Abstract:
An image projector employs a prism assembly that receives incident light, illuminates a reflective light modulator, and receives therefrom reflected imaging light for direction toward a small-diameter light entry pupil of a light-weight, compact projection lens. The prism assembly includes compensating and output prism components having opposed surfaces separated by an air gap. The prism assembly sets up a correct illumination angle on the DMD and then separates the incident illumination light from the reflected imaging light. Each on-state micromechanical DMD mirror reflects the illumination light nearly normal to a horizontal datum plane, forming reflected imaging light that reflects off the air gap and reenters the prism assembly through the output prism component. A focusing lens disposed between the prism assembly and the DMD refracts the reflected imaging light into a converging imaging light bundle compatible with the small diameter light entry pupil of the projection lens.

Description:
This application is a continuation-in-part of U.S. patent application Ser. No. 09/405,425, filed Sep. 22, 1999, for IMAGE PROJECTION SYSTEM PACKAGED TO OPERATE LYING FLAT WITH A VERY LOW PROFILE, which claims the benefit of U.S. Provisional Application No. 60/134,473, filed May 17, 1999. 
    
    
     FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     None 
     TECHNICAL FIELD 
     This invention relates to image display systems and, in particular, to an image projection system implemented with a reflective light modulator and having an optical system that reduces weight and packaging profile. 
     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  10 , which 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. 
     Reducing its height is one step in achieving true portability for image projector  10 . Reducing its weight (mass) would be another beneficial step toward true portability. The image projector market is demanding projectors that can be carried along with a companion laptop computer in a briefcase. 
     SUMMARY OF THE INVENTION 
     An object of this invention is, therefore, to provide a truly portable image projector apparatus. 
     Another object of this invention is to provide an image projector weighing less than about 2.27 kilograms (five pounds) and having a height less than about 6.35 centimeters (2.5 inches). 
     A further object of this invention is to provide an image projection optical system suitable for use with a light-weight and compact projection lens. 
     This invention is suitable for use in an image projector employing a reflective light modulator, such as a DMD, and a prism assembly that illuminates the DMD and receives a reflected imaging light bundle for directing toward a projection lens. 
     The prism assembly includes compensating and output prism components having opposed surfaces separated by an air gap. The prism assembly sets up a correct illumination angle on the DMD and then separates incident illumination light from reflected imaging light by total internal reflection discrimination. 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 a compound angle that directs the light toward the DMD at the correct angle for illumination. For each micromechanical mirror of the DMD in its on-state, the illumination light reflects nearly normal to the horizontal datum plane to form imaging light that reenters the prism assembly through the output prism component. Because the angle of incidence at the air gap is greater than the critical angle, the imaging light reflects off the air gap and propagates through the output prism component. 
     A focusing lens of this invention is disposed between the prism assembly and the DMD to refract all the reflected imaging light into a converging imaging light bundle as it propagates into a small diameter light entry pupil of a light-weight, compact projection lens. 
     In an alternative embodiment, the focusing lens is implemented as a curved surface on the side of the prism assembly facing the reflective light modulator. This embodiment reduces weight, complexity, and space, but adds to the cost and complexity of the prism assembly. 
     An advantage of the focusing lens of this invention is that the projection lens requires a light entry pupil having a diameter that is only 30 to 50 percent the diameter of prior art entry pupils. 
     Another advantage of the focusing lens of this invention is that the projection lens has a mass and a length that is about 50 to 75 percent of prior art projection lenses. 
     Additional objects 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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A,  1 B,  1 C, and  1 D are respective isometric, frog 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 representative embodiment of this 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, alternative focusing lens embodiment, and a reflective light modulator implemented in the embodiment of FIGS. 2A-2E. 
     FIG. 4 is a simplified pictorial side view of a prior art prism assembly, reflective light modulator, and projection lens showing a telecentric ray tracing through the prism assembly that requires a large light entry pupil for the projection lens. 
     FIG. 5 is a simplified pictorial side view of a prism assembly, reflective light modulator, focusing lens, and reduced size projection lens of this invention showing a focused ray tracing through the prism assembly that requires a relatively small light entry pupil for the projection lens. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     U.S. patent application Ser. No. 09/405,425, filed Sep. 22, 1999, for IMAGE PROJECTION SYSTEM PACKAGED TO OPERATE LYING FLAT WITH A VERY LOW PROFILE, which this application relies for an earlier filing date under 35 U.S.C. §120, is incorporated herein by reference. 
     FIGS. 2A,  2 B,  2 C,  2 D, and  2 E are respective isometric, frontal, side elevation, top plan, and rear end views of an image projection system  110  suitable for explaining the use of this invention. Projection system  110  includes a high power lamp  112  positioned at a focus of an elliptical reflector  114  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 arc lamp, which is suitable for use in an image projector to achieve its lifetime and lumen specifications. 
     Lamp  112  is positioned at the first focus of elliptical reflector  114 , which has a cold mirror surface that reflects forward only visible light, while 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 projection 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  have 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  includes 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  has a rectangular shape that is imaged through lens elements  124 ,  126 , and  128 , a prism assembly  130 , and a focusing lens  132  onto a light reflecting surface of a DMD  134 . Focusing lens  132  is described with reference to FIG.  5 . 
