Abstract:
An apparatus for projecting images onto a projection screen is disclosed. The apparatus incorporates novel features to reduce both cost and size while providing a high level of illumination uniformity at the projection surface. Specifically, the apparatus uses an illumination focusing group for providing a uniform level of light on the image to be projected. The illumination focusing group is designed to make use of lenses unsuitable for use as projection lenses, thus reducing scrap and the attendant cost thereof.

Description:
UNITED STATES GOVERNMENT RIGHTS  
       [0001] The United States Government has acquired certain rights in this invention through Government Contract No. NAS1-20219 awarded by the National Aeronautics and Space Administration. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates in general to the field of image projection, and more particularly, to the projection of images on a projection screen.  
           [0003]    Without limiting the scope of the invention, its background is described in connection with liquid crystal displays, as an example.  
           [0004]    The use of a combination of a light source and one or more lenses to project a small image onto a large screen is generally known in the art. This general method is commonly employed in the design of movie projectors, slide projectors and overhead projectors, as examples. Generally these designs incorporate a light source which illuminates an image printed on a planar object surface, such as a transparent film or similar medium. The light rays coming from the object surface are then focused on a screen or other projection surface by a lens or group of lenses. Generally, the image displayed on the projection surface is significantly larger than the image on the object surface, as the lens group performs both a magnification and a focusing function.  
           [0005]    With such an apparatus, it is important that the image projected on the surface have a substantially uniform illumination level. A non-uniform illumination level is manifested in the projected image as overly dark and overly bright regions, making the projected image uncomfortable or difficult for the viewer to read or discern. To address this problem, illumination lenses have been incorporated into the design. These lenses are designed to focus and direct the light onto the object surface with a uniform level of illumination. A more uniform level of illumination at the object surface generally results in a more uniform level of illumination in the image projected on the projection surface.  
           [0006]    Unfortunately, the cost of lenses represents a significant portion of the cost of a projection assembly. As such, the addition of illumination lenses to a design can represent a significant cost increase to the projection assembly.  
         SUMMARY OF THE INVENTION  
         [0007]    The following summary of the invention is provided to facilitate an understanding of some of the innovative features unique to the present invention, and is not intended to be a full description. A full appreciation of the various aspects of the invention can be gained by taking the entire specification, claims, drawings, and abstract as a whole.  
           [0008]    The present invention is designed to make use of lenses having the same design in both the illumination and projection groups. It is very important that the lenses in the projection group have very high tolerances, so as to avoid distortion or blurring of the projected image. It is less important, however, that the lenses of the illumination group meet the same level of precision. This invention makes use of these facts to reduce the cost of projecting devices incorporating the teaching herein. Specifically, the invention makes use of at least some lenses produced on a common manufacturing line in both the illumination and projecting groups.  
           [0009]    This invention allows the tolerances for the lens manufacturing line to be relaxed. The lenses produced are tested after manufacture and sorted according to quality. Lenses meeting the higher tolerances necessary for image projection are incorporated into the projection group, while lenses not conforming to projection tolerances are used in the illumination group, thus saving cost.  
           [0010]    The novel features of the present invention will become apparent to those of skill in the art upon examination of the following detailed description of the invention or can be learned by practice of the present invention. It should be understood, however, that the detailed description of the invention and the specific examples presented, while indicating certain embodiments of the present invention, are provided for illustration purposes only because various changes and modifications within the spirit and scope of the invention will become apparent to those of skill in the art from the detailed description of the invention and claims that follow. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    The accompanying figures, in which like reference numerals refer to identical or functionally-similar elements throughout the separate views and which are incorporated in and form part of the specification, further illustrate the present invention and, together with the detailed description of the invention, serve to explain the principles of the present invention.  
         [0012]    [0012]FIG. 1 is a drawing of a transmissive projection device incorporating the present invention;  
         [0013]    [0013]FIG. 2 is a drawing of a reflective projection device incorporating the present invention;  
         [0014]    [0014]FIG. 3 is a drawing of a projection unit incorporating a light source, a projection device and a projection screen in accordance with the present invention; and  
         [0015]    [0015]FIG. 4 is a drawing of one embodiment of a compact polarizer in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts which can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention and do not delimit the scope of the invention.  
