
 light 66G and a light valve for blue light 66B. Thus, red light, green
 light and blue light made to strike the respective light valves 66R, 66G,
 66B are spatially modulated by the respective light valves 66R, 66G, 66B.
 Then, after passing through the respective light valves 66R, 66G, 66B,
 spatially modulated red light, green light and blue light enter the
 dichroic mirror 60, where they are subjected to a process of color
 synthesis and then projected onto the screen by means of the projector
 lens 67 as synthesized light.
 FIG. 17 is a schematic illustration of still another embodiment of image
 display apparatus according to the invention that comprises a single plate
 reflection type light valve and adapted for polarization mode conversion.
 This embodiment of image display apparatus comprises a cuboidal dichroic
 mirror 70, a green light emitting diode unit 71G arranged vis-a-vis a
 surface 70G of the dichroic mirror 70, a red light emitting diode unit 71R
 arranged vis-a-vis a surface 70R of the dichroic mirror 70 perpendicular
 to the surface 70G and a blue light emitting diode unit 71B arranged
 vis-a-vis a surface 70B of the dichroic mirror 70 parallel to the surface
 70R. Each of the light emitting diode units 71R, 71G, 71B comprises a
 light source having a plurality of light emitting diodes and a same number
 of coupling lenses arranged in front of the respective light emitting
 diodes.
 This embodiment of image display apparatus according to the invention
 additionally comprises a polarization mode conversion means 72 arranged
 vis-a-vis the surface of the dichroic mirror 70 parallel to the surface
 70G, an integrator 72, a condenser lens 74, a polarizing beam splitter 75,
 a light valve 76 arranged on the light path of light reflected by the
 polarizing beam splitter 75 and a projector lens 77 arranged on the light
 path of light reflected by the light valve 76.
 With the embodiment of image display apparatus illustrated in FIG. 17, red
 light, green light and light emitting emitted respectively from the light
 emitting diode units 71R, 71G, 71B enter the dichroic mirror 70, where
 they are subjected to a process of color synthesis. Color-synthesized
 light is then made to enter the polarization mode conversion means 72
 typically including a light valve and a half wavelength plate, where it is
 subjected to a process of polarization mode conversion. Light subjected to
 the process of polarization mode conversion is then made to enter the
 polarizing beam splitter 75 by way of the integrator 73 and the condenser
 lens 74.
 Of the light that enters the polarizing beam splitter 75, only the fraction
 thereof, or S-polarized light, that is polarized in the direction
 perpendicular to the plane of incidence is reflected by the plane of
 reflection of the polarizing beam splitter 75 and enters the light valve
 76. The reflection type light valve 76 modifies the state of polarization
 of incident light for each of the pixels according to the image to be
 displayed and reflects it toward the polarizing beam splitter 75. Of the
 light reflected by the light valve 77, the fraction thereof, or
 P-polarized light, that is polarized in the direction parallel to the
 plane of incidence passes the polarizing beam splitter 75 and enters the
 projector lens 77, which projects it onto the screen.
 FIG. 18 is a schematic illustration of still another embodiment of image
 display apparatus according to the invention that comprises a single plate
 transmission type light valve and adapted for polarization mode
 conversion.
 This embodiment of image display apparatus comprises a cuboidal dichroic
 mirror 80, a green light emitting diode unit 81G arranged vis-a-vis a
 surface 80G of the dichroic mirror 80, a red light emitting diode unit 81R
 arranged vis-a-vis a surface 80R of the dichroic mirror 80 perpendicular
 to the surface 80G and a blue light emitting diode unit 81B arranged
 vis-a-vis a surface 80B of the dichroic mirror 80 parallel to the surface
 80R. Each of the light emitting diode units 81R, 81G, 81B comprises a
 light source having a plurality of light emitting diodes and a same number
 of coupling lenses arranged in front of the respective light emitting
 diodes.
 This embodiment of image display apparatus according to the invention
 additionally comprises a polarization mode conversion means 82 arranged
 vis-a-vis the surface of the dichroic mirror 80 parallel to the surface
 80G, an integrator 82, a condenser lens 84, a light valve 85 and a
 projector lens 86.
 With the embodiment of image display apparatus illustrated in FIG. 18, red
 light, green light and light emitting emitted respectively from the light
 emitting diode units 81R, 81G, 81B enter the dichroic mirror 80, where
 they are subjected to a process of color synthesis. Color-synthesized
 light is then made to enter the polarization mode conversion means 82
 typically including a light valve and a half wavelength plate, where it is
 subjected to a process of polarization mode conversion. Light subjected to
 the process of polarization mode conversion is then made to enter the
 light valve 85 by way of the integrator 84 and the condenser lens 84.
 Light that enters the light valve 85 is spatially modulated by the light
 valve 85 and then projected on the screen by means of the projector lens
 86.
