Abstract:
An image display is illuminated by a light source employing a plurality of low power light emitting devices. The lifetime and light intensity of the light sourceare increased by orders of magnitude. The illumination provided by the light source is made uniform to efficiently utilize the light source and so obtain optimum projection results.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority benefit of Taiwan application Serial no. 87108195, filed May 26, 1998, the full disclosure of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates in general to an illumination device, and more particularly, to an illumination device used in a display. 
     2. Description of the Related Art 
     The conventional projection apparatus has been widely applied in front projection type or large panel rear projection type displays. The required source has to provide enough luminance. Typically, light sources such as halogen lamps, arc lamps, for example. high pressure mercury lamps, metal halogen lamps, xenon lamps are employed. These lamps have the advantage of high luminance, but on the other side, have the disadvantages of high electricity consuming, short lifetime, and high temperature. These light sources are thus selected only while a high luminance is required. 
     FIG.  1 A and FIG. 1B show two types of conventional illumination devices. FIG.  1 A is a schematic drawing of a projection apparatus disclosed in U.S. Pat. No. 5,418,583. A first lens array  20  and a second lens array integrator  30  are used to uniformize a light source  10 , and to project the light source  10  onto a liquid crystal display (LCD) light valve  40 . FIG. 1B is a schematic drawing of a projection apparatus disclosed in U.S. Pat. Nos. 4,656,562 and 5,634,704. A glass rod integrator  25  and a lens  35  are used to uniformize a light source  10 , and to project the light source  10  onto a liquid crystal display (LCD) light valve  40 . In both of the projection apparatus shown in FIG.  1 A and FIG. 1B, a halogen lamp or various type of arc lamp is used as the light source  10 . Therefore. the projection apparatus has very high electricity consumption, a short life time, and high temperature and is not suitable for use in a small scale display. 
     The diagonal length of a desktop display screen is typically in a range of about 20 inch to 30 inch. The required luminance of the light source is not as high as the front projection type or large panel back projection type displays. Moreover, the desktop screen is often in a ON status, so the light source has to keep supplying a light. Therefore, the conventional light source with a high electricity consumption, short lifetime, and a high operation temperature is not suitable to apply in a desktop display screen. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an illumination device which employs a low power light emitting device as a light source. The light emitting device has the properties of low electricity consumption, long lifetime, and low operation temperature, that is, low thermal consumption. By superposing several light emitting devices on a light valve, the lifetime and the light intensity of a light source are increased of orders in magnitude. In addition, with the advantage of low operation temperature, the optical lens used in the illumination device can be selected from plastic material, so that the fabrication cost is lowered. 
     It is another object of the invention to provide an illumination device which includes a uniformizing means and a polarization means to efficiently apply the light supplied by the light source, and to obtain an optimum results of projection. 
     It is a further object of the invention to provide an image projection apparatus. The illumination device provided above is used as a light source. Therefore, the projection apparatus can obtain a high efficient projection display with a low fabrication cost. 
     To achieve the above-mentioned objects and advantages, an illumination device and an image projection apparatus using the illumination device are provided. The illumination device comprises at least a light emitting device. A light emitted from the light emitting device is uniformized by a uniformizing means. The light source includes an LED. Being uniformized by the uniformizing means, the light projects on a light valve such as a liquid crystal display to display an image. In the invention, various types of illumination uniformizing means can be employed and are introduced in the section of the detailed description of the preferred embodiments. By the illumination uniformizing means, an incident light can be re-distributed or converted into a way of back light panel to achieve the objective of being uniform. 
     In addition, most of the light valves can only receive a single type of polarization light. The illumination device thus further comprises a polarization converter to convert a light into a useful polarization type of light. Therefore, the efficiency of the light source is enhanced. 
