Patent Publication Number: US-11035528-B2

Title: Light emitting device with diffuser and light reflector and projection system having the same

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     This invention relates to the field of projectors, and in particular, it relates to a light emitting device and a projection system using the same. 
     Description of the Related Art 
     Traditional light sources such as fluorescent light, incandescent light, super-high performance light or xenon light cannot easily achieve high efficiency and long life. With the development of solid state light sources, light emitting diodes (LED) and semiconductor lasers are gradually entering the illumination and display market. 
     There are typically two ways to generate white light or color light: One is to directly use color light sources such as red, green or blue LED to provide color light, or to combine these color light to generate white light; the other is based on wavelength conversion, by exciting wavelength conversion materials (such as phosphors) with an excitation light to generate converted light, and then combining the excitation light or converted light to generate white light. 
     Taking phosphors as an example, conventional light sources that use wavelength conversion based method and LED chips typically provide the phosphor materials on a surface of the LED chip, so that the converted light that travels backwards through the phosphor layer and scatters toward the LED can be reflected by the LED chip to exit from the side of the phosphor layer, whereby the light output efficiency is enhanced. A shortcoming of this structure is that, the heat generated by the LED chip and the heat generated by the phosphor layer interfere with each other, which lowers the light emitting efficiency of the LED chip and the light conversion efficiency of the phosphor, and can even reduce the life of the LED device. 
     This shortcoming similarly exists in devices that directly use color light sources to provide color light. For example, when laser sources are directly used to provide color lights, because laser light is strongly coherent, the pixels in the projected image on the screen will show speckles due to coherency, so the image cannot be properly displayed. Therefore, in laser projectors, de-coherence devices or methods need to be used to eliminate the coherent speckles. In current technologies, to enhance light output efficiency, typically a diffusing element is disposed on the surface of the laser source, so that the light that travels backwards through the diffusing element toward the laser source can be reflected by the surface of the laser source to exit from the side of the diffusing element. However, the heat generated by the laser source and the heat generated by the diffusing element interfere with each other, which lowers the light emitting efficiency of the laser source, and can even reduce the life of the laser source. 
     To overcome the above shortcomings, U.S. Pat. No. 7,070,300 B2 describes a method that separates the LED and the phosphor materials, as shown in  FIG. 1 . An excitation light from one or more LED source  102  is collimated by a collimating device  108 , and a wavelength selection filter  110  reflects the excitation light to another collimating device  114  which focuses the light onto a phosphor plate  112 . The converted light from the phosphor plate  112  passes through the wavelength selection filter  110  to become the output light of the light source. In this device, relying on the different wavelengths of the excitation light and the converted light, the light path of the excitation light and the converted light are separated using the wavelength selection filter  110 ; so that while increasing the light output efficiency, the converted light is prevented from traveling back to the LED chip. Therefore, the heat generated by the LED chip and by the phosphor will not interfere with each other, which solves the above-described problem of the conventional technology. 
     A problem of the technology scheme described in the above patent is that, if the phosphor is changed to a diffusing element for eliminating coherency, because the coherent light and incoherent light have the same wavelength, the incoherent light emitted from the diffusing element toward the coherent light source will travel back to the coherent light source along the same path of the coherent light, so the incoherent light cannot be output from the light source device. Thus, the goal of increasing light output efficiency and the goal of reducing interference of the heat generated by the coherent light source and the diffusing element are in conflict with each other, and both goals cannot be achieved at the same time. 
     SUMMARY OF THE INVENTION 
     The present invention solves the above problem of the conventional technology, by providing a light emitting device and a projection system employing the same which improves light output efficiency; at the same time, most of the incoherent light from the diffusing element travelling toward the coherent light source will not return to the coherent light source along the same path of the coherent light, so that the interference of the heat generated by the laser source and the heat generated by diffusing element is reduced. 