 With either of the image display apparatus illustrated in FIGS. 17 and 18,
 light is split into two fluxes of light as a result of polarization mode
 conversion so that apparently it comprises twice as many light emitting
 diodes as it actually has. Thus, with the embodiments of image display
 apparatus illustrated in FIGS. 15, 17 and 18, for light emitted from each
 of the monochromatic light emitting diodes to efficiently irradiate the
 light valve, the total surface area of the light emitting region of the
 light emitting diode is preferably not greater than
 (L.sub.LV1.times..theta..sub.LV1
 /NA.sub.LED).times.(L.sub.LV2.times..theta..sub.LV2 /NA.sub.LED).
 Additionally, the light emitting region of each of the light emitting
 diodes preferably has a contour similar to that of the light valves.
 The total surface area of the light emitting regions of the light emitting
 diodes can be reduced by half to consequently reduce the overall
 dimensions of the image display apparatus when the technique of
 polarization mode conversion is employed. Additionally, a same level of
 brightness can be produced with a power consumption rate that is only a
 half of that of an apparatus that does not employ the technique of
 polarization mode conversion.
 The light sources of an image display apparatus according to the invention
 preferably show an enhanced level of color purity. Specific examples of
 devices that can be used for the light sources of an image display
 apparatus according to the invention include semiconductor laser devices
 and organic electro luminescence devices besides light emitting diodes.
 In the case of a light emitting diode or a semiconductor laser device
 adapted to emit red light, it is preferably made of a material containing
 at least an element selected from a group of elements including B, Al, Ga,
 In and Ti and at least an element selected from a group of elements
 including N, P, As and Sb. Particularly, the use of a compound selected
 from a group of compounds including AlGaAs or those including AlGaInP is
 preferable.
 On the other hand, in the case of a light emitting diode or a semiconductor
 laser device adapted to emit green light, it is preferably made of a
 material containing at least an element selected from a group of elements
 including B, Al, Ga, In and Tl and at least an element selected from a
 group of elements including N, P, As and Sb or a material containing at
 least an element selected from a group of elements including Be, Mg, Zn
 and Cd and at least an element selected from a group of elements including
 O, S, Se and Te. Particularly, the use of a compound selected from a group
 of compounds including AlGaN or those including ZnCdSe/ZnMgSSe is
 preferable.
 Finally, in the case of a light emitting diode or a semiconductor laser
 device adapted to emit blue light, it is preferably made of a material
 containing at least an element selected from a group of elements including
 B, Al, Ga, In and n and at least an element selected from a group of
 elements including N, P, As and Sb or a material containing at least an
 element selected from a group of elements including Be, Mg, Zn and Cd and
 at least an element selected from a group of elements including O, S, Se
 and Te. Particularly, the use of a compound selected from a group of
 compounds including AlGaN or those including ZnCdSe/ZnMgSSe is preferable.
 Additionally, a compound selected from a group of compounds including SiC
 and other similar compounds of the IV group elements may be used for a
 light emitting diode or a semiconductor laser device operating as light
 source for the purpose of the invention.
 Alternatively, a light emitting diode or a semiconductor laser device
 operating as light source for the purpose of the present invention may be
 made of a material such as p-Si or p-Ge.
 If an organic electro luminescence device is used as light source for the
 purpose of the invention, it may preferably have a multilayer structure of
 materials selected from DST, TPD, CuPc, Alq, MTDATA, PPV, CN-PPV, PTPDMA,
 PTPDES, PVK, PVOXD, BeBq, ZnBq and rubrene, on which an electrode of ITO
 or MgIn is formed.
 The transmission type light valve of an image display apparatus according
 to the invention may be an STN (super twisted nematic) liquid crystal
 display device, a ferromagnetic liquid crystal display device or a polymer
 dispersion type liquid crystal device. Such a device may be driven by
 means of a simple matrix drive system or an active matrix drive system,
 either of which may feasibly be used for the purpose of the invention.
 On the other hand, the reflection type light valve of an image display
 apparatus according to the invention may be a reflection type liquid
 crystal display device comprising a glass or silicon substrate and a drive
 electrode or an active device for driving TN (twisted nematic) mode liquid
 crystal, ferromagnetic crystal liquid or polymer dispersion type liquid
 crystal. Alternatively, the reflection type light valve may be a
 reflection type liquid crystal display device adapted to apply a voltage
 to liquid crystal by irradiating the latter with light by way of a
 photoconductive film. Still alternatively, the reflection type light valve
 may be a reflection type display device having a structure that changes
 its shape and state as a function of the electric field applied to it such
 as a grating light valve.
 INDUSTRIAL APPLICABILITY
 In an image display apparatus according to the invention, light emitted
 from the light source can be made to irradiate the light valve effectively
 and efficiently. Therefore, according to the invention, there is provided
 an excellent image display apparatus that shows an enhanced efficiency of
 irradiation of light as emitted from the light source.