         [0017]    The general features of a projection device designed according to the present invention are shown in FIG. 1. A projection device, generally designated  100 , comprises a rear illumination focusing group  104 , an object surface  106 , a first projection focusing group  108 , and a second projection focusing group  110 . In operation, the illumination focusing group  104  receives light from a light source, here designated  102 . The projection device  100  is shown in FIG. 1 having a generally linear orientation for clarity, but there is nothing in the nature of the invention necessitating such a layout. The projection device  100  could be constructed to have an “L” or an “U” shape, for example, through the use of mirrors, prisms or other devices used for changing the direction of light rays without departing from the invention.  
         [0018]    In a preferred embodiment, the illumination focusing group  104  focuses the light received from the light source  102  onto the object surface  106  in a uniform, telecentric manner. In various embodiments, the light source  102  could comprise, for example, an aperture lamp, a light pipe or a fiber optic device. One advantage of this design, however, is that no additional optics, such as light pipes or lens arrays, are required to make the light uniform. Conventional designs generally require additional optics, such as lens arrays, for this function. Without the illumination focusing group  104 , the object surface  106  would generally be illuminated in a less-uniform manner, owing to imperfections in the light source or sources  102 . Non-uniform illumination of the object surface  106  would manifest itself as undesirable light and dark regions in the image projected on a surface.  
         [0019]    The illumination focusing group  104  is shown in FIG. 1 as comprising a rear illumination focusing lens  114 , an intermediate illumination focusing lens  116  and a front illumination focusing lens  118  as an illustration. Although the illumination focusing group  104  is described in connection with lenses  114 ,  116  and  118  as shown in FIG. 1, there is no requirement that the illumination focusing elements necessarily comprise the types or number of lenses shown, or that they comprise lenses at all. Any one or more of focusing elements  114 ,  116  or  118  could alternatively, without departing from the spirit of the invention, be implemented as fresnel lenses, for example, or as curved mirrors or any of the numerous other devices or combination thereof known in the art of focusing light.  
         [0020]    After passing through the illumination focusing elements  114 ,  116 , and  118 , light passes through a first plano-convex field lens  130  and into the object surface  106 . The first piano-convex field lens  130  ensures that the light propagating the object surface  106  is telecentric.  
         [0021]    The design shown in FIG. 1 makes use of a transmissive object surface  106  (e.g., film, a slide, a negative, etc.) That is, the light rays falling on the object surface  106  pass through the object plane  106  and into the projection groups  108  and  110 . Generally, the object surface  106  will have an image displayed thereon. The object surface  106  can comprise a photographic slide, as an example. In the preferred embodiment, the object plane  106  is a liquid crystal display panel under the control of some electronic apparatus such as a personal computer. In this embodiment, the image on the object surface  106  can be varied by providing different electronic signals to the liquid crystal display panel.  
         [0022]    In an embodiment using a liquid crystal display panel as an object surface  106 , the light passing through the liquid crystal display panel must be polarized before striking the display panel. This can be accomplished, for example, through the use of a polarized filter in the light path such as is represented by element  112 , but any of the devices known in the art of light polarization, for example those disclosed elsewhere in this application, could be employed successfully without departing from the basic invention.  
         [0023]    After passing through the object surface  106 , the light rays pass through the second plano-concentric field lens  132 , the first projection focusing group  108 , and the second projection focusing group  110 . The focusing groups  108  and  110  project and focus the light rays passing through the object surface  106  onto a display screen (not shown).  
         [0024]    The first projection focusing group  108 , as represented in FIG. 1, comprises a rear projection focusing lens  120 , an intermediate projection focusing lens  122  and a front projection focusing lens  124  as an illustration. Although the projection focusing group  108  is described in connection with lenses  120 ,  122  and  124  as shown in FIG. 1, there is no requirement that the projection focusing elements necessarily comprise the types or number of lenses shown, or that they comprise lenses at all. Any one or more of focusing elements  120 ,  122  or  124  could alternatively, without departing from the spirit of the invention, be implemented as fresnel lenses, for example, or as curved mirrors or other any of the numerous other devices or combination thereof known in the art of focusing light.  
         [0025]    The second projection focusing group  110 , as represented in FIG. 1, comprises a rear projection focusing lens  126  and a front projection focusing lens  128  as an illustration. Although the projection focusing group  110  is described in connection with lenses  126  and  128  as shown in FIG. 1, as with the illumination group  104  and the first projection group  108 , there is no requirement that the projection focusing elements necessarily comprise the types of, or number of, focusing devices shown in FIG. 1.  