     The present invention provides a light emitting device, which includes: 
     a coherent light source for emitting a coherent light; 
     a diffusing element having a first surface and a second surface opposite each other, for diffusing the coherent light from the coherent light source to generate an incoherent light; and 
     a light guide element disposed on the side of the first surface of the diffusing element, for guiding the coherent light emitted by the coherent light source to incident on the first surface of the diffusing element to form a first light path, and for guiding a portion of the incoherent light from the first surface of the diffusing device to exit via the first light path, for guiding a remaining portion of the incoherent light from the first surface of the diffusing device to exit via a second light path, and for separating the first and second light paths; 
     further, the luminous flux of the incoherent light exiting via the first light path is less than the luminous flux of the incoherent light exiting via the second light path. 
     The present invention also provides a projection system that includes the above light emitting device. 
     Compared to conventional technology, the present invention has the following advantages: 
     In embodiments of the present invention, by using the light guide element to guide the coherent light to the diffusing element via the first light path, and to guide a majority of the incoherent light from the first surface of the diffusing element to become output of the light emitting device via a second light path that is separate from the first light path, the output efficiency of the light emitting device is enhanced, and at the same time, the majority of the incoherent light form the first surface of the diffusing element will not return to the coherent light source along the light path of the coherent light, so that the inference between the heat generated by the incoherent light and the heat generated by the coherent light source is reduced. This improves the light emitting efficiency and life of the coherent light source. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates the structure of a conventional light source which uses LED and phosphor to generate highly efficient color lights. 
         FIG. 2  illustrates the structure of a light emitting device according to an embodiment of the present invention. 
         FIG. 3  illustrates the structure of a light emitting device according to another embodiment of the present invention. 
         FIG. 4  illustrates the structure of a light emitting device according to another embodiment of the present invention. 
         FIG. 5  illustrates the structure of a light emitting device according to another embodiment of the present invention. 
         FIG. 6  illustrates the structure of a light emitting device according to another embodiment of the present invention. 
         FIG. 7  illustrates the structure of a light emitting device according to another embodiment of the present invention. 
         FIG. 8  illustrates the structure of a light emitting device according to another embodiment of the present invention. 
         FIG. 9  illustrates the structure of a light emitting device according to another embodiment of the present invention. 
         FIG. 10A  illustrates the structure of a light emitting device according to another embodiment of the present invention. 
         FIG. 10B  illustrates the structure of a light emitting device according to another embodiment of the present invention. 
         FIG. 11  illustrates the structure of a light emitting device according to another embodiment of the present invention. 
         FIG. 12  illustrates the structure of a light emitting device according to another embodiment of the present invention. 
         FIG. 13  illustrates the structure of a light emitting device according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     To solve technical problems of the conventional technologies, embodiments of the present invention provide a light emitting device, which includes: a coherent light source for generating a coherent light; a diffusing element having a first surface and a second surface opposite each other, for diffusing the coherent light from the coherent light source to generate an incoherent light; and a light guide element disposed on the side of the first surface of the diffusing element, for guiding the coherent light emitted by the coherent light source to incident on the first surface of the diffusing element to form a first light path, for guiding a portion of the incoherent light from the first surface of the diffusing device to exit via the first light path, for guiding a remaining portion of the incoherent light from the first surface of the diffusing device to exit via a second light path, and for separating the first and second light paths; further, the luminous flux of the incoherent light exiting via the first light path is less than the luminous flux of the incoherent light exiting via the second light path. 
     Considering the near Lambertian distribution characteristics of the incoherent light scattered by the diffusing element, and the relatively small etendue of the coherent light incident on the diffusing element form the coherent light source, embodiments of the present invention takes advantage of the difference in etendue between the coherent light source and the diffusing element. It uses the light guide element to guide the input light path of the coherent light into the output light path of the diffusing element, and at the same time, limits the luminous flux of the incoherent light guided by the light guiding device and exiting via the first light path to be smaller than the luminous flux of the incoherent light exiting via the second light path. This way, the majority of the incoherent light from the first surface of the diffusing element (i.e. the majority of the incoherent light emitted from the diffusing element toward the coherent light source) exits via the second light path, instead of escaping excessively via the first light path, i.e., it will not return to the coherent light source along the input path of the coherent light. Therefore, the interference between the heat generated by the coherent light source and the heat generated by the diffusing element is reduced, and the light emitting efficiency and life of the coherent light source are improved. 