     Both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG.  1 A and FIG. 1B show two conventional illumination devices; and 
     FIG. 2A shows a first embodiment of an illumination device according to the invention; 
     FIG. 2B shows a light distribution lens used in the illumination device shown in FIG. 2A; 
     FIG. 2C shows a modification of the first embodiment; 
     FIG. 3A shows a second embodiment of the invention; 
     FIG. 3B shows an example of the arrangement of the LED light source shown in FIG. 3A; 
     FIG.  3 C and FIG. 3D show two examples of scattering mechanism of the illumination uniformizing means shown in FIG. 3A; 
     FIG. 3E shows a modification of the second embodiment; 
     FIG. 4 shows a third embodiment of the invention; 
     FIG. 5A shows a fourth embodiment of the invention; 
     FIG.  5 B and FIG. 5C shows two types of light converging lens used in the fourth embodiment; 
     FIG. 6A shows an example of multiple sets of light emitting device light source incident to a back light plate; 
     FIG. 6B is a cross section view along the line II-II′ in FIG. 6A; 
     FIG. 7A shows a fifth embodiment of the invention; 
     FIG. 7B shows the operation mechanism of the polarization means shown in FIG. 7A; 
     FIG. 8A shows a sixth embodiment of the invention; 
     FIG. 8B shows the operation mechanism of a wedged glass rod shown in FIG. 8A; and 
     FIG. 9 shows an image projection apparatus comprising an illumination device provided in the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention provides an illumination device comprising a light source and an illumination uniformizing means which uniformizes the light emitted from the light source. The light source comprises light emitting devices (LED), organic light emitting diodes (OLED), laser diode (LD), electroluminescence devices (EL), field emission display (FED), and cold cathode fluorescence lamp (CCEL). The light emitted from the light emitting devices is non-uniformly distributed and converged in a certain area. By the illumination uniformizing means, the light emitted from the light source is distributed uniformly to project on a light valve such as a LCD, for example, a projection type LCD or a projection type DMD, or a transmission type LCD, to display the image. In other words, the illumination uniformizing means utilize a distribution lens to rearrange the non-uniform incident light or in a way of back light panel to uniformize the incident light. 
     Moreover, most light valves only accept one type of polarization light. The illumination device thus further comprises a polarizer, or a polarization means. The light which in unacceptable for the light valve is thus polarized to an acceptable polarized light, for example, an S-polarized light is converted into a P-polarized light. The light emitted from the light source can thus be utilized efficiently. 
     First Embodiment 
     In FIG. 2A, an illumination device in the first embodiment according to the invention is shown. The illumination device  200  comprises a light source  202  and an illumination uniformizing means. The light source  202  includes a planar array of light emitting devices  204 , while the illumination uniformizing means comprises a planar light distribution lens array  206  and a light converging lens  210 . The light distribution lens array  206  further comprises a number of lenses  208 , and the optical axis of the light emitting device  204  is overlapped with the optical axis of the corresponding light distribution lens  208 . The light converging lens  210  is used to project a light emitted from the light distribution lens array  206  to a light valve  212 . The light valve  212  can be an LCD. 
     Since the light emitting device emits a non-uniform light, the light intensity along its optical axis is more intensive than the position far away from the optical axis. The theory of uniformizing the light emitted from the light emitting device by the light distribution lens  204  and the light converging lens  210  is introduced as follows. 
     In FIG. 2B, a set of the light emitting device  204  and a light distribution lens  208  are shown. The middle part of the light distribution lens  208  is a negative lens, that, the light intensity along the optical axis emitted from the light emitting device  204  is distributed to the rimland of the light valve  212 . The edge parts of the light distribution lens  208  are positive lens or lens with smaller curvature, so that the light emitted from the edge part of the light emitting device  204  is only diverted or bent with a small angle. As a consequence, the non-uniform light emitted from the light emitting device  204  is rearranged and re-distributed as a uniform light. 
     Through the light converging lens  210 , the light from each of the light distribution lens  208  in the light distribution lens array  206  is directed, overlapped and projected onto the whole LCD light valve. A uniform projection light is thus distributed on the whole LCD light valve. 
