Abstract:
A retractable screen system suitable for portable, hand held computing and communication units joins a number of optical sheets to form a rollable, extendible and retractable projection sheet. The resulting folded optical system provides a lightweight, portable unit. Telescoping arms allow the screen to be extended and stowed quickly and easily. High screen resolution is afforded by multiple mechanisms that scan concurrently. Beam indexing of the screen permits the precise alignment via feedback, and an embodiment fit for military, command and control applications.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation in part of currently U.S. Non-Provisional application Ser. No. 09/611,707, entitled “Compact Rear Projection System Using Birefringent Optics” filed Jul. 7, 2000, now U.S. Pat. No. 6,561,649 which claims priority from U.S. Provisional Patent Application No. 60/143,058, entitled “Compact Rear Projection System Based upon a Curved Turning Mirror and Anamorphic Projection” filed Jul. 9, 1999, and from which the present application also claims priority. The present application, in addition, claims priority from the U.S. Provisional Patent Application No. 60/178,332, entitled “Collapsible Rear Projection Display” filed Jan. 27, 2000. All of these applications are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to image projection systems, and more particularly to rear projection systems with retractable screens. 
     BACKGROUND 
     There has been a recent surge in the demand for hand held computing and communication devices, such as personal assistants and cellular telephones. Providing these devices with image projection capability requires small and lightweight projection systems. The screen for such a system should be designed for portability without sacrificing, despite its small dimensions, acceptable resolution. The screen, moreover, while small, should be larger in the deployed state than screens currently used in these portable devices. These displays generally provide an unacceptable viewing area for high resolution. 
     SUMMARY OF THE INVENTION 
     A rear-projection system according to the principles of the invention has a retractable rear-projection screen that is retractable into or extendible from a base of the host device. An example of a base is a rear-projection television or a hand-held device such as a cellular phone, personal assistant or a portable computer appliance, e.g., with wireless Internet access. A projector within the base projects light onto the rear of the screen when the screen is extended. The viewer views a resulting image on the front of the screen. The screen can be stowed into the base when the screen is not in use, so that the device is portable and easily stored. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the invention can be obtained from the following description in conjunction with the drawings, in which: 
         FIG. 1  shows a side view of a retractable screen system according to the principles of the invention; 
         FIGS. 2A and 2C , and  2 B and  2 D show, respectively, ray traces and corresponding polarization state diagrams exemplary of the invention; 
         FIG. 3  shows an optical system according to the principles of the invention; 
         FIG. 4  shows a scanning system according to the principles of the invention; 
         FIG. 5  illustrates a scanning system according to the principles of the invention; 
         FIG. 6  shows a polarizing scanner according to the principles of the invention; 
         FIG. 7  illustrates a scanning system according to the principles of the invention; 
         FIGS. 8A-D  illustrates scanning traces, a nd a phase diagram, for the embodiment of  FIG. 7 ; 
         FIG. 9  illustrates a scanner according to the principles of the invention; 
         FIG. 10  shows details of a reader used in the scanner of  FIG. 9 ; 
         FIGS. 11A and 11B  show a positioning diagram and a scanline timing diagram, respectively, for the reader of FIG.  10 . 
         FIGS. 12A and 12B  show a side-view of a scanner and a top view of a holographic disk of the scanner, respectively, according to the principles of the invention; 
         FIG. 13  shows a side view of an embodiment of a retractable screen according to the principles of the invention; and 
         FIG. 14  shows a side view of an embodiment of a retractable screen system according to the principles of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     A hand-held projection screen system  100  according to the principles of the invention is shown, in side view, by FIG.  1 . The hand-held unit  100  includes a housing  102  that contains an image formation module  104 , an image information memory  106 , a bus  108 , a take-up roller  110 , an internal pulley  112 , a front telescoping arm pair  114 , a rear telescoping arm pair  116 , and a projection sheet  118 . The image formation module  104  and the image information memory  106  are located within the housing  102 . The image formation module  104  comprises, for example, a scanner having a laser and a light modulator. The image information memory  106  contains information, such as pixel data, for forming an image. The memory  106  also includes means for reading from and, optionally, writing to the memory  106 . The image formation module  104  reads information from the image information memory  106  via the bus  108 . The information in the memory  106  may be generated internal to the unit  100  or communicated from outside the unit  100 , as by an antenna. 
