Abstract:
An image projection system implemented with a projector engine using a reflective light modulator, preferably a Digital Micromirror Device (DMD), operates lying flat with very low profile on a support table. The invention overcomes the disadvantage of previous DMD projectors that require either tilting all or part of the projection system 45 degrees relative to a support table top or packaging the projection system in a thick box that allows light to impinge on the DMD from above or below its light reflecting surface. This is accomplished with a prism assembly that sets up the correct illumination angles for the DMD and directs imaging (output) light along approximately the same vector as that of illumination (input) light incident to the prism assembly. The illumination light and imaging light do not propagate in a common plane within the prism assembly, but the vectors of the illumination light entering and the imaging light exiting the prism assembly are approximately the same. An alternative preferred embodiment of the prism assembly includes a light escape window through which illumination light reflected by the DMD in its off-state escapes from the prism assembly in a direction away from the projection lens. An implementation using a third prism optically fixed to an output prism or forming an integral part of an enlarged output prism is especially advantageous because it can provide a three-point mounting of the prism assembly to the floor of the interior of the projection system housing.

Description:
RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 60/134,473, filed May 17, 1999. 
    
    
     TECHNICAL FIELD 
     This invention relates to image display systems and, in particular, to an image projection system implemented with a reflective light element and packaged to operate lying flat with very low profile on a support table. 
     BACKGROUND OF THE INVENTION 
     The following description is presented with reference to an image projector implemented with a reflective light modulator of a digital micromirror device (DMD) type but is applicable also to image projectors implemented with other types of reflective light modulators. Image projectors currently implemented with DMDs require that the projector housing or DMD-illuminating light beam-directing optics contained within the projector housing be tilted at a 45 degree angle relative to a support table on which the image projector rests. This is done to cause the illuminating light to impinge on the DMD from either above or below its light reflecting surface and thereby provide a correct orientation of the DMD relative to a projection screen on which an image can be viewed. Inclining the projector or its components causes the projector to occupy an undesirably tall space when it is in use. Currently available single DMD projectors are taller than 10 cm in their operating positions. Using a tilting mechanism to thin the profile to less than 10 cm requires a tilting mechanism that raises the operating height by a corresponding amount. 
     FIGS. 1A,  1 B,  1 C, and  1 D are respective isometric, frontal, side elevation, and top plan views of such a prior art image projector. With reference to FIGS. 1A,  1 B,  1 C, and  1 D, a prior art 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 that lens elements  24 ,  26 , and  28  image onto a DMD  30 . 
     The illumination beam propagating from integrator tunnel  22  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  and exits lens element  26  toward a prism assembly  40 . Prism assembly  40  is composed of prism components  42  and  44  that are spaced apart by an air space interface  46 . After reflection by mirror  32 , principal ray  16  of the illumination beam strikes a surface of lens element  28 . 
     An incident light beam derived from principal ray  16  propagates through prism component  42  and, by total internal reflection, reflects off of a surface  50  at air space interface  46  to form a reflected incident light beam. The reflected incident beam propagates through prism component  42  to strike DMD  30 . DMD  30  in its “on” light reflecting state (on-state) reflects an imaging light beam propagating normal to the plane of DMD  30  through prism component  42  and, without total internal reflection, through air space 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. DMD  30  in its “off” light reflecting state (off-state) reflects light by total internal reflection off of a face  68  of prism component  44 . 
     The angles of the faces and the shapes of prism components  42  and  44  are selected so that the incident light beam, reflected incident light beam, and imaging light beam propagating within prism assembly  40  are coplanar. The arrangement of the components of image projector  10  results in the upward inclination of prism assembly  40  and thereby dictates for a housing (not shown) of projector  10  a minimum height that is greater than a minimum height that would be possible with an uninclined prism assembly and principal rays  16  and  62  propagating along essentially the same vector. 
     SUMMARY OF THE INVENTION 
     The invention is an image projection system implemented with a projector engine using a reflective light modulator, preferably a Digital Micromirror Device (DMD), and operating lying flat with very low profile on a support table. The invention overcomes the above-described disadvantage of previous DMD projectors that require either tilting all or part of the projection system 45 degrees relative to a support table top or packaging the projection system in a thick box that allows light to impinge on the DMD from above or below its light reflecting surface. This is accomplished with a prism assembly that sets up the correct illumination angles for the DMD and directs imaging (output) light along approximately the same vector as that of illumination (input) light incident to the prism assembly. 
