Abstract:
This invention involves a lighting arrangement for color image projection with at least two lighting units, whose light will hit image-forming elements, such as DMDs or grating light valves via optical elements, so that a subsequent optical projection system will project a multi-colored image on a projection surface. This invention shows that the illuminating optical paths will hit one or more image-forming elements from different directions and that once they pass the image-forming element or elements, they will be combined into one common optical projection path.

Description:
RELATED APPLICATION  
       [0001]     The present application claims the benefit of priority to German Patent Application No. 10 2005 061 182.6 filed on Dec. 21, 2005. Said application is incorporated by reference herein.  
       FIELD OF THE INVENTION  
       [0002]     The invention relates to lighting arrangement to project color images with at least two lighting units via optical elements. The light passes through image-forming modulators, such as DMDs or grating light valves via optical elements, so that a subsequent optical projection system will project a multi-colored image on a projection surface.  
       BACKGROUND OF THE INVENTION  
       [0003]     There are a number of known lighting arrangements for color image projection, which use only one image-forming element (single-chip arrangement) or multiple image forming elements (multi-chip arrangements).  
         [0004]     DE 10127617 A1, for example, describes a projection arrangement with one lighting unit to create an illuminated field. Here, an image-forming element (light modulator) modulates the light coming from the lighting unit at the image-forming element. Then, an image is projected into the intermediate image layer via an optical projection system. The optical projection system has imaging optics with a mirror and a lens between the image-forming element and the mirror. Here, the light coming from the image-forming element and reflected by the mirror (optical projection path) transits the lens a second time.  
         [0005]     Usually, lighting concepts assume broadband light sources. With multi-chip systems, the light emitted by a light source is distributed to the different chips by corresponding systems. With single-chip systems, a color wheel modulates the light.  
         [0006]     Furthermore, there are known micro display systems that do not use light sources with a broad spectrum but narrow band sources, such as LEDs or laser light sources. Analogous to the use of broadband light sources, in order to illuminate a display, the different sources in a spectrum are overlapped by color separators; thus, similar lighting concepts are used, just like with conventional lighting.  
         [0007]     Arrangements of this type have the disadvantage of being relatively large, and it is often very difficult to house these in the limited space of a device. Furthermore, the combination of different spectrums also represents high requirements for the color separators used, especially when three or more light sources must be overlapped.  
       SUMMARY OF THE INVENTION  
       [0008]     Based on these disadvantages, this invention intends to further develop a lighting arrangement for color image projection by using monochromatic light sources with the goal of allowing for an effective overlapping of light sources while at the same time reducing the size of the system using relatively simple resources.  
         [0009]     This task is fulfilled by a lighting arrangement such as the one described at the beginning of this document. An arrangement of this type means that the light beams reach one or more image-forming elements separately from at least two directions and will subsequently be combined into one projection beam after they have passed the image-forming element.  
         [0010]     Using reflective image-forming elements, where the beam deflections of the illuminating light do not concur with the reflections on the layer of the image-forming element, creates an advantage.  
         [0011]     The optical paths for illumination have different spectrums, while the spectrums of the different illumination paths can be disjunctive or partially overlapped.  
         [0012]     The invention shows advantageous arrangements for single-chip and multi-chip formations:  
         [0013]     When using only one image-forming element, it is advantageous to use a prism arrangement with two air gaps (double-sided TIR prism). Here, the positioning of the air gap is determined by opposite angles to the common reference layer; one optical path is directed toward each air gap, which consists of one or more basic colors and the two optical paths are separately directed toward the image-forming element. The image-forming element combines the optical paths of the lighting system into the optical projection path.  
         [0014]     For practical purposes, a system is planned that will allow a synchronization between the image-forming element and the optical paths of the lighting system. The image-forming element will then be able to modulate the alternatively illuminated optical paths. Especially when a DMD is used as an image-forming element, this can mean that the on and off statuses of the DMD are interchanged for the two optical paths.  
         [0015]     In order to generate additional color portions, the invention creates a system of time-controlled color overlapping within one or both optical paths. This will allow the generation of three-color setup or of more colors.  
         [0016]     The orientation of two separate optical paths toward the image-forming element allows the time overlapping of relatively close or even transcending spectrums by means of the image-forming element. Contrary to conventional lighting arrangements, no dichroite is used for the overlapping of the two optical paths of the lighting system to the optical projection system, which would enlarge its size.  