     In FIG. 2C, a modification of the illumination device shown in FIG. 2A is shown. The light distribution lenses  208  of the light distribution lens array  206  are arranged in a curved shape with a curvature, where the light converging lens  210  shown in FIG. 2B is not included. The profile of the light distribution lens array  206  is like a concave lens having a central axis I-I′. The optical axis  208   a  of each light distribution lens  208  is directed towards a center point O of the light valve  212 . The center point O is located at the central axis I-I′. Again, each light distribution lens  208  of the light distribution lens array  206  is disposed with a light emitting device  204 . The profile of the light source  202  which comprises light emitting devices  204  is similar to it of the light distribution lens array  206  to provide a light to the illumination device  200 . 
     In the first embodiment, the non-uniform light emitted from the light emitting devices is uniformized by an illumination uniformizing means. The illumination uniformizing means comprising a planar light distribution lens array and a light converging lens, or alternatively, a curved light lens array can be used instead of the above combination. A Fresnel type lens may also be used to replace the combination of the light distribution lens and the light converging lens. In addition to the Fresnel type lens, distribution lens such as holographic optic element (HOE) or binary optical device may also be employed. The distribution lenses and the focus means can assembled as a compound optical lens apparatus. The compound optical lens may also be a holographic type. The binary optical device may be formed by microelectronic fabrication technique based on principle and theory of optical diffracton and computer technique. The binary optical device comprises a double value type device, for example, device through which only light with two phases can pass, or a multi-value device which has multi-phase characteristic. 
     With a low power light source, the power consumption is low. Therefore, the light distribution lenses and the light converging lens can be fabricated from plastic material. In addition to the low power consumption, the illumination device provided in the first embodiment has a further advantage of low fabrication cost. 
     Second Embodiment 
     FIG. 3A shows a second embodiment of the invention. A back light panel type of uniformizing means is used to uniformize a light emitted from a light source. 
     An illumination device comprises a light source  302  and an illumination uniformizing means  310 . The light source  302  comprises an array of light emitting devices  306  shown as FIG.  3 B. The uniformizing means  310  includes a wedged back light penal. The uniformizing means comprises an incident plane  312 , a bottom surface  314 , a top plane  318 , and a side plane  316  opposite to the incident plane  312 . The side plane  316  includes a mirror to reflect a light emitted from the light source  302 . The bottom plane  314  further comprises a scattering pattern as shown in FIG. 3C or FIG.  3 D. 
     The bottom plane  314  of the illumination uniformizing means  310  has both functions of transmitting and scattering a light. When a light emitted from the light source  302  is incident into the illumination uniforming means  310  through the incident plane  312 , the light is totally reflected by the bottom plane  314  and the projection plane  318  and travelling in between. When the light is incident on the scattering pattern of the bottom plane  314 , the light is scattered to transmit through the projection plane  318  to the light valve  320 . 
     It is known that the illumination intensity of the light emitted from an light emitting devices decreases with increasing distance. Therefore, the light reaching the incident plane  312  is stronger than the light reaching the side plane  316 . To achieve the objective of obtaining a uniform light distribution on the light valve  320 , the light scattered from the bottom plane  314  closer to the incident plane  312  has to be weaker than the light scattered from the bottom plane  314  closer to the side plane  316 . The scattering pattern designed as FIG.  3 C and FIG. 3D can achieve the object. As shown in the figure, the scatter pattern is gradually condensed from the incident plane to the side plane. 
     By the design of the scattering pattern on the bottom plane  314 , a non-uniform light emitted from the light emitting devices  204  is uniformized to project onto the light valve  320 . 
     In addition, the light incident on the side plane  316  is reflected to be utilized iteratively to enhance the efficiency of the light source. 
     FIG. 3E shows a modification of the second embodiment. A set of light converging means  330  is disposed on the projection plane  318  to collimate the light scattered by a large angle. The light converging means  330  includes a lens array. To identify the center of each lens of the light converging means  330  and each center of the corresponding scattering pattern, the operation can thus be optimized. 
     In this embodiment, a wedged back light panel is used as an illumination uniformizing means instead of the light distribution lens array in the first embodiment. Similar to the first embodiment, with the low power light source such as light emitting devices, a low operating temperature is reached. Therefore, the optical elements can be fabricated from plastic material. In addition to the low power consumption, the illumination device provided in the first embodiment has a further advantage of low fabrication cost. 