     The prism assembly includes compensating and output prism components having opposed surfaces separated by a light beam separation boundary, which is preferably an air space. The prism assembly sets up a correct illumination angle on the DMD and then separates illumination light from imaging light by total internal reflection discrimination. In a preferred embodiment, illumination light travels upwardly at 8 degrees relative to the surface of a support table (hereafter referred to as the horizontal datum plane) and in a direction such that its projection onto the horizontal datum plane is parallel to the projection of the optical axis of a projection lens that receives light exiting the prism assembly. The illumination light enters the prism assembly and reflects by total internal reflection off a top surface of the compensating prism component. The top surface has relative to the three-dimensional DMD coordinate system a compound angle that directs the light toward the DMD at the correct angle for illumination. In a preferred embodiment, the angle of this first reflected light beam is tilted 24 degrees (16 degrees in the prism glass) from the normal of the horizontal datum plane and is less than the critical angle of the glass from which the first prism component is formed at the air gap interface surface. The projection of this first reflected light beam onto the horizontal datum plane is rotated 40 degrees from the projection of the optic axis of the projection lens onto the same horizontal datum plane. The light passes, therefore, through the air space between the first and second prism components. For each micromechanical mirror of the DMD in its on-state, the illumination light reflects at 4 degrees from the normal of the horizontal datum plane to form imaging light, the projection of which onto the horizontal datum plane is parallel to the projection of the optical axis of the projection lens. The imaging light reenters the prism assembly through the output prism component. Because the angle of incidence at the air gap interface surface is greater than the critical angle, the imaging light reflects off the air gap and propagates through the output prism component. The imaging light exits the prism assembly, traveling upwardly at +4 degrees from the horizontal datum plane toward a projection lens. The illumination light and imaging light do not propagate along a common plane within the prism assembly, but the vectors of the illumination light entering and the imaging light exiting the prism assembly are approximately the same. 
     The DMD in a preferred implementation is positioned face up and, therefore, can be mounted on one printed circuit board that covers the interior bottom of the projection system. This arrangement is less expensive than the alternative of using a high-density connector at right angles to the DMD control electronics for the printed circuit board and support surface to hold the DMD. 
     An alternative preferred embodiment of the prism assembly includes a light escape window through which light reflected by the DMD in its off-state escapes from the prism assembly in a direction away from the projection lens and is not absorbed into the prism. The light escape window is preferably either a third prism optically fixed to or an integral part of the prism assembly or a faceted array optically fixed to the prism assembly. An implementation using the third prism is especially advantageous because it can provide a three-point mounting of the prism assembly to the floor of the interior of the projection system housing. 
     Additional objects and advantages of the present invention will be apparent from the following detailed description of preferred embodiments thereof, which proceeds with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A,  1 B,  1 C, and  1 D are respective isometric, frontal, side elevation, and top plan views of a prior art image projector. 
     FIGS. 2A,  2 B,  2 C,  2 D, and  2 E are respective isometric, frontal, side elevation, top plan, and rear end views of a preferred embodiment of the present invention. 
     FIGS. 3A,  3 B,  3 C, and  3 D are respective isometric, frontal, side elevation, and top plan views showing the spatial arrangement of a prism assembly and a reflective light modulator implemented in the embodiment of FIGS. 2A-2E. 
     FIG. 4 is a diagram showing the coordinate system for the prism assembly of FIGS. 3A-3D. 
     FIG. 5 is an isometric view of a prism assembly that implements in the prism assembly of FIGS. 3A-3D a light window for unwanted light to escape from the prism assembly in a direction away from the projector lens. 
     FIG. 6A is an exploded fragmentary isometric view showing the spatial arrangement of the DMD mounted below and the prism assembly of FIG. 5 (with a light escape window area as an integral part of the output prism component) mounted in an optics chassis that is mountable in the projection system of FIGS. 2A-2E; FIGS. 6B and 6C are top plan views of an optics chassis, respectively, with and without the prism assembly of FIGS. 3A-3D installed; and FIG. 6D is a top plan view of the optics chassis with the prism assembly of FIG. 5 installed. 