         [0017]     Another advantage of this invention is to realize the illumination of the image-forming elements via an intermediate imaging system.  
         [0018]     Here, the solution described in the “State-of-the-art” section is equipped with two light sources that can be time-modulated in order to illuminate the image-forming element via the optical paths that are oriented in different directions.  
         [0019]     Another embodiment includes multiple intermediate imaging systems in order to create an overlap between partial images in the area of the intermediate image. This could be made possible by using dichroites or polarization beam splitters. If you generate two intermediate images with different polarization, you can create 3D effects with the respective auxiliary measures.  
         [0020]     It is also advantageous to design a two-piece optical projection system including a projection lens and a field lens. Here, the field lens will be used to illuminate an image-forming element as well as to project the modulated image (field lens design). Here, at least two lighting units are arranged so that their optical paths can be illuminated via the field lens from different directions and combined in the modulated optical projection path. The field lens can also be designed as a complex optical system consisting of different optical elements.  
         [0021]     When using three image-forming elements (multi-chip systems), the invention includes a prism arrangement consisting of at least four partial prisms. Three of these prisms are arranged so that the even surface of a partial prism is parallel to the image-forming element.  
         [0022]     Another surface of the partial prism is used for the entrance of the light of a basic color, while the third surface of each partial prism incorporates a fourth partial prism. Here, the composite surfaces of the fourth partial prism, which are in contact with the composite surfaces of the first and second prism, are coated with color separating layers. The color portions reflected by the image-forming elements are thus overlapped into a common optical projection path.  
         [0023]     With only four partial prisms, each image-forming element is illuminated via another path (optical path), while the optical projection path is overlapped by all three image-forming elements by the color separating layers. The prisms, via which the optical paths reach the different chips, are designed to generate total reflections wherever they touch the air gaps of the enclosed prism. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]     The following examples will describe the lighting arrangement in this invention in more detail. The figures depict the following:  
         [0025]      FIG. 1 : a first prism arrangement to illuminate an image-forming element;  
         [0026]      FIG. 2 : a second prism arrangement to illuminate an image-forming element;  
         [0027]      FIG. 3 : a prism arrangement to illuminate three image-forming element;  
         [0028]      FIG. 4 : an intermediate imaging system with two lighting units;  
         [0029]      FIG. 5 : an arrangement with one field lens to be used for illumination and projection. 
     
    
     DETAILED DESCRIPTION  
       [0030]      FIG. 1  shows a combination of three connected prisms  1 ,  2  and  3 , where air gaps  4  and  5  are present between the composite surfaces of prisms  1  and  2  and between prisms  2  and  3 . Prisms  1 ,  2  and  3  are designed so that their composite surfaces have exact opposite angles (β=β) to a reference plane. An optical path  7  from a monochromatic light source, such as the color green, penetrating into prism  3 , hits air gap  5  and is reflected by this gap onto an image-forming element  8 , such as a DMD. A second optical path  9  of a monochromatic light source, such as a red one, reaches the air gap  9  through prism  2  and is also reflected onto the image-forming element  8 . Then, the unification of the two optical paths  7  and  9  takes place at the image-forming element  8  to form the common optical projection path  10 . Via a switch arrangement (not pictured), the different monochromatic light sources can be switched on and off, so that the different color channels can be modulated separately. The triggering of the image-forming element  8  takes place so that the on and off status is interchanged between the optical paths  7  and  9 . Also feasible is an overlapping of single color portions in the front area of the lighting system, so that a three-color setup or a setup with even more colors can be generated via the image-forming element  8 .  
         [0031]     Another alternative of the prism arrangement is shown in  FIG. 2 . This includes prisms  11 ,  12 ,  13  and  14 .  
         [0032]     Analogous to the arrangement in  FIG. 1 , air gaps  4 ′ and  5 ′ are present, where the optical paths  7 ′ and  9 ′ are reflected totally toward the image-forming element  8 ′. This is where the unification into a common optical projection path  10 ′ takes place. The positioning of the composite surfaces characterized by the air gaps  4 ′ and  5 ′ between prisms  12  and  14  as well as between prisms  13  and  14  is defined by the angles α′ and β′.  