     The Third Embodiment 
     When a light valve can only accept a certain type of polarized light, only half of the incident light can be used, therefore, the illumination effect is not satisfactory. 
     In FIG. 4, a third embodiment of the invention is shown. The element and theory of this embodiment are basically the same as those in the second embodiment. The additional element is a polarization converter  410  is disposed between the light valve  320  and the illumination uniformizing means  310 . Therefore, the light unacceptable to the light valve  320  is converted into an acceptable type of polarized light to the light valve  320 , so that the light emitted from the light source can be fully utilized. 
     The polarization converter  410  comprises ¼ wavelength plates  414  at one plane thereof and a plurality of polarization beam splitters  412 . Each polarization beam splitter  412  is disposed on a corresponding ¼ wavelength plates  414  and arranged in a zigzag form. The adjacent polarization beam splitters  412  are arranged with a right angle between each other. 
     As mentioned in the second embodiment, when a light emitted from the light source  302  is incident into the illumination uniforming means  310  through the incident plane  312 , the light is totally reflected by the bottom plane  314  and the projection plane  318  and travelling in between. When the light is incident on the scattering pattern of the bottom plane  314 , the light is scattered to transmit through the projection plane  318  to the light valve  320 . The scattered light comprises both P-polarized  420  and S-polarized lights  422 . 
     In the case that the light valve only accepts a P-polarized light  420 , the S-polarized light  422  thus cannot be utilized at all. Traveling through the polarization beam splitter  412 , the P-polarized light  420  and the S-polarized light  422  of the scattered light are split. The P-polarized light  420  passes through and projects on the light valve  320 , while the S-polarized light  422  is reflected by two adjacent polarization beam splitter  412 . Passing through the ¼ wavelength plates  414 , the S-polarized is polarized into a circular polarized light and traveling back to the illumination uniformizing means  310 . When the circular polarized light is scattered by the scattering pattern on the bottom plane  314 , passing through the polarization converter  410 , the circular polarized light is then polarized into a P-polarized light to be accepted by the light valve  320 . Thus, the light emitted from the light source  302  can be fully utilized and projected into the light valve  320  efficiently. 
     Furthermore, the light converging lens, for example, a lens array, can also be dispose on the projection plane  318  of the illumination uniformizing means  310 . The scattered light with a large scattered angle can thus be collimated to project on the light valve  320 . 
     In addition to the advantages mentioned in the first and the second embodiments, by disposing a polarization converter, a light emitted from a light source can be fully transformed and projected onto the light valve, so that a better illumination is obtained. 
     The Fourth Embodiment 
     FIG. 5A shows the fourth embodiment of the invention. The elements and arrangement of the fourth embodiment are substantially the same as those in the third embodiment. The difference between the third and the fourth embodiments is the polarization converter  516  employed between the illumination uniformizing means  310  and the light valve  320 . 
     In FIG. 5A. a polarization converter  510  comprises a plurality of polarization beam splitters  512  and ½ wavelength plates  514 . The polarization beam splitters  512  are parallel from each other, while the ½ wavelength plates  514  are disposed on every other polarization beam splitters  512  on a plane  516  of the polarization converter  510 . 
     Again, when a light emitted from the light source  302  is incident into the illumination uniforming means  310  through the incident plane  312 , the light is totally reflected by the bottom plane  314  and the projection plane  318  and travelling in between. When the light is incident on the scattering pattern of the bottom plane  314 , the light is scattered to transmit through the projection plane  318  to the light valve  320 . The scattered light comprises both P-polarized  530  and S-polarized lights  532 . 
     Considering a light valve  320  accepts a P-polarized light only. When the scattered light reaches the polarization beam splitter  512 , the P-polarized light  530  pass through to illuminated the light valve  320 , while the S-polarized light  532  is reflected by two adjacent polarization beam splitters  512 . The reflected S-polarized light  532  then travels through the ½ wavelength plate  514  to be converted to a P-type polarized light and thus to illuminate the light valve  320 . This embodiment convert all the incident light from the light source into a type of polarized light which can be accepted by the light valve, therefore, a high efficiency is obtained. 