     FIGS. 7A and 7B are respective side and frontal isometric views showing an alternative design for the prism assembly of FIGS. 3A-3D. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIGS. 2A,  2 B,  2 C,  2 D, and  2 E are respective isometric, frontal, side elevation, top plan, and rear end views of a preferred embodiment of an image projection system  110  of the present invention. With reference to FIGS. 2A,  2 B,  2 C,  2 D, and  2 E, projection system  110  includes a high power lamp  112  positioned at a focus of an elliptical reflector  114  having an F-number of approximately F/1 to produce a high intensity illumination beam that is characterized by a principal ray  116 . Lamp  112  is preferably a 120 watt, high pressure mercury lamp, which is suitable for use in an image projector to achieve its lifetime and lumen specifications. The mercury 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. 
     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 . The second focus of elliptical reflector  114  is positioned one-half the distance between the front face of a rotating color wheel disk  118  of a color wheel assembly  120  and an integrator tunnel  122 . As shown best in FIGS. 2B and 2E, elliptical reflector  114  is tilted upwardly 5 degrees from a horizontal datum plane to minimize the height of projection system  110 . Color wheel disk  118  rotates at about 7,200 rpm, which is twice the system video image refresh rate, to sequentially display red, green, and blue images on a projector screen (not shown). Color wheel disk  118  may also include a white segment that functions to increase lumens while decreasing color saturation. 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. 
     Integrator tunnel  122  creates at its output end a uniform illumination pattern and facilitates delivering the illumination light past the motor of color wheel assembly  120  so that the motor does not create a shadow in the illumination. Integrator tunnel  122  is composed of a solid glass rod that relies on total internal reflection to transfer light through it. Integrator tunnel  122  may also include a cladding that supports the integrator tunnel without disrupting total internal reflection. The uniform illumination pattern of light propagating from the output end of integrator tunnel  122  is of rectangular shape and is imaged through lens elements  124 ,  126 , and  128  onto a light reflecting surface of a DMD  130 . Integrator tunnel  122  is rotated 8 degrees about its major axis to correct for rotation in the illumination on DMD  130 , which rotation is caused by a prism assembly  140  described below. DMD  130  is preferably a Texas Instruments Model DMD 1076 spatial light modulator composed of a rectangular array of aluminum micromechanical mirrors, each of which can be individually deflected at an angle of ±10 degrees about a hinged diagonal axis. The deflection angle (either positive or negative) of the mirrors is individually controlled by changing the memory contents of underlying addressing circuitry and mirror reset signals. Lens element  128  is tilted upwardly 6 degrees from the horizontal datum plane and rotated −10 degrees about the vertical axis to partly correct for distortion caused by oblique illumination of DMD  130 . A beam direction turning mirror  132  positioned between an exit face of lens element  126  and an entrance face of lens element  128  turns the beam direction in an X-Z plane (FIGS. 3A-3D) by about 90 degrees within the housing of projection system  110 . 
     Illumination light exiting lens element  128  enters a prism assembly  140  that is comprised of a first or compensating prism  142  and a second or output prism component  144  that are spaced apart by an air space interface  146 . Prism assembly  140  allows DMD to lie flat when in operation. Prism assembly  140  sets up the correct illumination angle on DMD  130  and separates by total internal reflection discrimination the illumination light from the imaging light reflected by DMD  130  in its on-state. The illumination angles for DMD  130  are controlled by the angles of the faces of prism assembly  140 . Prism assembly  140  refracts and reflects the incident light bundle so that the DMD  130  is illuminated from a corner with a projection angle partly built into the output light bundle. After the illumination light reflects off DMD  130  in its on-state, imaging light exits prism assembly  140  along essentially the same propagation direction as that of illumination light entering prism assembly  140 . Because of the many degrees of freedom in prism assembly  140 , light can enter it roughly parallel to a support table and in line with a projection lens. In a preferred case, the DMD can be placed on a large support surface and a single printed circuit board that covers the bottom of the projector. This provides a cost-effective solution because it eliminates the need for a high-density electrical connector otherwise required between the printed circuit board and an off-board DMD. 
