Abstract:
An integrated prism-based optical device for a projection system performs light integration, beam shaping, and input/output beam separation. The integrated optical device preferably includes a light integrator tunnel with a first lens located adjacent one end and a prism located adjacent an opposite end. A second lens is located adjacent another side. The individual elements are preferably secured together using an index matching adhesive to form a unitary optical device. The integrated optical device is compact and provides uniform, high intensity light to the display device. The integrator tunnel may have a wedge or frustum shape eliminating the need for the first lens. The integrated optical device allows for a projector that requires fewer parts, is lighter weight, more compact, less costly, and more easily assembled than prior projection systems.

Description:
RELATED APPLICATIONS 
     None 
     FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     None 
     TECHNICAL FIELD 
     This invention relates to color sequential video and multimedia projectors and more particularly to a prism-based optical engine that performs light beam integration, beam shaping, and input/output light separation. 
     BACKGROUND OF THE INVENTION 
     Projection systems have been used for many years to project motion pictures and still photographs onto screens for viewing. More recently, presentations using multimedia projection systems have become popular for conducting sales demonstrations, business meetings, and classroom instruction. 
     In a common operating mode, multimedia projection systems receive analog video signals from a personal computer (“PC”). The video signals may represent still, partial-, or full-motion display images of a type rendered by the PC. The analog video signals are typically converted in the projection system into digital video signals that control a digitally driven image-forming device, such as a liquid crystal display (“LCD”) or a digital micro mirror device (“DMD”). 
     It is becoming more important to provide a display system that is compact but that provides a high-quality image. Significant effort has been invested into developing projectors producing bright, high-quality, color images. However, it is difficult to obtain a suitable projected image, especially when using compact portable color projectors in a well-lighted room. For example, current display systems are heavy and bulky because of the number of required optical elements and the placement and spacing of the optical elements. 
     FIG. 1 shows such a system. An image projector  10  includes a high power lamp  12  positioned at the focus of an elliptical reflector  14  to produce a high intensity illumination beam characterized by a principal ray  16  that propagates through a rotating color wheel disk  18  of a color wheel assembly  20 . Disk  18  includes at least three sectors, each tinted in a different one of three primary colors to provide a field sequential color image capability for image projector  10 . The illumination beam propagates through an integrator tunnel  22  to create at its output end a uniform illumination pattern. 
     The illumination beam propagates from integrator tunnel  22  through lens elements  24  and  26  and is directed by a mirror  32  that is inclined so that the illumination beam propagates upwardly at a 45 degree angle relative to the plane of the supporting table for image projector  10 . After reflection by mirror  32 , principal ray  16  propagates through lens element  28  toward a prism assembly  40 . Prism assembly  40  is composed of prism components  42  and  44  that are spaced apart by an interface  46 . 
     An incident light beam derived from principal ray  16  propagates through prism component  42  and, by total internal reflection, reflects off of a surface at air space interface  46  to form a reflected incident light beam. The reflected incident beam propagates through prism component  42  to strike light valve  30 . Light valve  30  reflects an imaging light beam propagating normal to the plane of light valve  30  through prism component  42  and, without total internal reflection, through interface  46  into prism  44  to exit through an exit face  60  of prism component  44 . The imaging light beam that passes through exit face  60  is characterized by a principal ray  62  and propagates through a projection lens  64  to a projector screen (not shown) to display an image to a viewer. 
     As can be seen, this architecture employs many optical elements to integrate the light and to reflect, redirect, and transmit various polarized light or light rays depending on whether they propagate in a direction toward a display device. These optical elements add weight and bulk to the display system and are more costly to manufacture and more time consuming to assemble. Additionally, the arrangement of the components, such as, for example, the necessary upward inclination of prism assembly  40 , dictates for a housing (not shown) a minimum height that is greater than desired. 
     What is needed, therefore, is a compact, light weight, low-profile multimedia projection system that achieves a bright, high-quality projected image at a relatively low cost. 
     SUMMARY OF THE INVENTION 
     An object of this invention is, therefore, to provide a multimedia display device having a simplified prism-based optics system. 
     Another object of the invention is to provide a multimedia projector in which the prism assembly is a totally integrated system that performs light integration, beam shaping, and input/output beam separation. 
     A further object of the invention is to provide a multimedia projector that eliminates the need for optical components to relay the image from the integrator onto a display device. 
     Still another object of the invention is to provide a multimedia projector that can be used with a transmissive or reflective LCD or a DMD. 
     Yet another object of this invention is to provide a multimedia projector that requires fewer parts, is lighter weight, more compact, less costly, and more easily assembled than prior projection systems. 
     A frame sequential color display projection system of this invention includes an arc lamp having a predetermined power rating for providing a source of polychromatic light that propagates through a color wheel that sequentially provides R, G, B, and optionally, W light colors during respective sequential time periods. A display controller is synchronized with the color wheel to generate color image data during the respective time periods. The light propagates through an integrated prism-based optical device that performs light integration, beam shaping, and input/output beam separation to create a uniform illumination pattern and to direct light toward or away from a display device. 