     Residing nearly fully within the housing  102  when stowed into a stowed position, and less fully within the housing  102  when extended into an deployed position, is the projection sheet  118 . A front telescoping arm pair  114  and a rear telescoping arm pair  116  support the projection sheet  118 . Because  FIG. 1  is a side view, it shows one of the arms of the front telescoping arm pair  114  in extended position—the other arm of the pair is hidden from view behind the arm shown. The same holds for the rear telescoping arm pair  116 —only one arm of the pair is shown in  FIG. 1. A  front pulley  126  joins the distal ends of the arms of the front telescoping arm pair  114  and is attached to an end of the projection sheet  118 . The projection sheet  118  includes a first sheet  120 , which joins a second sheet  122  and a third sheet  124  end-to-end to form a continuous film so that, in effect, the front telescoping arm pair  114  is attached to an end of the third sheet  124 . When, as pictured in  FIG. 1 , the front telescoping arm pair  114  is extended, it, along with the front pulley  126 , frames the third sheet  124 . The third sheet  124  extends between the front telescoping arm pair  114 , into the housing  102 , and around the internal pulley  112 . Also extending around the internal pulley  112  is the first sheet  120 , which continuously joins the first sheet  124 . The first sheet  120  extends out of the housing  102  at a tilted angle to roll around a rear pulley  128  that connects the distal ends of the rear telescoping arm pair  116 . The rear pulley  128  and the rear telescoping arm pair  116  together frame the second sheet  122 . The second sheet  122 , at one of its ends, continuously joins the first sheet  120  and rolls around the rear pulley  128 . At the opposite end, the second sheet rolls around the take-up roller  110  after entering the housing  102 . The take-up roller  110  is biased, as by a spring, to take up the slack in the projection sheet  118 . Collapsing the front telescoping arm pair  114  and the rear telescoping arm pair  116  causes the projection sheet  118  to roll around the internal pulley  112  and the rear pulley  128  onto the take-up roller  110 , making the unit  100  easy to store or transport. 
     The three sheets that make up the projection sheet  118  have different optical characteristics. The first sheet  120  is a polarization-dependent reflector that selectively transmits and reflects light based on polarization state of the light. The second sheet  122  is a multi-layered laminated structure. When implemented as two layers, the internal layer is a quarter-wave retarder, which rotates the polarization state of the incident light by 90 degrees. The backing layer is a metallic mirror. A multi-layered design to reduce light leakage in the folded optics is discussed below. The third sheet  124  is also a laminated structure, having multiple layers that include a collimating, light-incident layer and a plastic layer that may include diffusers. Details of this folded optical configuration are provided below. 
     A folded circularly polarized system according to the principles of the invention provides for reduced cabinet depths in comparison to known projection systems, and is suited for the dimensions of a hand-held unit. Projected images traverse a folded light path such that light is incident on the transmission screen at a projection quality angle (for example, having a magnitude less than the Brewster angle). In one embodiment, a polarization-dependent reflector acts as a mirror in the optical path for light of a particular polarization state. After the polarization-dependent reflector reflects the projected light, the polarization state is manipulated to a transmissible polarization state. The projected image can then pass through the polarization-dependent reflector to the transmission screen and then to the viewing audience. Due to the geometry of the optical path, the light incident on the transmission screen is a projection quality incident angle. 
       FIG. 2A  illustrates a ray trace  200  for a circularly polarized system exemplary of principles of the invention. The optical configuration includes an image projector  202 , mirrors  206  and  210  and a transmission screen  208 . The projection image rays  212 ( a-l ) are shown as arrows on the ray trace  200 . The projection system also includes a quarter-wave polarizer  204 . In operation, the projector  202  projects an image in the direction of the turning mirror  206 . The projected image is shown as rays  212 ( a-c ) leaving the projector  202  in the direction of the mirror  206 . The quarter-wave material  204  is interposed between the projector  202  and the turning mirror  206 , and the projected image rays  212 ( a-c ) pass through the quarter-wave material  204  before striking the reflective surface of the turning mirror  206 . Upon passing through the quarter-wave material  204 , the projected light  212 ( a-c ) becomes circularly polarized. 
     The turning mirror  206  directs the projected images toward the transmission screen  208 . The rays  212 ( f, i  and  l ) leaving the turning mirror  206  remain circularly polarized but have a handedness opposite to that of the rays incident on the mirror  206 . The departing rays are incident on the transmission screen  208 . The transmission screen  208  includes a layer of quarter-wave polarizing material and a layer of polarization-dependent reflective material. Upon passing through the screen&#39;s quarter-wave material, the projected rays  212  ( f, i  and  l ) again become linearly polarized. The polarization state is such that the screen&#39;s polarization-dependent reflective material reflects this light. On the ray trace, these reflected rays  212 ( d, g  and  j ) are directed from the transmission screen toward the second mirror  210 . 
     The light reflected from the polarization-dependent reflector passes through the sheet of quarter-wave material, which circularly polarizes the light, prior to striking the second mirror  210 . The second mirror  210  changes the handedness of the polarization and directs this light  212 ( e, h  and  k ) back toward the transmission screen  208 . At the transmission screen  208 , the image again strikes the screen&#39;s layer of quarter-wave material, which linearly polarizes the light. In this instance, the polarization state is such that the screen&#39;s polarization-dependent material is transmissive, rather than reflective, and the rays are transmitted to the viewing audience. 