     Referring to  FIG. 2 , which illustrates the structure of a light emitting device according to an embodiment of the present invention. As shown in  FIG. 2 , in this embodiment, the light emitting device  100  includes a coherent light source  110  for generating a coherent light, a diffusing element  120  for scattering the coherent light form the coherent light source  110  to generate an incoherent light, and a light guide element  140 . The diffusing element  120  includes a first surface  121  and a second surface  122  opposite each other. The light guide element  140  is disposed on the side of the first surface  121 , i.e. the first surface  121  of the diffusing element is closer to the light guide element  140  than the second surface is. 
     To ensure that the coherent light source has a relatively small etendue, the coherent light source  110  is preferably a laser diode. The coherent light source can also be light emitting diodes or other types of light sources. A light homogenizing device which smoothes and shapes the light can be used to guide the coherent light from the coherent light source  110  to the light guide element  140 . The light homogenizing device may be a fly-eye lens array or a hollow or solid light rod. 
     The scattering mechanisms of diffusing elements typically include volume scattering and surface scattering. Surface scattering diffusing element achieve scattering by using surface microstructures of a transparent material to refract and reflect light. Such diffusing elements can be divided into single-sided microstructure and double-sided microstructure. The microstructures may be formed by sandblasting the surface of a glass substrate, or by chemical corrosion of a glass surface, or by heat-press forming on a plastic substrate. If a single-sided microstructure diffusing device is used, preferably, the coherent light is incident onto the surface that has the microstructures (i.e. the first surface is the surface with microstructures) and exits from the smooth surface. This achieves relatively high transmission rate. Volume scattering achieves diffusion by mixing micro particles that have different refractive indices or are opaque into the body of the diffusing element. 
     In this embodiment, the light guide element  140  is a second reflective element  140  that has an aperture  130  and a reflecting surface (not labeled in the drawing) surrounding the aperture. In this embodiment, the second reflective element  140  is specifically an arc shaped reflective element having the aperture  130  and the reflecting surface. The second reflective element  140  can also be a planar reflector, saw-tooth shaped reflecting device or a curved reflecting device of other shapes. These preferred shapes will be described in more detail later. 
     The aperture  130  guides the coherent light from the coherent light source  110  by transmission onto the first surface  121  of the diffusing element  120  to form the first light path, and guides a portion of the incoherent light from the first surface  121  of the diffusing element  120  to exit via the first light path. The reflective surface of the second reflective element  140  is used to guide the remaining incoherent light from the first surface  121  by reflection to exit via the second light path. The incoherent light reflected by the reflective surface of the second reflective element  140  and the incoherent light from the second surface  122  of the diffusing element  120  together constitute the output light of the light emitting device  100 , so that the light output efficiency of the light emitting device is increased. An optical element may be provided at the output end of the light emitting device to collect, smooth and shape the incoherent light. 
     Further, the second reflective element  140  separates the first light path and the second light path using the aperture  130  and the reflective surface. Specifically, the first light path is from the diffusing element  120 , via the aperture  130  of the second reflecting device  140 , to the coherent light source  110 ; the second light path is from the diffusing element  120  via the reflective surface of the second reflecting device  140 , and reflected by the reflective surface to travel away from the coherent light source  110 , so as to be separated from the first light path. By separating the first and second light paths, the light output efficiency of the light emitting device is improved, and at the same time, the incoherent light exiting via the second light path will not return to the coherent light source along the first light path. 
     Because the etendue of the coherent light emitted by the coherent light source  110  is relatively small, and the incoherent light after scattering by the diffusing element  120  has a near Lambertian distribution and has an etendue much larger than that of the coherent light, the area ratio of the aperture  130  of the second reflective element  140  to the reflective surface can be controlled to be a relatively small value, so that the majority of the incoherent light from the first surface  121  of the diffusing element can be reflected by the reflective surface of the second reflective element  140  to become output and be effectively utilized, while a small portion of the incoherent light leaks through the aperture  130  and become lost. The ratio of lost light is within an acceptable range. Preferably, the area of the aperture of the second reflective element  140  is less than or equal to ¼ of the area of the reflective surface of the second reflecting device  140 . 