         [0026]    The illumination group  104  and projection groups  108  and  110  employed in this invention each incorporate at least one lens having a common design and manufacture with the other group. In other words, at least one lens in the projection groups  108  and  110  has the identical design as at least one lens in the illumination group  104 . In the embodiment shown in FIG. 1, for example, the illumination group  104  and first projection group  108  are designed to use lenses of the same manufacture, although this is only one particular embodiment of the numerous combinations possible in accordance with the present invention.  
         [0027]    The primary advantage of this design is the cost savings associated with it. As noted above, this invention makes use, in the illumination group  104 , of parts that would otherwise be scrapped, or would require rework to be used in either of the projection groups  108  or  110 .  
         [0028]    In any manufacturing process, the cost of a part generally increases as the acceptable tolerances for that part are tightened. The increased cost can result, for example, from a lower yield of conforming parts from a given process. As tolerances are tightened, it is intuitive that fewer parts will fall within those tolerances and be considered conforming parts. Non-conforming parts are either scrapped or reworked, either of which adds to the final cost of the conforming parts. In order to increase yield, better manufacturing methods can be employed, but more consistent manufacturing processes are generally more expensive, thus increasing costs nonetheless.  
         [0029]    The fact that this design uses one or more common lenses between the illumination group  104  and projection groups  108  and  110  means that significant costs can be saved in the manufacture of the lenses. The reason for this is that, as discussed above, the purpose of the illumination group  104  is to provide a uniform level of illumination to the object surface  106 . The tolerances necessary to perform this task are much looser than the tolerances necessary to project a clear, focused and uniform image on a projection screen.  
         [0030]    In the embodiment shown in FIG. 1, lenses  114 ,  116  and  118  are identical in manufacture to lenses  124 ,  122  and  120 , respectively. Lenses  124 ,  122  and  120  would be taken from a group of lenses selected due to conformance with a tighter set of tolerances than those met by lenses  114 ,  116  and  118 . This illustration should not be interpreted as limiting the invention, however. In some embodiments, the illumination group  104  and projection groups  108  and  110  share only one or two focusing elements rather than a complete focusing group.  
         [0031]    A second embodiment of the invention, comprising a projection device  200 , is shown in FIG. 2. Projection device  200  comprises a light source  202  supplying light, represented by light ray  224 , to a first polarizing group comprising elements  204 ,  206  and  208 .  
         [0032]    In this embodiment, the polarizing group comprises a polarized half mirror  204  oriented to reflect one polarized component  226  of each light ray  224  into the illumination group  210 . The polarized half mirror  204  is designed to pass light having one polarity while reflecting light having a polarity orthogonal to the polarity passed. All polarized components  226  passing into the illumination group  210  have a uniform polarization. An orthogonal component  228  of each light ray  224  passes through the half mirror  204  and strikes the full mirror  206 , which is oriented to reflect the orthogonally polarized light  206  through the half wave plate  208  into the illumination group  210 . The full mirror  206  could alternatively be a polarized mirror having a polarity orthogonal to the half mirror  204 , with essentially the same effect.  
         [0033]    The half wave plate  208 , the design of which is well known in the art of optics, is constructed to rotate the polarity of the light rays  228  by 90 degrees as they pass through it. It can be seen, then, that after passing through the half wave plate  208 , light ray  228  will be polarized in the same orientation as light ray  226 , so that all of the light passing into illumination group  210  is polarized with the same orientation. This is but one illustrative embodiment of a polarizing device. Any of a number of polarizing devices well known in the art could be employed for this function with successful results.  
         [0034]    Polarized light passes through the illumination group  210  and into the prism block  212 , where it is reflected by the polarized half mirror  214  onto, for example, a field lens  216  and liquid crystal display  218 . It should be appreciated that illumination of image surfaces other than liquid crystal displays (e.g., photographs, slides, samples, etc.) may be imaged. The polarized half mirror  214  is designed to pass light having one polarity while reflecting light having a polarity orthogonal to the polarity passed. The polarized half mirror  214  shown in FIG. 2 is designed and oriented to reflect the light polarized by the polarizing elements  204 ,  206  and  208 . Liquid crystal display  218  holds the image that is to be projected onto the projection screen. Liquid crystal display  218  rotates the polarization of the light by 90 degrees and reflects the light back through the field lens  216  back into the prism block  212 . The light, now having a polarity orthogonal to its earlier orientation, passes through the polarized half mirror  214 , out of the prism block  212  and into the first projection focusing group  220 . The light passes through the first projection focusing group  220  and second projection focusing group  222 , which together focus and expand the image onto a projection screen (not shown).  