     As illustrated by the ray trace  200 , the optical elements are used to create an optical folder that permits a shallower cabinet depth than in conventional rear projection systems. In a conventional system, the image rays ( 212   f, i  and  l ) are incident on the transmission screen  208  at angles A, B and C. The incident angle C, measured from the normal to the transmission screen, is greater than A. As the field of view increases, the incident angle increases. Using the optical folder described above, the optical path is folded such that the incident angle for substantially all the image rays is equal to or less than a projection quality incident angle. 
       FIG. 2B  is a polarization state diagram illustrating exemplary polarization states corresponding to the ray trace  200  of FIG.  2 A. The light leaving the projector is linearly polarized, as represented by the polarization state symbol  220 . The light is shown having a plane of vibration in the Y axis. This light passes through the quarter-wave material  204 , which circularly polarizes the light. In the diagram, this is shown as a right-handed circular polarization state symbol  222 , which indicates the polarization and handedness of the polarization. This circularly polarized light strikes the turning mirror  206 , which reflects the light as left-handed circularly polarized light  224 . The reflected light strikes the quarter-wave material on the transmission screen  208 . This quarter-wave material linearly polarizes the light, as shown by the linear polarization state symbol  226 . In this case, however, the light is polarized in a different plane of vibration  226  than the projector light  220 . 
     The linearly polarized light is reflected by the polarization-dependent material in the transmission screen  208  and again passes through the quarter-wave material in the screen  208 . The quarter-wave material circularly polarizes the light  228 . The second mirror  210  reflects this circularly polarized light, changing the handedness of the polarization  230 . The mirror  210  directs this light toward the screen  108  where it again strikes the quarter-wave material. The quarter-wave material linearly polarizes the light  232 . The plane of vibration in this polarization state  232  is such that the polarization-dependent reflector is transmissible. The light passes through the material and the transmission screen to the person or persons viewing the screen. 
     In the circularly polarized system shown in  FIG. 2A , the projector can be any source capable of projecting light. The mirrors  206  and  210  can be conventional mirrors, such as a mylar film mirror stretched over a metal frame, or, as will be explained hereafter, the mirrors can be dielectric mirrors. The screen  208  can be composed of multiple layers, including lenticular lens layers and Fresnel lens layers, in addition to the materials described above. The polarization-dependent layer can be a polarization-dependent reflecting film such as 3M&#39;s Dual Brightness Enhancement Film (DBEF). The quarter-wave polarization material can be any of a number of known polarizing materials or in retardation films for use in optical systems. 
     The circularly polarized system  250 , for which a single exemplary ray trace is shown in  FIG. 2C , is adaptable for the optical configuration used in the hand-held unit  100  shown in FIG.  1 . The system  250  consists of a linear polarizing projector  252 , a mirror  254 , a projection screen  256 , a polarization dependent reflector  258 , and quarter wave sheets  260  and  262 . 
     In operation, the linear polarizing projector  252  projects a linearly polarized beam  264 , in a polarization state one. The beam  264  is left-handedly polarized to a polarization state two as it passes through the quarter wave sheet  260 . Reflection from the mirror  254  rotates the beam  264  as right-handedly polarized in a polarization state three. Passage of the right-handedly polarized beam  264  through the quarter wave sheet  262  rotates the polarization of the beam  264  to a linearly polarized state represented as a polarization state four. The plane of vibration of the rotated beam  264  is such that the linearly polarized beam  264  reflects from the polarization dependent reflector  258 . The reflection rotates the beam  264  into a right-handedly polarized beam in a polarization state five. The beam  264  transforms into a left-handedly polarized beam in a polarization state six as it reflects from the mirror  254 . Passage through the quarter wave sheet  262  rotates the beam  264  into a linear polarization represented by a polarization state seven. The plane of vibration of the linearly polarized beam  264  is now oriented so that the beam  264  passes through the polarization dependent reflector  258  and onto the screen  256 .  FIG. 2D  illustrates polarization state symbols for each of the polarization states one through seven. 
       FIG. 3  shows a circularly polarized system  300  that corresponds in principle to the system  250  of  FIG. 2C , and represents an implementation of the optical configuration used in the hand-held unit  100  of FIG.  1 . In the circularly polarized system  300 , a light control film  302  and a collimator  304  are used to reduce ghosting and scatter. As in  FIG. 2C , the quarter-wave material  310  and polarization-dependent reflector  314  are not part of the screen  316 , although, here, there is only one sheet of quarter-wave material  310 . As in the circularly polarized systems  200  and  250 , the projector  306  and mirrors  308  and  312  can be conventional mirrors or dielectric stacks. Both the projector  306  and the mirrors  308  and  312  are part of the image formation module  104  of FIG.  1 . The solid  320  and broken  322  arrows indicate ray traces, where the solid arrows show the desired light travel of the system  300  and where the broken arrows indicate leakage from the polarization-dependent reflector  314  and the quarter-wave material  310 . Leakage may arise because a realization of the retardance characteristics of these elements is angle and wavelength dependent. 