     Compared to conventional technologies, in the present embodiment, by using the aperture  130  of the light guide element  140  to guide the coherent light along the first light path to the diffusing element  120 , and by using the reflective surface of the light guide element  140  to guide the majority of the incoherent light from the first surface of the diffusing element  120  to exit via the second light path which is separate from the first light path and become the output of the light emitting device, the light output efficiency of the light emitting device is improved, and at the same time, the majority of the incoherent light from the first surface  121  of the diffusing element will not return to the coherent light source along the same path of the coherent light, so that the interference between the heat generated by the coherent light source and the heat generated by incoherent light is reduced, which improves the light emitting efficiency and life of the coherent light source. 
     Referring to  FIG. 3 , which illustrates the structure of a light emitting device according to another embodiment of the present invention. A main difference from the embodiment of  FIG. 2  is that, in this embodiment, the light emitting device  200  additionally includes a first reflective element  150  and a light collecting device  160 . The first reflective element  150  reflects the incoherent light from the second surface  122  of the diffusing element  120  toward the second reflective element  140 , so that all of the incoherent light from the diffusing element travel toward the second reflective element  140 . In this embodiment, the diffusing element  120  is a transmission type diffusing element as an example. To ensure that all incoherent light from the diffusing element travel toward the second reflective element  140 , the first reflective element  150  is needed to reflect the incoherent light from the second surface  122  of the diffusing element. In other embodiments, the first reflective element  150  may be omitted; rather, by increasing the thickness of the diffusing element  120 , the incoherent light generated from the coherent light by the first surface of the diffusing element cannot penetrate to the second surface of the diffusing element, so that all the incoherent light generated by the diffusing element exits the first surface toward the second reflective element  140 . 
     Preferably, the second reflective element  140  is a hemisphere or a part thereof. The location where the coherent light is incident on the diffusing element  120  is at a first point near the spherical center of the hemisphere, and the light input port of the light collecting device  160  is located at a second point near the spherical center, where the first point and second point are symmetrical with respect to the spherical center. The coherent light from the coherent light source  110  is incident on the diffusing element  120  through the aperture  130 . The incoherent light from the second surface  122  of the diffusing element  120  is reflected by the first reflective element  150 , and travels toward the light guide element  140  together with the incoherent light from the first surface  121  of the diffusing element. The reflective surface of the second reflective element  140  reflects a majority of the incoherent light to the light input port of the light collecting device located at the second point near the spherical center. The majority of the incoherent light is collected by the light collecting device  160  and output as the output light of the light emitting device  200 . Meanwhile, a small portion of the incoherent light from the diffusing element  120  transmits through the aperture of the second reflective element  140  and become lost. 
     Preferably, the aperture  130  is an offset aperture of the hemisphere shaped second reflective element  140 , so that the coherent light from the coherent light source  110  is incident on the diffusing element  120  perpendicularly. This way, if the diffusing element  120  falls off of the first reflective element  150 , the coherent light will be reflected by the first reflective element  150  back into the aperture  130  of the second reflective element  140  and back to the coherent light source, and will not be reflected to the reflective surface of the second reflective element  140  and become output to the light collecting device, which can harm the human eyes. 
     Preferably, the second reflective element  140  may also be a semi-ellipsoid of a part thereof. The location where the coherent light is incident on the diffusing element  120  is at the first focal point of the semi-ellipsoid, and the light input port of the light collecting device  160  is located at the second focal point of the semi-ellipsoid. A majority of the incoherent light from the diffusing element is reflected by the reflective surface of the second reflective element  140  to the second focal point of the semi-ellipsoid, and is collected by the light collecting device  160  and output as the output light of the light emitting device  200 . Similarly, the aperture  130  is preferably an offset aperture of the semi-ellipsoid shaped second reflective element  140  so that the coherent light from the coherent light source  110  is incident on the diffusing element  120  perpendicularly. 