         [0035]    The illumination group  210  and projection groups  220  and  222  employed in this embodiment each incorporate at least one lens having a common design and manufacture as at least one lens in the other group. In other words, at least one lens in the projection groups  220  and  222  has the identical design as at least one lens in the illumination group  210 . In the embodiment shown in FIG. 2, the illumination group  210  and first projection group  220  are designed to use lenses of the same manufacture, although this is only one particular embodiment of the numerous combinations possible in accordance with the present invention.  
         [0036]    As with the embodiment shown in FIG. 1, the primary advantage of this design is the cost savings associated with it. As noted above, this invention makes use, in the illumination group  210 , of parts that would otherwise be scrapped, or would require rework to be used in either of the projection groups  220  or  222 .  
         [0037]    A full projection system, generally designated  300 , is shown in FIG. 3. The projection system  300  comprises a light source  302  providing light to a projection unit  200  similar to that described in FIG. 2. The projection unit  200  projects an image onto a projection surface  308 . The scale of the image on the projection surface  308  is generally considerably larger than the source image on the object surface. The outside edges  304  and  306  of the light pattern projected onto the projection surface diverge so as to expand the scale of the projected image. In certain embodiments, the size of the image displayed on the projection surface  308  is adjusted by moving the projection unit  200  closer or further away from the projection surface  308 .  
         [0038]    A compact polarizer of the type used in the present invention is shown in FIG. 4 and generally designated  400 . Polarizer  400  comprises a polarized half mirror  402 , a full mirror  404 , and a half-wave plate  406 . Light from a light source, such as aperture lamp  408  is emitted with a random polarization. Each ray of light such as ray  410  has both a vertical component  412  and a horizontal component  414 . The following discussion focuses on the path of a single light ray for clarity, but it is well known in the art that light source such as lamp  408  emits a plurality of light rays traveling in a multitude of polarizations.  
         [0039]    The magnitude of the two polarization components  412  and  414  is related to the polarization and amplitude of the ray  410 . Ray  410  travels from the lamp  408  to the polarized half mirror  402  where it impinges thereon at point  416 . Polarized half mirror  402  is transparent to the vertically polarized component  412  and reflective to the horizontally polarized component  414  of each light ray  410 . The reflected horizontal component is designated  422  in FIG. 4. In the preferred embodiment, the polarized half mirror  402  is disposed at an angle of approximately 45 degrees from a vector connecting the lamp  410  to the center point  416  of the half mirror  402 , so as to reflect most of the light received from lamp  408  at an approximately 90 degree angle. In an alternate embodiment, the polarized half mirror  402  could be designed to reflect the vertical component  412  and pass the horizontal component  414  without departing from the spirit of the invention.  
         [0040]    Assuming a random, uniform distribution of polarization, reflected component  422  will comprise approximately one half of the light emitted from lamp  408 . The remaining portion of the light passes through the polarized half mirror  402  to the full mirror  404 . The light  424  impinging on the full mirror  404  represents the vertically polarized component of light ray  410 . The vertical component  424  impinges on the full mirror  404  at point  418  and is reflected into the half wave plate  406 . The vertical component  424  impinges on the half wave plate  406  at point  420 . The half wave plate  406  rotates the polarization of the vertical component  424  by 90 degrees, so that, after passing through the half wave plate, the rotated component  426  has the same polarization as reflected component  422 . It will be apparent to one of skill in the art that, through the use of this polarizer, nearly 100% of the light can be uniformly polarized, with minimum losses along the light path.  
         [0041]    The embodiments and examples set forth herein are presented to best explain the present invention and its practical application and to thereby enable those skilled in the art to make and utilize the invention. Those skilled in the art, however, will recognize that the foregoing description and examples have been presented for the purpose of illustration and example only. Other variations and modifications of the present invention will be apparent to those of skill in the art, and it is the intent of the appended claims that such variations and modifications be covered. The description as set forth is not intended to be exhaustive or to limit the scope of the invention. Many modifications and variations are possible in light of the above teaching without departing from the spirit and scope of the following claims. It is contemplated that the use of the present invention can involve components having different characteristics. It is intended that the scope of the present invention be defined by the claims appended hereto.