     In operation, the projector  306  outputs linearly polarized light  320 , which is reflected by the turning mirror  308  toward the quarter-wave material  310  and the second mirror  312 . The quarter-wave material  310  circularly polarizes the light, and the mirror  312  reflects the light toward the polarization-dependent reflector  314 . The handedness of the light changes when reflected by the mirror  312 , and the quarter-wave material changes the polarization state to linear. The polarization-dependent reflector  314  reflects the light back to the quarter-wave material  310  and mirror  312 . As shown by broken rays  322   a-e , however, some light is not reflected due to the characteristics of the polarization operative elements  310  and  314 . These rays transmit to the screen  316 . The reflected light  320  traverses the folding path again, and the polarization state becomes transmissive by the polarization-dependent reflector  314 . 
     For the system  300  of  FIG. 3 , the light  322   d-e  that leaks through the polarization-dependent reflector  314  is incident at a significantly different angle than the light  320  that again traverses the folding path. A collimator  304  and light control film  302  operate to reduce the artifacts that can be caused by the mirror and quarter-wave material realization. The light control film  302  operates to absorb light incident at unwanted angles. For purposes of explanation, the film  302  is shown having slats  303  made of light-absorbing material. Light striking the slats is absorbed. The collimator  304 , which can be a cylindrical lens, collimates the desired light  320  to the acceptance angle of the light control film  202 . In this configuration, the light control film  202  can be interposed between one of the Fresnel lenses in the screen and a diffusing or scattering surface. Light control films of suitable characteristics are known, such as light control films manufactured by 3M Corp. Negative birefringence films also can be used to correct for phase shifts introduced by the polarization operative elements (similar to their use in LCD panels). 
     The quarter-wave material  310  in  FIG. 3  can have broad angle and broad bandwidth capabilities. A broad angle film can be constructed from liquid crystal material and negative birefringent corrector films. Liquid crystals (LC) can act as birefringent agents. For example, combining LC material with binders yields an adjustable retardation material. Electric fields are used to cure the binders, fixing the LC&#39;s molecular orientation and, therefore, the material&#39;s retardation. The binder can be an ultraviolet light curable polymer. Negative birefringent corrector films, such as those used in LCD displays, can be combined with the LC retardation film to make the film broad angle. 
     The retardation characteristic of LC material is a function of the angle of incidence. In the fold system of  FIG. 3 , the angle of incidence on the quarter-wave material  310  is a function of position; however, in small regions the angle of incidence to the quarter-wave material  310  varies only slightly. The retardance characteristic of the LC material can be tailored point by point for the mean of local angles of incidence. This sets the retardance characteristic for a specific center wavelength and a range of angles about the local mean for the incident light. The LC material characteristic for light reflected from the mirror  312  can be calculated for a separate sheet prepared in the same manner. The retardance characteristic of the incident film is then adjusted to account for the effect of the second film by subtracting the contribution of the second film for light incident at the angle of reflection. The retardance of the second film is likewise adjusted by subtracting the contribution of the first film for light incident at the angle of incidence. The sheets can be laminated to form a “bi-film” optimized for the local range of angles. 
     The “bi-film” can be effectively bonded to the second mirror  312  in the system  300  of FIG.  3 . To implement the “bi-film” with a dielectric mirror, the film is optimized for some wavelength, such as blue light. The film can then be bonded to a mirror produced using Giant Birefringent Optics (GBO) reflective in the blue region but transmissive in the red and green. Two additional“bi-film” layers are then prepared with retardance adjusted for the green band and red band respectively, with appropriate backing mirrors reflective in the proper wavelength. The result is a dielectric stack that implements appropriate retardance and reflection characteristics for the desired wavelengths of light and for the range of angles in the optical system. 
     A saddle surface, collimating scanning system  400  according to the principles of the invention is shown in FIG.  4 . The system is shown in a Y-Z cross-section for ease of explanation. The point source scanning projector  402  is shown projecting toward a mirror  404 . Both the projector  402  and the mirror  404  are part of the image formation module  104  of FIG.  1 . The mirror  404  collimates the point source beamlets so that the light striking and reflecting off the mirror  404  projects properly upon the X and Y directions of the birefringent material. The light is incident on the quarter-wave retardation plate  406  and a second mirror  408 . Light reflected from the second mirror  408  is directed to the polarization-dependent reflector  410  for eventual transmission by the screen  412 . As in previously described embodiments, the screen can include a collimator for aligning desired light with a light control film  414 . The mirror  408  is produced using GBO, as described with reference to  FIG. 3 , which causes the polarization directions of a ray propagating toward a point on the quarter-wave material  406  having non-zero X and Y coordinates to change. 