         [0033]      FIG. 3  shows a design alternative with four prisms  15 ,  16 ,  17  and  18  and three image-forming elements  19 ,  20  and  21 .  
         [0034]     One even surface each of the prisms  15 ,  16  and  17  is arranged parallel to the respective image-forming element  19 ,  20  and  21 . Another surface of each prism  15 ,  16  and  17  is used for the light intrusion of a basic color, while the third surface of each of the three prisms  15 ,  16  and  17  abuts the fourth prism  18 . The composite surfaces of the fourth prism  18 , which are in contact with the composite surfaces of prisms  15  and  16 , are coated with color separating layers  22  and  23 . Optical path  24 , which is marked by the basic color red, reaches the image-forming element  19  via the first outer surface of prism  15 . The portion (projection light) reflected by the image-forming element  19  hits the color separating layer  22  via the second outer surface of prism  15 , and overlaps with the color portion of the optical path marked by the color green, which is reflected by the image-forming element  20 . This reflected color portion also makes up the common optical projection path  26 , which is also hit by the color portion of the optical path  27  marked by the color blue, which is reflected by the image-forming element  21 , via the color separating layer  23 . To illuminate the image-forming elements  19  and  20 , air gaps  28  and  29  are located between the composite surfaces of prisms  16  and  18  and the composite surfaces of prisms  17  and  18 , so that the optical paths  25  and  27  can be totally reflected in the direction of the image-forming elements  20  and  21 . Due to the condition of the total reflection for the optical paths  25  and  27 , the selection of materials for prisms  16  and  17  and the necessary lighting angles at the image-forming elements  20  and  21 , the angles α 1  and α 2  of the prisms  16  and  17  are defined.  
         [0035]     The described prism combinations are only examples for a multitude of possible combinations, which unite several optical light paths into one optical projection path once they have passed the image-forming element or image-forming elements.  
         [0036]     In addition to the shown alternatives with one or three image-forming elements, other alternatives can be realized, such as two image-forming element configurations.  
         [0037]      FIG. 4  shows an intermediate imaging system with one lighting unit  30  and one lighting unit  31 , which are marked by three field points each. The optical paths  32  and  33 , which are emitted by the lighting units  30  and  31 , hit an image-forming element  36  via illumination systems  34  and  35  (not described in detail); there, they are combined into one common image-modulated optical projection path  37 . An intermediate image is created on image layer  42  via an optical imaging system located behind the image-forming element  36 , which consists of lenses  38 ,  39  and  40  as well as a mirror  41 . The deflecting mirror  41  (pupil of intermediate image) directs the modulated optical projection path  37  through the lenses  40 ,  39  and  38  into the intermediate image layer  42  a second time.  
         [0038]     Compared to prism combinations with relatively long optical paths, an arrangement of this kind has the advantage that the actual projection lens can be designed without long optical paths and that no reflection conditions from the lighting within the projection lens must be taken into consideration. This allows the development of small and simple projection lenses. A setup of this kind bears advantages, especially for a device concept with a number of different projection lenses for different areas of use (focal length, zoom factor, lens shift).  
         [0039]      FIG. 5  shows an example with a two-part projection lens, consisting of a projection lens  43  and a field lens  44 . Here, the field lens  44  is used for the lighting of an image-forming element  45  as well as for the projection of the image modulated by the image-forming element  45  (field lens design). Also, with this layout alternative, two optical paths  46  and  47  are planned to illuminate the image-forming element  45 .  
         [0040]     The image-forming element  45  is illuminated via the lighting units  48  and  49 , which are depicted as cones of light. For this, the optical path  46  emitted by the lighting unit  48  is deflected at the deflection mirror  50  toward the field lens  44 , defined there and then pointed to the image-forming element  45 . Analogous to this beam line, the optical path  46 , which originates in the lighting unit  49  and travels toward the field lens  44  via a deflection mirror  51 , becomes optical path  47  and hits the image-forming element  45 . Due to the double function of field lens  44 , the image modulated by the image-forming element  45  is directed to the projection lens  43  in a common optical projection path  52 . This alternative has the advantage that the different elements can be integrated into fairly small modules and that undesired reflections, which can occur with prism combinations, are avoided.