     More specifically, in this invention, the second reflective element  140  is the reflective wall of a hollow structure that has an aperture. The reflective wall has a reflective coating coated on the inner surface, and the aperture is an opening on the reflective wall. It should be understood that the reflective coating can also be coated on the outer surface of the reflective wall. 
     Further, in this embodiment, the light collecting device  160  is a hollow light guide. In fact, the light collecting device  160  of this embodiment can also be a lens, lens set, hollow light guide, solid light guide, hollow compound parabolic concentrator, solid compound parabolic concentrator, or their combinations. 
     Referring to  FIG. 4 , which illustrates the structure of a light emitting device according to another embodiment of the present invention. One difference from the embodiment of  FIG. 3  is that, in this embodiment, the second reflective element  140  of the light emitting device  300  is a solid transparent body  330  coated with a reflective coating  331  on its outer curved surface, and the aperture  332  is an opening  332  in the reflective coating  331 . Preferably, there is an air gap between the diffusing element  120  and the solid transparent body  330  (not shown in the drawing), to increase the output brightness of the light emitting device. Similar to the embodiment of  FIG. 3 , the solid transparent body  330  is preferably a hemisphere or semi-ellipsoid. Here, the air gap between the diffusing element  120  and the solid transparent body  330  has a thickness preferably less than 1% of the radius of the hemisphere or 1% of the longest semi-principle axis of the semi-ellipsoid, which can effectively increase the output brightness of the light emitting device. 
     Referring to  FIG. 5 , which illustrates the structure of a light emitting device according to another embodiment of the present invention. One difference because this embodiment and the embodiment of  FIG. 3  is that, in the light emitting device  400 , the aperture  130  is a gap on the edge of the second reflective element  140 , and the coherent light from the coherent light source  110  is incident on the diffusing element  120  through the gap. It should be understood that the second reflective element  140  of this embodiment may be a solid transparent body coated with a reflective coating on its outer curved surface 
     Referring to  FIG. 13 , which illustrates the structure of a light emitting device according to another embodiment of the present invention. One difference because this embodiment and the embodiment of  FIG. 3  is that, in the light emitting device  1200 , the second reflective element  140  is a saw-tooth shaped reflective element, which includes two saw-tooth surfaces  1301  and  1302 , each being a part of a set of concentric spheres. It can be understood with reference to the illustration of  FIG. 3  that, the saw-tooth surfaces  1301  and  1302  can respectively function as an arc shaped reflective element, so the saw-tooth shaped reflective element can be considered a nested combination of a set of arc shaped reflective element s, which has the same reflecting effect as one arc shaped reflective element with respect to the incoherent light from the first surface of the diffusing element. A difference between the saw-tooth shaped reflective element and an arc shaped reflective element is that, the saw-tooth shaped reflective element occupies a smaller space and has a more compact structure. 
     Referring to  FIG. 6 , which illustrates the structure of a light emitting device according to another embodiment of the present invention. One difference from the embodiment of  FIG. 3  is that, in this embodiment, the second reflective element of the light emitting device  500  is a planar reflective element  540  having an aperture  530  and a reflective surface surrounding the aperture  530 . The planar reflective element  540  reflects the incoherent light from the diffusing element so that the incoherent light is output in a direction which is at an angle relative to the coherent light. 
     Because the incoherent light has a near Lambertian distribution, preferably, this embodiment further includes a lens set  570  to collect the incoherent light from the diffusing element  120  and collimate it onto the planar reflective element  540 . A majority of the collimated incoherent light is reflected by the reflective surface of the planar reflective element  540  and becomes the output light of the light emitting device  500 . More preferably, the projection area of the aperture  530  on the output light spot of the lens set  570  is less than ¼ of the area of the output light spot, so as to reduce the amount of incoherent light leaked out through the aperture  530  and to increase the light output efficiency of the light emitting device  500 . It should be understood that, other light collecting devices described earlier can be used to replace the lens set  570  in order to collect the incoherent light from the diffusing element  120  and relay it to the planar reflective element  540 . 