     In the preferred embodiment, the integrated optical device includes a light integrator tunnel with a first lens abutting one end of the light integrator tunnel. A prism is positioned in an abutting relationship at one side to the opposite end of the integrator tunnel, and a second lens abuts another side of the prism. Preferably, the individual elements are secured together using an index matching adhesive that matches the refractive index of the optical elements, which are typically made of glass so that together the integrator tunnel, prism, and lenses form a unitary element. 
     The integrated optical device is positioned within the projection system with one end adjacent to the color wheel and the opposite end closely adjacent to the display device. The close proximity of the integrated optical device to the display device eliminates the need for additional optical elements to reimage the light onto the display device. Additionally, placement of the integrated optical device adjacent the display device provides uniform, high intensity light from the color wheel to the display device. 
     In addition to integrating the incoming light from the lamp, the integrated optical device decreases the angular direction of the light as it propagates through the integrated optical device. This allows the use of an F 1  reflector, which is a small reflector with a short working distance resulting in a smaller and more compact projection device. The second lens element reduces polarization losses associated with some light valves but may be eliminated depending on the type of light valve used. 
     The integrator tunnel may have a wedge or frustum shape that eliminates the need for the first lens because the wedge or frustum shape performs the function of reducing the incident angle of the incoming light. Depending on the type of light valve employed, it may not be necessary to use lenses. However, it may be necessary to use a lens between the prism and display device to reduce polarization losses associated with some light valves. 
     Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof that proceeds with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified pictorial plan view of a prior art multimedia projector. 
     FIG. 2 is a simplified pictorial plan view of a multimedia projector showing a preferred embodiment of the unitary optical integrator. 
     FIG. 3 is a simplified pictorial plan view of a multimedia projector showing an alternative embodiment of the unitary optical integrator. 
     Additional aspects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof, which proceeds with reference to the accompanying drawings. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 2 shows a preferred embodiment of an image projection system  110  of this invention that includes a high power arc lamp  112  positioned at a focus of an elliptical reflector  114  having an F-number that produces a high intensity illumination beam that is characterized by a principal beam  116 . Arc lamp  112  is preferably a 270 watt, high pressure mercury arc lamp, which is suitable for use in an image projector to achieve its lifetime and lumen specifications. The mercury arc lamp has a nominal 1.3 mm arc gap, which contributes to high efficiency operation of the projector engine of image projection system  110 . The small size of the arc gap impacts the alignment of the lamp arc to the rest of the optical system and increases the importance of the stability of the arc itself. Arc lamp  112  is preferably a model SHP 270, manufactured by Phoenix, located in Himeji City, Japan. 
     Arc lamp  112  is positioned at the first focus of elliptical reflector  114 , which has a cold mirror that reflects forward only visible light. Much of the infrared and ultraviolet light is transmitted and absorbed in the housing of elliptical reflector  114 . Color wheel disk  118  includes R, G, and B segments and is rotated by a motor  124  at about 5,600 to 7,500 rpm, which is twice the range of system video image refresh rates, to sequentially display R, G, and B images on a projector screen (not shown). Color wheel disk  118  may also include a W (actually clear) segment that functions to increase lumens. All segments of color wheel disk  118  carry ultraviolet reflective coatings to prevent ultraviolet light from reaching ultraviolet light sensitive components in the optical system. The beam  116  propagates through an integrated optical device  126  and onto a light valve  128 . The light valve  128  reflects an imaging light beam  130  propagating normal to the plane of light valve  128  to pass through an exit face  132  of the integrated optical device  126 . The imaging light beam  130  that passes through the exit face  132  propagates through a projection lens  134  to a projector screen (not shown) to display an image to a viewer. 
     In the preferred embodiment of FIG. 2, the integrated optical device  126  includes a first lens  136 , an integrator tunnel  138 , a prism  140 , and a second lens  142 . These elements are preferably made of glass and are located in an abutting relationship. Preferably, they are secured together by an index matching adhesive that has a refractive index matching that of the glass. Integrator tunnel  138  has an input end  144  and an output end  146  and creates at its output end  146  a uniform illumination pattern and facilitates delivery of the illumination light beam to the prism  140 . Integrator tunnel  138  is composed of a solid glass that relies on total internal reflection to transfer light through it. The integrator tunnel  138  has a substantially constant cross section along its length so that the input end  144  and output end  146  have substantially the same dimensions. The light enters the input end  144  through first lens  136 . The first lens  136  decreases the angular direction of the light as it propagates from the color wheel  118  through the first lens  136  and to the integrator tunnel  138 . The light propagates through the integrator tunnel  138  and is reflected many times within the integrator tunnel  138  in order to homogenize the light into a beam having a uniform intensity. Integrator tunnel  138  may also include a cladding that supports the integrator tunnel  138  without disrupting total internal reflection. The uniform illumination pattern of light propagating from the output end  146  is of a shape that corresponds to the light valve  128 . In other words, the integrator tunnel  138  shapes the beam through internal reflectance to have the same aspect ratio as the light valve  128 . 