     FIGS. 3A,  3 B,  3 C, and  3 D are respective isometric, frontal, side elevation, and top plan views of prism assembly  140 . With reference to FIGS. 3A,  3 B,  3 C, and  3 D, principal ray  116  of the illumination beam propagates generally in the X direction and strikes an entrance surface  148  of prism component  142  upwardly at an  8  degree angle relative to a horizontal datum plane, which in FIGS. 3A,  3 C, and  3 D is the X-Z plane. An incident beam derived from principal ray  116  and characterized by a principal ray  116   i  propagates through prism component  142  and, by total internal reflection, reflects off a top surface  150  of prism component  142 . Top surface  150  of prism component  142  is set at a compound angle relative to the coordinate system (FIGS. 3A-3D and FIG. 4) that directs principal ray  116   i  toward DMD  130  at a 24 degree angle measured relative to the normal of the X-Z horizontal plane and the projection of principal ray  116   i  onto the X-Z plane at a 40 degree angle of rotation from the X-axis. The principal ray angle is less than the critical angle at the air gap interface surface that is characteristic of the glass from which prism component  142  is formed. Principal ray  116   i  passes, therefore, without total internal reflection through air space interface  146  to strike DMD  130 . 
     The controller, which is an integral component of DMD  130 , provides electrical signals to direct the micromechanical mirrors of DMD  130  to the desired light reflecting states. In their on-state, the micromechanical mirrors of DMD  130  receive the incident beam and reflect an on-state reflected light beam characterized by a principal ray  116   r . The micromechanical mirrors in their on-state reflect principal ray  116   r  at a 4 degree angle relative to the normal of the X-Z horizontal plane and the projection of principal ray  116   r  onto the X-Z horizontal plane is parallel to the X-axis. The 4 degree off-normal angle causes principal ray  116   r  to strike a top surface  152  of prism component  144  at an angle that is greater than the characteristic critical angle of the glass from which prism component  144  is formed. 
     Principal ray  116   r  by total internal reflection reflects off top surface  152  at air space interface  146  and propagates through prism component  144  to an exit surface  160 . An imaging beam derived from principal ray  116   r  of the on-state reflected light beam is characterized by a principal ray  162  and propagates through exit surface  160 . Principal ray  162  propagates generally in the X direction, traveling upwardly at a +4 degree angle relative to the X-Z plane. The imaging beam propagates toward a projection lens  164  to a projector screen (not shown) to display an image to a viewer. 
     In their off-state, the micromechanical mirrors of DMD  130  receive the incident beam and reflect an off-state reflected light beam characterized by a principal ray  116   o . The micromechanical mirrors in their off-state reflect principal ray  116   o  at a 44 degree angle relative to the normal of the X-Z horizontal plane and the projection of principal ray  116   o  onto the X-Z horizontal plane at a 42 degree angle of rotation from the X-axis. The 44 degree angle causes principal ray  116   o  to propagate onto a side surface  170  of prism component  144 . Side surface  170  is coated with an absorptive coating such as black paint, so that principal ray  116   o  will not be internally reflected by side surface  170 , but will be absorbed by the absorptive coating. The absorptive coating functions to prevent off-state light from otherwise reflecting by total internal reflection off side surface  170  and entering projection lens  164  as stray light. The stray light would scatter inside the lens barrel, propagate through projection lens  164 , and be projected onto the display screen. 
     FIG. 4 is a diagram showing the DMD coordinate system for prism assembly  140 . FIG. 4 indicates the coordinate vector directions defining the angular inclination of top surface  150  of prism component  142 , which establishes the illumination angle for DMD  130  and the vector direction of the parallel opposed surfaces of prism components  142  and  144  at air space interface  146 . Air space interface  146  has a thickness controlled by spacer balls embedded in bonding material or by other means. The bonding material is placed outside the optically active area. 