     In addition, an optical elements may be provided at the output end of the light emitting device  500  to collect, smooth and shape the incoherent light, which will not be described in more detail here. 
     Referring to  FIG. 7 , which illustrates the structure of a light emitting device according to another embodiment of the present invention. One difference from the embodiment of  FIG. 6  is that, in this embodiment, the light guide element of the light emitting device  600  includes a third reflective element  630  and a transparent medium surrounding the third reflective element  630  (in this embodiment, the transparent medium is specifically air surrounding the third reflective element  630 ). The third reflective element  630  is used to guide the coherent light from the coherent light source  110  by reflection to incident onto the first surface of the diffusing element  120  to form a first light path, and to guide a portion of the incoherent light from the first surface of the diffusing element  120  to exit via the first light path; the transparent medium surrounding the third reflective element  630  guides the remaining portion of the incoherent light from the first surface of the diffusing element  120  by transmission to exit via a second light path, where the incoherent light exiting via the second light path constitutes the output light of the light emitting device  600 . The light guide element separates the first and second light paths by using the third reflective element  630  and the transparent medium. 
     Similar to the embodiment of  FIG. 6 , preferably, this embodiment further includes a lens set  570  to collect and collimate the incoherent light form the first surface of the diffusing element  120 . More preferably, the projection area of the third reflective element  630  on the output light spot of the lens set is less than ¼ of the area of the output light spot, so as to reduce the amount of incoherent light leaked by reflection of the third reflective element  630 , and increase the light output efficiency of the light emitting device  600 . It should be understood that, other light collecting devices described earlier can be used to replace the lens set  570 , the light collecting device being located between the diffusing element  120  and the third reflective element  630 , in order to collect and relay the incoherent light from the first surface of the diffusing element  120 . An optical elements may be provided at the output end of the light emitting device  600  to collect, smooth and shape the incoherent light, which will not be described in more detail here. 
     Referring to  FIG. 8 , which illustrates the structure of a light emitting device according to another embodiment of the present invention. In this embodiment, the light guide element of the light emitting device  700  includes a second reflective element  140  having an aperture  130  and a reflective surface surrounding the aperture. The aperture  130  guides the coherent light from the coherent light source  110  by transmission onto the first surface of the diffusing element  120  to form the first light path, and guides a portion of the incoherent light from the first surface of the diffusing element  120  to exit via the first light path. Different from the embodiment of  FIG. 3 , in this embodiment, the first reflective element  150  is not provided; the incoherent light generated by the diffusing element  120  which travels toward its second surface directly exits the second surface to become the output light of the light emitting device. The reflective surface of the second reflective element  140  guides the incoherent light from the first surface  121  of the diffusing element  120  by reflection back to the first surface  121  of the diffusing element  120 ; this incoherent light transmits the diffusing element and exits the second surface  122  of the diffusing element  120  to become the output light of the light emitting device. 
     Preferably, the second reflective element  140  is an arc shaped reflective element having an aperture and a reflecting surface surrounding the aperture. The arc shaped reflector has a hemispherical shape, and the location where the coherent light is incident on the diffusing element  120  is at the spherical center. The incoherent light from the first surface of the diffusing element  120  travels towards the second reflective element  140 , which reflects a majority of the incoherent light towards the diffusing element  120  located at the spherical center. Further, this embodiment includes a light collecting device  160 ; the light input port of the light collecting device  160  is located on the side of the second surface of the diffusing element  120  (i.e., the second surface  122  is closer to the light collecting device  160  than the first surface  121  is), for collecting the incoherent light from the diffusing element  120 . 
     In addition, more specifically, the second reflective element  140  of this embodiment is the reflective wall of a hollow structure that has an aperture. The reflective wall has a reflective coating coated on the inner surface, and the aperture is an opening on the reflective wall. It should be understood that the second reflective element  140  can also be a saw-tooth shaped reflective element, which includes at least two saw-tooth surfaces each being a part of a set of concentric spheres, where the diffusing element is located at the spherical center of the concentric spheres. 