     In the saddle surface, collimating scanning system  400  of  FIG. 4 , the collimating element is the mirror  404 , which has a saddle surface. The saddle surface accomplishes two goals. It collimates light to optimally orient the polarization to minimize leakage through the system&#39;s birefringent elements. It also implements a second cylindrical power that increases the effective field of the scan. For example, given a fifteen degree divergence from the source to the mirror  404 , the second cylindrical power provides an effective 30 degree divergence angle, thereby increasing the vertical scan dimension. The same result is achievable with two cylindrical elements (mirrors or lenses), rather than one saddle shaped element. The magnification for these elements can be in one or two directions and need not be the same in both directions. 
     A cylindrical optic, collimating scanning system  500 , shown in  FIG. 5 , serves as an alternative scanning system. In the system  500 , which is included within the image formation module  104  of  FIG. 1 , light from a projector  503  reflects off a scanning mirror  502  onto a collimating optical element  510 . The light incident on the scanning mirror  502  is polarized in the Y and Z directions as indicated by the polarization state symbol  504 . The reflected ray diverges into a beam represented by multiple rays  506   a-d , as would be analogous to the divergence of a point source projection. The polarization of the scanned beam is represented for a typical ray  506   c  by a polarization state symbol  508 . The rays strike the collimating optical element  510  that collimates the light  512   a-d , resulting in effective line source projection. The polarization state of the collimated light  512   a-d  is shown by the polarization state symbol  514  for a typical ray  512   c . The collimator optimally orients the polarization to minimize leakage through the system&#39;s birefringent elements. A separate, orthogonal, cylindrical-powered scanner  501  reflects the line source projection, to create a two-dimensional scan having an effective field. 
     In operation, and referring back to  FIG. 1 , image information in the image information memory  106  is conveyed along the bus  108  to the image formation module  104 . Based on the information read, the image formation module  104  projects a linearly polarized light beam  130  that is incident on the projection sheet  118 , and specifically on the second sheet  122 . The second sheet  122  reflects the beam  130  to the first sheet  120 . The first sheet  120  reflects the beam  130  based on the current polarization state of the light. The second sheet  112  again reflects the beam  130 . The beam  130  now has a polarization state that allows it to pass through the first sheet  120  and onto the third sheet  124 , which serves as a viewing screen. 
       FIG. 6  demonstrates how a linearly polarized beam  130 , required for the folded optics, can be produced from an unpolarized beam. A linear polarizing scanner  600 , which is included within the image formation module  104 , includes a laser  606  driven by a video signal drive  604  connected to a signal monitor  602 . Use of a signal monitor in a feedback scheme is discussed later. The image formation module  600  further includes a focusing lens  610 , a polarizing cube  612 , a half-wave plate  614 , a biprism  620  and a line scanning mirror  622 . The laser  606  outputs a laser beam  608 . The focusing lens  610  collimates the beam  608  to make it nearly circular in cross-section. The focusing lens  610  also gives the beam  608  a cone angle that will cause the beam  608  to focus at the screen. This slowly-converging beam  608  passes into a polarizing cube  612  that splits it into two beams having orthogonal polarization states. One of the two beams, an orthogonally polarized beam  616 , passes through the polarizing cube  612 . The other of the two beams, an orthogonally polarized beam  618 , reflects from a half-wave plate  614 , and is thereby converted to the same polarization state of that of the orthogonally polarized beam  616 . A biprism  620  toes in the beams  616  and  618  to make them meet at the viewing screen  124  after reflecting off a line scanning mirror  622 , the saddle surface mirror  404  and the first  120  and second  122  sheets of the projection sheet  118 . For color implementation, the image formation module  600  can include three lasers, such as a red, a green and a blue one. By tilting the mirrors with slightly different tilt angles, or by using a triprism, each of the three lasers can be toed in to meet at the screen  124 . 
     The blank retrace on each scan results in a duty cycle of approximately 50%, even if alternate frames are traced in reverse order, i.e., top to bottom, then bottom to top, etc. Higher resolution at the viewing screen may require faster writing. U.S. Pat. No. 5,629,790 to Neukermans et al. is directed to micro scanning mirrors for use in a display. Neukermans discloses a reciprocating scanning mirror embodiment that cancels overall torque, thereby reducing vibration and the need for a weighty scanner to absorb the vibration. The disclosure is limited to two scanning mirrors, and corresponding beams. Although two beams write faster than one, more writing speed can achieve higher resolution at the viewing screen. 