     Moreover, a set of light converging lens  520  can also disposed on the projection plane  318  of the illumination uniformizing means  310 . The set of light converging lens  520  comprises a plurality of collimate cylindrical lenses  522  and mirrors  524 . The collimate cylindrical lenses  522  and the mirrors  524  are disposed alternately on the projection plane  318 . That is, each collimate cylindrical lens  522  is arranged between two mirrors  522 , while each mirrors  524  is located between two collimate cylindrical lenses  522 . Each collimate cylindrical lens  522  can be replaced by a rows of lens  522 ′ as shown in FIG.  5 C. Each of the mirrors  524  is disposed on the projection plane  318  aligned with a corresponding ½ wavelength plate  514 . In addition, by aligning the optical center of each light converging lens  520  with the center of each scatter pattern, a better operation performance is obtained. 
     To lower the fabrication cost, the collimate cylindrical lens  522  can be replaced by Fresnel lens, or the collimate cylindrical lens  522  and mirrors  524  can be fabricated by material like plastic. 
     In addition, the collimate cylindrical lens  522  may also be replaced by holographic optic element or binary optical device. 
     The embodiment effectively utilize the light emitted from the light source to illuminate the light valve with a low fabrication cost. 
     From the second to the fifth embodiment, a back light panel type illumination uniformizing means is used with a single light emitting device as a light source to illuminate a light valve. In practical use, more than one light emitting devices can be used as the light source to illuminate a light valve. 
     FIG. 6A shows a top view of an illumination device adapting four light emitting devices as a light source. The light valve and the polarization converter are not shown in the figure. Four light emitting devices  302   a ,  302   b ,  302   c , and  302   d  are used in this embodiment to emit a light onto four side planes of an illumination uniformizing means respectively. According to the specific requirements of a practical application, any number of the light emitting devices can be applied as a light source. 
     FIG. 6B is a cross sectional view alone the cutting line II to II′ in FIG.  6 A. Since the light intensity decays with distance, the scatter pattern has to be gradually intensively distributed as increasing the distance to the light source. When two lights of light emitting devices  302   a  and  302   c  are incident onto two opposite side planes, two back light panels  310  are required as shown in FIG.  6 B. 
     The Fifth Embodiment 
     FIG. 7A shows a fifth embodiment of the invention, in which an integrator is used as an illumination uniformizing means. 
     In FIG. 7A, an illumination device  700  comprises a light emitting device light source module  710 , a light integration array, that is, an integrator  720 , a polarization converter  730 , and a light converging lens  740 . The light source module  710  further comprises an array of Light emitting devices  712 . The integrator  720  comprises a plurality of columnar light converging lens  722  into a lens array, and each of the columnar light converging lens corresponding to one LED  712 . Each light emitted from the LEDs is to incident the corresponding columnar light converging lens  720  and converged thereby. By stacking the light converged on each projection plane  722 ′, the light is uniformized to project to the polarization converter  730 . The polarization converter  730  converts the incident light into a polarization type of light acceptable for a light valve  720 . Before reaching the light valve  720 , the light travels through a light converging means  740  disposed in front the polarization converter  730  to be converged. 
     The incident plane and the projection plane of each columnar light converging light can be either a spherical plane or a non-spherical plane. A Fresnel type lens can be used as the columnar light converging lens  722  and the light converging means  740 . 
     The mechanism of the polarization converter  730  is shown as FIG.  7 B. The theory of the polarization converter  730  is the same as it of the polarization converter  510  introduced in the fourth embodiment. Considering the light valve  720  accepts a P-polarized light only. The P-polarized  736  light from the integrator  722  travels through a polarization beam splitter  732  of the polarization converter  730  to illuminated the light valve  720 , while the S-polarized light  738  is reflected by two adjacent polarization beam splitters  732 . The reflected S-polarized light  738  then travels through the ½ wavelength plate  734  to be converted to a P-type polarized light and thus to illuminate the light valve  720 . This embodiment convert all the incident light from the light source into a type of polarized light which can be accepted by the light valve, therefore, a high efficiency is obtained. 