     Referring to  FIG. 9 , which illustrates the structure of a light emitting device according to another embodiment of the present invention. One difference from the embodiment of  FIG. 8  is that, in this embodiment, the second reflective element of the light emitting device  800  is a solid transparent body  330  coated with a reflective coating  331  on its outer curved surface, and the aperture  332  is an opening  332  in the reflective coating  331 . Preferably, the solid transparent body  330  has a hemispherical shape, and there is an air gap between the diffusing element  120  and the solid transparent body  330 . The air gap has a thickness preferably less than 1% of the radius of the hemisphere or 1% of the longest semi-principle axis of the semi-ellipsoid, which can effectively increase the output brightness of the light emitting device. 
     Preferably, this embodiment further includes a lens set  570  disposed on the side of the second surface of the diffusing element  120 , so that the incoherent light from the diffusing element  120  is collimated by the lens set  570  and outputted. In fact, the other types of light collection devices described earlier may be used to replace the lens set  570 . 
     Further, other embodiments of the present invention modify the above-described embodiments; i.e., the light emitting device additionally includes a driving device for driving the diffusing element, so that the illumination spot of the coherent light incident on the diffusing element acts upon the diffusing element along a predetermined path. This can avoid the high temperature of the diffusing element caused by the coherent light incident on the same location of the diffusing element for a long time period, and can increase the useful life of the diffusing element. Preferably, the driving device may be a rotating plate, and the diffusing element may be mounted on the rotating plate, so that the diffusing element moves with the rotating plate in a circular manner, and the light spot of the coherent light incident on the diffusing element acts upon the diffusing element along a circular path. Of course, the driving device may drive the diffusing element to move in other ways, such as a linear motion. 
     Referring to  FIG. 10A , which illustrates the structure of a light emitting device according to another embodiment of the present invention. The embodiment of  FIG. 10A  is another reflective-type embodiment, modified based on the embodiment of  FIG. 3 . The light emitting device  900 A of this embodiment further includes a rotating plate  980 . Both the diffusing element  120  and the first reflective element  150  are ring shapes which are concentric with the rotating plate; they are disposed on the rotating plate  980  and rotate with it. The first reflective element  150  is located between the driving device  980  and the diffusing element  120 . The first reflective element  150  may also be a part of the rotating plate  980 . The incoherent light reflected by the reflecting surface of the second reflective element  140  transmits through the rotating plate  980  and is collected by the light collecting device  160  for output. The light collecting device  160  can also be located on and extension line outside of the periphery of the rotating plate  980 , i.e., the incoherent light reflected by second reflective element can be incident directly into the light collecting device  160  without passing through the rotating plate  980 . 
     Referring to  FIG. 10B , which illustrates the structure of a light emitting device according to another embodiment of the present invention. The embodiment of  FIG. 10A  is another reflective-type embodiment, modified based on the embodiment of  FIG. 8 . The light emitting device  900 B of this embodiment further includes a rotating plate  980 . The diffusing element  120  is a ring shape which is concentric with the rotating plate; it is disposed on the rotating plate  980  and rotates with it. The region of the rotating plate  980  which carries the diffusing element  120  is formed of a transparent material, so that a portion of the incoherent light from the diffusing element directly transmits through the rotating plate  980  to be output, while another portion of the incoherent light is reflected by the reflecting surface of the second reflective element  140  back to the diffusing element, and exits its second surface and the rotating plate  980  to be output. This embodiment further includes a lens set  570 , located on the side of the second surface of the diffusing element. The lens set  570  collimates the incoherent light and outputs it. 
     Further, another embodiment of the present invention provides a light emitting device where the coherent light source includes at least two sub-light sources respectively for generating at least two colored lights, and a light combining device for combining the light from the at least two sub-light sources into one light beam. This embodiment is described in more detail below. 