     A multi mirror-pair, torque-canceling, scanning configuration  700 , shown in  FIG. 7 , provides a light-weight scanner that writes at high speed, by reciprocating mirror motions in each pair of mirrors while maintaining a temporal phase relationship among all the mirrors. The configuration  700  includes an outer gimbal  702 , an inner gimbal  704 , and scanning mirrors  706 ,  708 ,  710  and  712 . Horizontal torsion bars  714  axially and rotatably connect the inner gimbal  704  to the outer gimbal  702 . Vertical torsion bars  716  axially and rotatably connect the mirror  706  to the inner gimbal  704 . Also axially and rotatably connected to the inner gimbal  704  are mirrors  708 ,  710  and  712  by vertical torsion bars  718 ,  720  and  722 , respectively. The image formation module  104  includes, in addition to the configuration  700 , one source of modulated laser light directed at each of the mirrors  706 ,  708 ,  710  and  712  and either the saddle surface mirror  404  or the scanner  501 , mirror  502  and element  510 . 
     In operation, a central stripe electrode  726  attracts inner portions of the mirrors  706  and  708 , while the outer stripe electrodes  724  and  728  repel outer portions of the mirrors  706  and  708 . This causes the mirrors  706  and  708  to turn inward. The central electrode  726  and the outer electrodes  724  and  728  alternates repel and attract to turn the mirrors  706  and  708  alternately inward and outward. Similar reciprocal motion occurs with the mirrors  710  and  712  in response to attraction and repulsion by a central stripe electrode  732  and the outer stripe electrodes  730  and  734 . Modulated laser light reflecting from the mirrors  706 ,  708 ,  710  and  712  subsequently reflects from the saddle shape mirror  404  and through the folded optics, ultimately arriving at the viewing screen  124 . 
       FIG. 8A  is a timing diagram of the scanning of mirrors  706  and  708  that demonstrates their operation. A trace  802  represents the scanning of the mirror  706 , and a trace  804  represents the scanning of the mirror  708 . During the time period between t 0  and t 2 , the scanner directed at the mirror  706  performs a blank retrace. In the same period, the scanner directed at the mirror  708  writes a scanline screen image on a viewing screen (not shown). During the time period between t 2  and t 4 , the scanner directed at the mirror  706  writes a scanline screen image, while the scanner directed at the mirror  708  performs a blank retrace. Each scanner thus proceeds through repeated cycles, each consisting of a writing phase followed by a blank retrace phase. The mirrors  706  and  708  are  180  degrees out-of-phase in writing scanline screen images. 
       FIG. 8B  shows the scan timing for the mirrors  710  and  712 . Specifically, a trace  806  represents the scanning of mirror  710 , and a trace  808  represents the scanning of mirror  712 . The scan timing for the mirrors  710  and  712  reciprocates, as shown above for the mirrors  706  and  708 . Also, the scan timing for mirrors  710  and  712  is delayed by one time period with respect to the scan timing for mirrors  706  and  708 . For example, the scanner directed at the mirror  712  begins scanning at time t 1  halfway through the scan of the scanner directed at the mirror  708 . 
       FIG. 8C  provides a composite timing diagram  800  of the scanning in FIG.  7 . The resulting scanline screen images are generally parallel and evenly spaced.  FIG. 8D  shows a cycle of scanning for a laser, with a broken line  810  representing blank retrace and a solid line  812  representing scanline screen image formation. At the time halfway between t 2  and t 3 , the trace  808  has created ¼ of the current scanline screen image. A phase  814  of the trace  808  is therefore halfway between two hundred seventy and three hundred sixty degrees, or three hundred fifteen degrees. By similar logic, a phase  816  of the trace  804  is forty five degrees. Similarly, a phase  818  of the trace  802  is two hundred twenty five degrees, and a phase  820  of the trace  806  is one hundred thirty five degrees. The four phases  814 ,  816 ,  818  and  820  reside at four respective equidistant phase locations on the cycle, i.e., the four phases are ninety degrees out-of-phase. At any point of time, this temporal phase relationship is maintained, delivering generally parallel and equally spaced scanline screen images. Although two pairs of reciprocating mirrors are shown, more than two pairs can be implemented for even higher writing speeds. 
     An alternative scanner is an electro-optical holographic scanner  900  as shown in FIG.  9 . The electro-optical holographic scanner  900  includes a disk  902 , and a plurality of radially-situated electro-optical (E-O) readers  910  responsive to a corresponding number of sources of modulated laser light sources  901 . The disk  902  includes a hub  904  that is surrounded by a track area  906  with a number of concentric tracks  908 . The plurality of readers  910  radially override and traverse the disk  902  to meet at the center. For simplicity of demonstration, four readers are shown, although practical considerations of writing speed and size may require more readers. In general, if there are n readers  910 , there are n+1 tracks  908 , as will be explained below. Each concentric track  908  contains a plurality of spaced holographic gratings  912 . Each holographic grating  912  is a reflective hologram that focuses and directs light to the viewing screen  124 . Light from the modulated light source  901  enters its corresponding reader  910 . The reader  910  reflects the light so that the light diffracts through the holographic grating  912 . The reader  910  then outputs the light through a light exit port  914  to a part of the viewing screen  124  at which the holographic grating  912  aims. 