     The prism  140  is located at the output end  146  of the integrator tunnel  138  at a side that is dimensioned to substantially match that of the output end  146 . The prism  140  is made of glass and has an interface  148  such as, for example, a PBS coating, an air gap creating a TIR interface, or other interfaces such as, for example, dichroic coatings, diffractive surfaces, waveplates, or polarizers depending on which type of display device is used. For example, in the preferred embodiment, the light valve  128  is a LCD. Skilled workers would know that if the display device is a LCD the prism  40  typically includes a PBS interface. The beam  116  propagates directly from the integrator tunnel  138  through the prism  140  and second lens  142  and onto the light valve  128 . The second lens  142  converges the light exiting the prism  140  so that the light properly strikes the surface of light valve  128 . The light valve  128  reflects the light beam back through the second lens  142 , which again converges the light to prevent internal reflection within the prism  140  to reduce polarization losses associated with the use of LCDs. The prism  140  redirects the light rays according to the polarization direction of the light either away from the projection lens  134  or through it to project an image onto a screen (not shown). 
     Tracing the path of light rays from the lamp  112  through the system will best help an understanding of the function of the integrated optical device  126  and its components. The light beam  116  from the arc lamp  112  is directed through the color wheel  118  and into the input end  144  of the integrator tunnel  138 . The integrator tunnel  138  concentrates and shapes the beam to provide uniform light intensity and to direct the light through the output end  146  and directly into the prism  140  to the light valve  128  at a substantially normal incident angle. For example, light entering the integrator tunnel  138  may have an incident angle of about 30 degrees. The light exiting the integrator tunnel  138  may have an incident angle of about 10 degrees. 
     The light rays that are received by the light valve  128  must be linearly polarized but light rays reflected from the light valve  128  having the same polarization direction must be excluded from the image forming beam directed to the projection lens  134 . As is known, application of voltage to the LCD causes a rotation of polarization of light rays so that only light rays in which polarization has been reversed are selected to form the projected image. This function is carried out in the prism  140 . 
     Although the elements are seen in FIG. 2 as being aligned so that the light travels in along a substantially straight path from the lamp  112  to the display device  128 , alternative arrangements are possible. For example, skilled workers would know to arrange the second lens  142  and the light valve  128  on another side  150  of the prism  140  as seen in phantom in FIG.  2 . 
     By positioning the integrator tunnel  138 , prism  140 , and first and second lenses  136  and  142  in an abutting relationship and, preferably, adhering them into a single unitary element, the integrated optical device  126  can be placed in close proximity to both the color wheel  118  and the light valve  128 . This provides a tight illumination pattern and provides for a more compact projection system. 
     The system described above is one in which the light valve  128  is a LCD. However, skilled workers would know and understand that the light valve  128  may be a DMD, in which case the prism  140  would have a TIR interface. Additionally, since polarization losses are not a concern with DMDs, the second lens  150  may be eliminated. 
     An alternative embodiment is seen in FIG. 3 in which an integrator tunnel  152  has a wedge or frustum shape having an input end  154  of smaller cross section than an output end  156 . The frustum shape is advantageous in that it performs the dual functions of decreasing the angular direction of the incoming light and integrating the light. In the previous embodiment of FIG. 2, these two functions were carried out by the first lens  136  and the integrator tunnel  138 . Thus, the wedge or frustum-shaped integrator tunnel  152  eliminates the necessity of having a lens at the input end  154 . The output end  156  delivers a uniform illumination light beam to a prism  158 . The prism  158  abuts the integrator tunnel  138  at one side having a dimension that substantially matches that of the output end  156 . It is preferred that the integrator tunnel  152  and prism  158  are secured with an index matching adhesive. As with the integrator tunnel of FIG. 2, wedge or frustum-shaped integrator tunnel  152  is composed of a solid glass that relies on total internal reflection to light through it. The light enters the narrow input end  154  and is reflected many times in order to homogenize the light into a beam having a uniform intensity. The uniform illumination pattern of light propagating from the wide output end  156  is of a shape that corresponds to the light valve  128 . In this embodiment, it is preferred that the light valve  128  be a DMD, in which case the prism  158  would have a TIR interface  160 . However, skilled workers would know that a LCD could be employed as the light valve  128 , in which case the prism  158  would have a PBS interface. Additionally, since polarization losses are a concern with the use of LCDs a lens element (shown in phantom in FIG. 3) is located at a side of the prism  158  facing the LCD. Furthermore, the location of the light valve  128  could be at another side  164  of the prism as shown in phantom in FIG.  3 . 
     It will be obvious to those having skill in the art that many changes may be made to the details of the above-described embodiments of this invention without departing from the underlying principles thereof. The scope of the present invention should, therefore, be determined only by the following claims.