     Referring to  FIG. 11 , which illustrates the structure of a light emitting device according to another embodiment of the present invention. One difference between this embodiment and the embodiment of  FIG. 10A  is that, in the light emitting device  1000 , the coherent light source  110  includes a first sub-light source  111  and a second sub-light source  112 . The first sub-light source  111  is a laser diode generating a red light, and the second sub-light source  112  is a laser diode generating a green light. The light emitting device  1000  additionally includes a light combining device  114 , which combines the lights respectively generated by the first sub-light source  111  and the second sub-light source  112  into one combined light beam. The combined light beam is incident on the diffusing element  120  via the aperture  130 . 
     More specifically, in this embodiment, the light combining device  114  is a dichroic filter that transmits red light and reflects green light. The red coherent light from the first sub-light source  111  transmits through the light combining device  114  to reach the aperture  130 , while the green coherent light from the second sub-light source  112  is reflected by the light combining device  114  to reach the aperture  130 . Of course, the light combining device  114  can also be a dichroic filter that reflects ref light and transmits green light, so that the red coherent light from the first sub-light source  111  is reflected to reach the aperture  130 , and the green coherent light from the second sub-light source  112  is transmitted to reach the aperture  130 . Therefore, the red coherent light and the green coherent light are combined into one light beam. 
     The red coherent light and green coherent light can simultaneously pass through the aperture  130 , and the output light of the light emitting device  1000  is a yellow incoherent light formed by the red incoherent light and the green incoherent light. Similarly, in another embodiment of the light emitting device, the coherent light source may further include a third sub-light source, such as a laser diode generating a blue coherent light. The light combining device may be a dichroic filter set formed in a cross shape, to combined the coherent lights from the first, second and third sub-light sources into one combined light beam and to guide it through the aperture  130 . This light emitting device can output a white incoherent light formed by combining the red, green and blue incoherent light. 
     In the above embodiments, the coherent light sources  110  are light sources that generate one color light (such as blue of yellow), and the light emitting device output one color incoherent light. The present invention may also be applied in situations that require a multiple color light sequence output. Therefore, another embodiment of the present invention provides a light emitting device, which further includes a control device for respectively controlling the on and off as well as light intensities of the two sub-light sources of the coherent light source. 
     Referring to  FIG. 12 , which illustrates the structure of a light emitting device according to another embodiment of the present invention. One difference between this embodiment and the embodiment of  FIG. 11  is that, the coherent light source  110  of the light emitting device  1100  further includes a third sub-light source  113 , which is a laser diode generating a blue coherent light. The light combining device  114  is a dichroic filter set, including two dichroic filter plates  1141  and  1142  disposed in parallel to each other. The dichroic filter set  114  combines the light from the first, second and third sub-light sources into one combined light beam. The combined light beam passes through the aperture  130  to incident on the diffusing element  120 . Specifically, the dichroic filter plate  1141  reflects the red coherent light generated by the first sub-light source  111  and transmits the green coherent light generated by the second sub-light source  112 , and both the red and green coherent light transmits through the dichroic filter plate  1142 ; the dichroic filter plate  1142  reflects the blue coherent light generated by the third sub-light source  113 . 
     The light emitting device  1100  further includes a control device (not shorn in the drawing), for respectively controlling the on and off as well as light intensities of the three sub-light sources of the coherent light source. For example, the control device may control the three sub-light sources that generate red, green and blue coherent light to turn on and off of in an order, so that the light emitting device outputs red, green and blue incoherent light in that order. The control device can also control the three sub-light sources that generate red, green and blue coherent light to turn on simultaneously, and control the light intensities of the three sub-light sources to change periodically, so that the color of the combined light generated by the light combining device changes periodically, and the light emitting device outputs an incoherent light whose color changes periodically. 
     The present invention also provides a projection system, including a light emitting device which may have the functions described in the above embodiments. 
     The above described embodiments of the present invention are exemplary only and do not limit the scope of the invention. Any equivalent structures and equivalent processes and variations based on the instant disclosure and drawings, or direct or indirect applications in other relevant technology areas, are all within the scope of patent protection of this invention.