       FIG. 10  shows an exemplary E-O reader  910  for use in the E-O holographic scanner  900  of FIG.  9 . An E-O reader  910  includes a polarizing tracker  1002  interposed between a polarization rotating reflector  1004  and a polarization rotating sheet  1006 . The polarizing tracker  1002  includes a polarizing cube  1008  for each track  908 . The polarizing cubes  1008  are sandwiched between electro-optical (E-O) cells  1010 . At the central end of the polarizing tracker is a turning mirror  1012  and a light exit port  1014 . 
     At any given time, a given reader  910  will be assigned to a particular track  908 . The reader  910  needs to be able to switch tracks quickly as the track area  906  rotates. The E-O cells  1010  that sandwich a corresponding polarizing cube  1008  are electrically switchable between two states. A first state preserves the polarization of light that passes through. A second state rotates the polarization, such as from P (parallel) to S (perpendicular) or from S to P. By electrically adjusting the states of the E-O cells that sandwich a polarizing cube  1008 , the track  908  corresponding to the cube  1008  is instantly assigned to the reader  910 . 
     To demonstrate, it is assumed that the modulated light source  901 , shown in  FIG. 10 , projects a P polarized laser beam  1016 . The E-O cell  1010  is set to preserve the P polarization. Thus, the beam  1016  passing into the polarizing cube  1008  remains P polarized when incident upon a prism facet  1018  in the polarizing cube  1008 . The prism  1018  facet passes P polarized light. Therefore, the beam  1016  passes to the E-O cell  1020 . The E-O cell  1020  is set to rotate the polarization to S. A prism facet  1022  of a polarizing cube  1024  does not pass, but reflects, the beam  1016 , because the beam  1016  now has an S polarization. The reflected beam  1016  passes through the polarization rotating sheet  1006 , which is implemented as a quarter-wave sheet. The beam  1016  is then diffracted by the holographic grating  912  at some angle and passes back to the polarizing cube  1024 . Since the two passes through the quarter-wave sheet  1006  have rotated the light back to P, the beam  1016  passes through the prism facet  1022 . The beam  1016  continues on to the polarization rotating reflector  1004 , which consists of a mirror layer  1026  and a polarization rotating layer  1028 . The beam  1016  passes through the layer  1028 , which is implemented as a quarter-wave sheet, and returns to the polarizing cube  1024 . As it passes through the quarter-wave sheet  1028  on its return its polarization is again made to be S. The S-polarized beam deflects off the prism facet  1022  and toward the turning mirror  1012 . Upon encountering the following E-O cell  1030  the polarization of the beam  1016  is once again converted to P. Thus, it is thus allowed to propagate to the turning mirror  1012 , which reflects the beam  1016  out the light exit port  1014 . The beam  1016  is reflected by an orthogonal scanner  1040  toward the projection sheet  118 . 
     Illustrated in  FIG. 11A  is a reader assignment scheme  1100  to demonstrate the operation of the readers  910 . The holographic gratings  1102 ,  1104 ,  1106  and  1108  in the respective tracks  1112 ,  1114 ,  1116  and  1118  denote where four respective readers  910  are reading at a given point of time. The gratings  1102  and  1106  lie on an axis  1120 , and the gratings  1104  and  1108  lie on an axis  1122 . Therefore, two of the four readers lie on the axis  1120  and the other two readers lie on the axis  1122 . As the track area  906  revolves, the four readers shift tracks to arrive at positions along their respective axes. The readers shift tracks inward in unison at the completion of each revolution of the track area  906 , except that shifting from the inner track  1124  occurs to the outer track  1112 . Thus, in a five-track configuration, five revolutions complete a cycle. In each cycle, a scanline screen image  302  is written to the viewing screen  124  based on the readings of a respective reader  910 . The turning mirrors  1012  of the four readers  910  are positioned in four different respective orientations, i.e., tilted slightly differently, to create four corresponding scanline screen images  302 . The cycles of the respective readers are out-of-phase by 90 degrees. The temporal phase relationship among the readers is maintained as the track area  906  revolves. 