     Similar to the above mentioned embodiments, the lens used in the embodiment may be made from plastic material which has a lower fabrication cost. The light converging lens may adapt holographic optical element or binary optical device. 
     In addition, each the light emitting devices do not have to correspond a certain one to the cylindrical light converging lenses, that is, more than one light emitting device may be disposed to correspond to a cylindrical light converging lens. 
     The Sixth Embodiment 
     FIG. 8A shows a sixth embodiment of the invention, in which a wedged glass rod is used to form an illumination uniformizing means. 
     In FIG. 8A, an illumination device comprises a light emitting device light source module  810 , a wedged glass rod array  820 , a lens array  830 , and a light converging lens  840 . The light emitting device light source module  810  comprises a number of light emitting devices  812 . The wedged glass rod array  820  comprises the same number of wedged glass rods  822  as the light emitting devices  812 . Each wedged glass rod  822  is disposed closely in front of a corresponding light emitting device  812 , so that each light emitted from each light emitting device  812  is collected by the corresponding wedged glass rod  822 . 
     In FIG. 8B, the light path collected by the wedged glass rod  822  is shown. In the wedged glass rod  822 , the light transmits in a way of total reflection by the inner surface of the wedged glass rod  822 . The light becomes a small angle deflected light while reaching the other end of the wedged glass rod  822 . The shape of the wedged glass rod  822  can be a cone shape as shown in the figure, or other shape with the same effect. That is, the wedged glass rod  822  has a small aperture for an incident light, that is, a smaller incident aperture, and a larger aperture at the other end of the wedged glass rod  822 , that is, a larger projection aperture. In some other applications, wedged glass rod having two apertures with the same dimension, for example, a columnar shape or a rectangular rod may also be employed. 
     The light travelling through each wedged glass rod  822  then reaches one of the lenses  832  of the lens array  830  to project on a light valve  850  uniformly. Before projecting on the light valve  850 , the light is converged by a light converging means  840 , so that all the light can be collected and projected on the light valve  850  efficiently. 
     Similar to the previous embodiment, each of the light emitting devices does not have to corresponding to a certain one of the wedged glass rod. More than one light emitting devices may be disposed and corresponding to only one wedged glass rod. 
     Fresnel type lens, holographic optical element, or binary optical device, can be adapted for fabricating lenses  832  and the light converging means  840 . Moreover, similar to the above mentioned embodiments, the lens used in the embodiment may be made from plastic material which has a lower fabrication cost. 
     The Seventh Embodiment 
     In FIG. 9, an image projection apparatus using an illumination device in the invention is shown. 
     As shown in the figure, the projection apparatus comprises three illumination devices with different light sources. A red light illumination device  900   a  is used for a red LCD light valve  910   a , a blue light illumination device  900   b  is used for a blue LCD valve  910   b , a green light illumination device  900   c  is used for a green LCD light valve  910   c . The illumination devices  900   a ,  900   b ,  900   c  can be selected from any of the above embodiments. The uniform red (R), blue (B), green lights emitted from the illumination devices  900   a ,  900   b ,  900   c  respectively are then traveling to a color synthesizing means  920 , which further comprises a spatially synthesizing means, for example, an X prism, a combination of dichroic mirrors and a sequentially synthesizing means, for example, time sequential controller. 
     In the X prism, the green light and the red light can transmit through the optical plane KK′, while the blue light is reflected therefrom. Whereas, the blue light and the green light can transmit through the optical plane JJ′, but the red light is reflected therefrom. The sequentilly synthesizing means includes a sequence order controller (not shown), for example, a time multiplex, to control the projection order of the three primary color lights. When the sequential frequency time sequential controller is fast enough, a required color by certain combination of the three colors is obtained. A color is thus obtained by synthesizing these three lights. The synthesized light is then projected on a screen via a projection object mirror  930  to display the image. 
     Other embodiments of the invention will appear to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples to be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.