     Integrator tunnel  122  is rotated  8  degrees about its major axis to correct for rotation in the illumination on DMD  134 , which rotation is caused by prism assembly  130 . DMD  134  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  134 . A beam direction turning mirror  136  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 prism assembly  130 , which 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  130  allows DMD  134  to lie flat when in operation. Prism assembly  130  sets up the correct illumination angle on DMD  134  and separates by total internal reflection discrimination the illumination light from the imaging light reflected by DMD  134  in its on-state. The illumination angles for DMD  134  are controlled by the angles of the faces of prism assembly  130  and the refraction of focusing lens  132 . Prism assembly  130  and focusing lens  132  refract and reflect the incident light bundle so that DMD  134  is illuminated from a corner with a projection angle partly built into the output light bundle. After the illumination light reflects off DMD  134  in its on-state, imaging light exits prism assembly  130  along essentially the same propagation direction as that of illumination light entering prism assembly  130 . Because of the many degrees of freedom in prism assembly  130 , light can enter it roughly parallel to a support table and in line with a projection lens. Preferably, DMD  134  is mounted on an etched 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  130 . 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 shown in FIGS. 3A-3D that directs principal ray  116   i  toward DMD  134  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  toward focusing lens  132  and DMD  134 . 
     FIGS. 3B and 3C further show an alternative embodiment in which focusing lens  132  is formed as a convex lens integral to the surface of output prism component  144  facing DMD  134 . 
     A controller, which is an integral component of DMD  134 , provides electrical signals to direct the micromechanical mirrors of DMD  134  to the desired light reflecting states. In their on-state, the micromechanical mirrors of DMD  134  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  134  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 shows a representative conventional prism assembly  180 , reflective light modulator  182 , and projection lens  184 . An incident light bundle  186  (shown in solid lines) enters prism assembly  180  telecentrically, reflects off an internal surface  188 , and propagates telecentrically toward reflective light modulator  182 , which reflects an imaging light bundle  190  that propagates telecentrically through prism assembly  180  and internal surface  188 , and propagates telecentrically toward projection lens  184 . 
     Because imaging light bundle  190  propagates through and exits prism assembly  180  telecentrically, projection lens  184  requires a light entry pupil having a diameter  192  compatible with the typical 0.5 inch (12.7 mm) to 1.25 in (31.75 mm) diagonal dimension of the reflective surface of reflective light modulator  182 . For example, the diameter of the entry pupil optics of prior art projection lens  64  (FIGS. 1A to  1 D) is about 1.6 inches (40 mm). 
     In contrast, FIG. 5 shows a preferred embodiment of this invention in which prism assembly  180  and reflective light modulator  182  are employed in combination with focusing lens  132  and a compact projection lens  200 . Incident light bundle  186  enters prism assembly  180  telecentrically, reflects off internal surface  188 , and propagates telecentrically toward reflective light modulator  182 . However, in this invention, focusing lens  132  refracts incident light bundle  186  as it propagates toward reflective light modulator  182 . A reflected imaging light bundle  202  is refracted again by focusing lens  132  causing imaging light bundle  202  to converge as it propagates through prism assembly  180  and exits toward compact projection lens  200 . 
     Because imaging light bundle  202  converges as it propagates through and exit prism assembly  180 , projection lens  200  requires a relative small light entry pupil having a diameter  204  that is about 30- to 50-percent the diameter of prior art entry pupils. For example, the diameter of the entry pupil optics of projection lens  164  (FIGS. 2A to  2 E) is less than about 0.8 inch (20 mm). Projection lens  200  is further advantageous because it has a mass and a length that is about 50 to 75 percent of prior art projection lenses. (Compare FIGS. 1 and 2.) 
     As shown by way of example in FIGS. 3B and 3C, to further reduce mass and profile, focusing lens  132  may, alternatively, be implemented as a curved surface on the side of prism assembly  130  facing DMD  134  or whatever reflective light modulator is employed. Likewise, referring to FIG. 5, focusing lens  132  may be similarly formed as a curved surface on the side of prism assembly  180  facing DMD  182 . The focusing lens may otherwise be positioned at, formed in, attached to, or bonded to a surface of any such prism assembly. 
     Skilled workers will recognize that portions of this invention may be implemented differently from the implementations described above. For example, prism assemblies  130  and  180  may include many types of prisms, such as a TIR prism, a polarization beam splitting prism, and a color combining prism (Philips prism or cube). The converging imaging light bundle propagating through the prism may also propagate through several optional optical elements (not shown), such as prisms, lenses, mirrors, and dichroic filters before entering the projection lens. 
     The implementation of this invention shown in FIGS. 2 and 3 is merely an illustrative example. More practical embodiments would employ the optical components of FIG.  5 . For example, depending on the application, prism  180  may be employed to allow mounting the reflective light modulator at a position facing a side or rear surface of the prism, thereby reducing the height or profile of the resulting projector. 
     The reflective light modulator employed with this invention may be a device other than a DMD, such as a reflective liquid crystal device. 
     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 DMD-based image projection systems, the focusing lens, prism assembly, and compact projection lens combination of this invention can be advantageously implemented in other types of image display systems. The scope of this invention should, therefore, be determined only by the following claims.