       FIG. 11B  is a timing diagram for scanline screen images  1150 ,  1152 ,  1154 ,  1156  and  1158 . At a time t 4 , the reader reading the grating  1102  is beginning the scanline screen image  1158 , and has just switched tracks after having completed the scanline screen image  1150 . The reader reading the grating  1108  has completed ¾ of the scanline screen image  1152 . The reader reading the grating  1106  has completed ½ of the scanline screen image  1154 . Finally, the reader reading the grating  1104  has completed ¼ of the scanline screen image  1156 . The state of scanline screen image completions are therefore 90 degrees out-of-phase. The temporal phase relationship of scanline screen image completions corresponds to the temporal phase relationship among the readers, and these relationships remain constant as the track area  906  revolves. 
     To yield high-resolution images requires that a scanline screen image of 2000 screen dots, corresponding to 2000 holographic gratings, be drawn in about 20 microseconds. This is the equivalent of tracing through a two meter track of holographic gratings at an effective linear rate of 95 kM/sec, which is unacceptable. A circle two meters in circumference has a diameter of 0.64 meters. A ten track system, with nine readers, reduces the size of the circular track to 64 mm or about 2.5 inches, and provides high writing speed. Moreover, no blank retrace exists, and thus there is no negative impact on the duty cycle caused by the reset time of a scanning mechanism. 
     Another alternative scanner is a tiling, holographic scanner  1200 , shown conceptually in  FIG. 12A. A  tiling, holographic scanner  1200  includes a laser array  1202  and a disk  1204  (shown in cross-section). A laser beam  1206  from a laser  1208  is reflected by an orthogonal scanner to a viewing screen. The laser beam  1206  traverses an angular range  1201  to create a scanline screen image segment  1203  on the viewing screen  124 . 
       FIG. 12B  shows a top view of a section  1201  of the disk  1204 . The disk  1204  has a plurality of concentric tracks. Shown are two tracks,  1212  and  1214 . Each contains a plurality of holographic gratings implemented as transmitting holograms. For example, a grating  1216  resides in the track  1212 , and a grating  1218  resides in the track  1214 . In  FIG. 12B , the lasers  1208  of the laser array  1202  are represented by broken lines. In the present implementation, each concentric track has  200  spaced holographic gratings. 
     In operation, with each revolution of the disk  1204 , each laser  1208  creates in unison, by reflection from the orthogonal scanner  1204 , a scanline screen image segment  1203 , so that the scanline screen image segments  1203  collectively form a scanline screen image. The orthogonal scanner  1204  forms the scanline screen images so that they are generally equally spaced and parallel. As in the previous embodiment, no blank retrace exists. 
     The lasers are generally spaced apart by a distance approximately equal to the length of a scanline screen image segment, but this need not be a restriction. Overlap and feathering of the scanline screen image segments  1203  can be employed to improve the images. A longer viewing screen can be covered by tiling together more lasers to form a two-dimensional array of lasers. This tiling approach is also amenable to mirror implementation. 
       FIG. 13  illustrates a hand-held unit with a beam index screen. A bus  1302  attaches to the beam index screen  124 , and extends through the front pulley  126 , down an arm of the front telescoping arm pair  114  and into the image formation module  104 . The retractable nature of the projection sheet  118  requires that precise alignment be available, when, for example, the unit is employed for command and control or military applications. For instance, for a colored viewing screen, red, blue and green phosphors may be employed. Fresnel elements on one side of the screen allow the incoming beam to impinge normally on the phosphors. Included in the phosphor layer would be black matrix areas or indexing stripes. Black matrix areas could be amorphous silicon solar cell material. When a beam  130  arrives at the beam index screen  124  and strikes, for example, an indexing stripe, a feedback signal is sent along bus  1302  to the image formation module  104 . Based on this feedback signal, the image formation module  104  aligns the beam  130 . Retraction of the front telescoping arm pair  114  allows the bus  1302  to collect slackly within the housing  102 . 
       FIG. 14  shows another embodiment of a hand-held unit with a retractable screen. This embodiment differs from the hand-held unit  100  in  FIG. 1  in that the projection sheet  118  is anchored not at the front pulley  126 , but at anchor  1402 . In addition, a take-up roller  1404  is substituted for the internal pulley  112 . Retraction of the front telescoping arm pair  114  causes the viewing screen  124  to roll up on the take-up roller  1404 . 
     A retractable screen, in accordance with the principles of the invention, can be made for units that are larger than hand-held devices. One application is roll away systems for both the home and commercial situations. The screen can roll away into a base such as a rear projection TV or a ceiling mount for home or military or electronic cinema applications. Thus, a large screen TV can be made both transportable and non-invasive upon living space, when not in use. In home theater applications, as in the other large image applications, a conventional image projector can be used as well as a scanner. 
     The foregoing descriptions are exemplary only, and are not intended to limit the scope of the invention. Modifications to these exemplary embodiments and substitutions for components in the exemplary embodiments may be apparent to one having ordinary skill in the art.