     FIG. 5 is an isometric view of an embodiment constituting a version of prism assembly  140  having a light escape window area. With reference to FIG. 5, a prism assembly  240  includes compensating prism  142  and output prism  144  described above in connection with prism assembly  140 , with a third prism component  272  optically bonded to side surface  170  of prism component  144 . The refractive indices of the cement and the opposed surfaces of prism components  144  and  272  are matched sufficiently to prevent high reflection at the glass-cement-glass interfaces. Prism component  272  effectively extends by about  20  percent the length of output prism  144  to allow the unwanted, off-state light to upwardly propagate through, and thereby not reflect at, a location represented by side surface  170 . Prism component  272  has a front surface  274  that functions as a light window for the unwanted, off-state light to escape prism assembly  240  so that off-state light is directed away from projection lens  164  (FIG.  2 D). The light window allows the unwanted, off-state light to reach and totally internally reflect off a top surface  276  of prism component  272  rather than a side surface  278  of prism component  272 , thereby eliminating a need for an absorptive coating on side surface  278 . Eliminating a need for applying an absorptive coating on the prism assembly embodiment shown in FIGS. 3A,  3 B,  3 C, and  3 D is desirable because an absorptive coating would tend to impart on prism assembly  140  thermal stresses that result in assembly deformation and misalignment. Removing the unwanted, off-state light by the presence of light window  274  also prevents unacceptable levels of stray light from degrading the quality of the projected image propagating through projection lens  164 . Skilled persons will appreciate that prism component  144  can be configured in an integrated design in which a single prism component has the shape of the exterior surfaces resulting from the bonding together of prism components  144  and  272 . 
     In either implementation, prism assembly  240  provides three-point structural stability when mounted in a projection system housing. 
     FIGS. 6A-6D illustrate the nature of the structural stability prism assembly  240  affords. FIG. 6A shows the spatial arrangement of DMD  130  mounted against an exterior bottom surface of, and prism assembly  240  fitted within, an optical component or optics chassis  282 . 
     FIGS. 6B and 6C show top plan views of optics chassis  282 , respectively, with and without prism assembly  140  installed. With particular reference to FIG. 6B, optics chassis  282  includes in a bottom surface  284  a rectangular opening  286  defined by boundary lines  288 . Opening  286  receives DMD  130  so that its light reflecting surface is aligned with and spaced apart from prism assembly  140 . A rectangular support frame  290  formed by four raised linear base landings  292  and set back from boundary lines  288  provides a base landing for prism assembly  140 . With particular reference to FIG. 6C, the active part of prism assembly  140  does not overlap rectangular opening  286  provided for DMD  130  in optics chassis  282 ; therefore, the base plane of prism assembly  140  rests on only two of the base landings  292 . This leads to slight tilt variations in the final position of prism assembly  140  as it is located in optics chassis  282  during manufacturing and thereby results in distortions in the projected image. Moreover, because a center of gravity  294  of prism assembly  140  extends beyond an edge line  296  of the base support plane created by the two base landings, prism assembly  140  is not dynamically stable and is vulnerable to high loads resulting from impact and vibration. Edge line  296  represents, therefore, a line of rotation of prism assembly  140  when it is mounted in optics chassis  282 . 
     FIG. 6D shows a top plan view of optics chassis  282  with prism assembly  240  installed. With reference to FIG. 6D, the presence of prism  272 , either optically fixed to or formed as an integral part of output prism  144 , effectively extends prism assembly  240  to cover a third base landing  292 . This added support point greatly reduces tilt variations and substantially improves the structural support of prism assembly  240  in optics chassis  282 . 
     Optics chassis  282  supports prism assembly  240  using a tertiary datum system that includes three base landings  292  and four side landings  298  positioned on the inner surfaces of adjacent optics chassis sidewalls  300 . Base landings  292  and side landings  298  create three mutually perpendicular datum planes within optics chassis  282 , to which prism assembly  240  is attached. 
     FIGS. 7A and 7B are respective side and front isometric views showing an alternative design for prism assembly  140 , which is implemented with a light escape window. With reference to FIGS. 7A and 7B, a prism assembly  340  substitutes for third prism component  272  a molded, faceted array  372  that is optically cemented to side surface  170  of prism component  144 . A face  374  of multiple facets  376  disperses the unwanted, off-state light as it exits side surface  170  of prism component  144  and propagates into a light absorber  378 . This is achieved by adding curvature to the facets  376  or by varying the face tilt angle of each facet  376  to refract the unwanted, off-state light to different locations on absorber  378 . The result is diminishing the concentration of incident light at any location on absorber  378  and to minimize an increase in its temperature and thereby mitigate thermal problems in image projection system  110 . 
     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. For example, although they are described with reference to image projection systems, the prism assembly designs and arrangements of the invention can be advantageously implemented in other types of image display systems. The scope of the present invention should, therefore, be determined only by the following claims.