Abstract:
A display illumination and viewing system has an illumination optical path and a viewing optical path coinciding along a portion of their lengths. A display is located at one end of the coinciding path portion. A first lens system is located on the coinciding path portion and a second lens system is located on the viewing optical path. An illumination assembly is located on the illumination optical path and off the coinciding path portion. The illumination assembly is spaced from the first lens system by a distance corresponding to a focal length of the first lens system. A reflective and transmissive element is located at an opposite end of the coinciding path portion to reflect light from the illumination assembly onto the coinciding path portion toward the display and to transmit light from the display along the viewing optical path. In another aspect of the invention, the image display system is operable in a color mode and a monochrome mode. Illumination circuitry is in communication with an illumination source and includes a switch operative to switch the illumination source between the color mode to provide a color display and the monochrome mode to provide a monochrome display.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit under 35 U.S.C. §112(e) of U.S. Provisional Application No. 60/140,327, filed Jun. 21, 1999, the entire disclosure of which is incorporated herein by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     The development of this invention was supported by R&amp;D Contract DAAN02-98-C-4026. The U.S. government may have certain rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     Miniature active matrix liquid crystal displays (AMLCD) require an illumination source. In systems using transmissive AMLCDs, a light source is placed behind the display. FIG. 1 shows a simple prior art approach in which a transmissive AMLCD  10  is provided with an light emitting diode (LED) backlight  20 . Rays from the backlight  30  propagate through the AMLCD and are modulated to produce an image. In some AMLCDs, color is obtained by sequentially loading red, green, and blue subframes into the AMLCD, and by simultaneously sequentially illuminating red, green, and blue LEDs. Sequential illumination is accomplished by providing current sequentially through one of the desired LED leads  40 . A beam shaping element  50 , such as a Fresnel lens, may be used to collimate the light. Other elements, such as diffusers or filters, may also be employed. AMLCDs and illuminators of this type are available commercially from Kopin Corporation. The viewing system for such displays may comprise simple optical magnifier optics, or a multi-stage optical system characterized by intermediate image planes between the stages. 
     Prior art miniature reflective AMLCDs (FIG. 2) use an illuminating system that is based on a beam splitter cube  70  adjacent to the display  60 . The beam splitter may comprise a polarization splitting coating  71  which serves to linearly polarize the illuminating light, and which also acts as the analyzer for the LCD. The polarizing beam splitter may alternatively be formed from polymer films. As with the transmissive AMLCD, optical elements  50  may be used to collimate, diffuse or filter the illumination. 
     FIG. 3 shows a more complex prior art reflective AMLCD system that includes a compact, simple magnifier added to the illuminator system for viewing a magnified image of the display  60  (U.S. Pat, No. 5,596,451). In this prior art device, a compact system is formed by using a single beam splitter  71  for illuminating and viewing the image from the AMLCD  60 . Although the beam splitter  71  is used for illumination and for viewing, mirror  42 , which provides the magnification, is not employed in the illumination system optical path. A lens or mirror to affect vergence of the illumination light is not needed because the light source  34  in this design is a broad area emitter. 
     For the case of reflective AMCLDs, projection systems have been described that employ efficient illuminators, based on lamps and collimating optics. Collimating optics provide efficient, uniform illumination of the reflective display (see, for example, U.S. Pat. No. 6,036,318). Collimation systems of this type are not employed in head-mounted displays owing to high weight and volume that results from the additional lenses and path length needed. 
     Collimating illumination optics are generally used in projection systems that employ high intensity lamps and projection lens systems, such as disclosed in U.S. Pat. No. 5,949,503. In some cases, such as in the patent cited, a portion of the projection optics may be used for illumination. For projection systems, this approach leads to improved illumination uniformity and improved contrast in the projected image. 
     SUMMARY OF THE INVENTION 
     This invention relates to the attainment of an improved illuminating system for reflective liquid crystal displays. The improvement is based on integrating the illumination system with the magnifying system and thus using a single set of optical elements for the two purposes of magnifying the image and illuminating the display. The invention also relates to a system for obtaining high brightness monochrome images which may be applied to reflective or transmissive liquid crystal displays. 
     More particularly, the invention provides a display illumination and viewing system comprising an illumination optical path and a viewing optical path. At least a portion of the illumination optical path coincides with at least a portion of the viewing optical path to form a coinciding path portion. A display comprising an active matrix liquid crystal display is located at one end of the coinciding path portion. A first lens system is located on the coinciding path portion and provides an image plane on the viewing optical path. A second lens system is located on the viewing optical path. 
     An illumination assembly, such as red, green, and blue LEDs, is located on the illumination optical path and off the coinciding path portion. The illumination assembly is spaced from the first lens system by a distance corresponding to the focal length of the first lens system. A reflective and transmissive element, such as a beam splitter, is located at an opposite end of the coinciding path portion to reflect light from the illumination assembly onto the coinciding path portion toward the display and to transmit light from the display along the viewing optical path. In this manner, the present invention provides a collimating illumination system for a head-mounted reflective AMLCD, offering uniform and efficient illumination, with less weight and volume than prior art systems. 
     In another aspect of the invention, the image display system is operable in a color mode and a monochrome mode. The display system comprises an active matrix liquid crystal display operable at a determined frame rate comprising sequential loading of red, green, and blue subframes. An illumination source comprising red, green, and blue light sources, such as LEDs, is disposed to illuminate the active matrix liquid crystal display. Illumination circuitry is provided in communication with the illumination source and includes a switch operative to switch the illumination source between the color mode to provide a color display and the monochrome mode to provide a monochrome display. In this manner, the present invention obtains increased brightness by providing the ability to switch the illuminator to a monochrome mode. In a further aspect, the invention also provides for adjusting illuminator brightness. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The invention will be more fully understood from the following detailed description taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a schematic illustration of a prior art transmissive active matrix liquid crystal display illumination system; 
     FIG. 2 is a schematic illustration of a prior art reflective active matrix liquid crystal display illumination system; 
     FIG. 3 is a schematic illustration of a prior art reflective active matrix liquid crystal display illumination system with magnifier; 
     FIG. 4 is a display system incorporating an illumination system and a viewing system; 
     FIGS. 5 a  and  5   b  are schematic illustrations of the optical principals of a display system according to the present invention; 
     FIG. 6 is a schematic illustration of an optical design of the display system of FIGS. 5 a  and  5   b;    
     FIG. 7 is a schematic illustration of a display system according to the present invention in use with an eyeglass display; 
     FIG. 8 is a block diagram and circuitry of an illumination control system according to the present invention; 
     FIG. 9 is a block diagram and circuitry of a further embodiment of an illumination control system according to the present invention; 
     FIG. 10 is a block diagram and circuitry of a still further embodiment of an illumination control system according to the present invention; and 
     FIG. 11 is a block diagram and circuitry of a still further embodiment of an illumination control system according to the present invention. 
    
    
     DESCRIPTION OF THE INVENTION 
     This invention comprises the integration of the viewing optics and the illumination optics in a single or multi-stage optical system. By integration we mean that some of the optical elements affecting vergence of rays that are used for the viewing optics also serve to collimate the light in the illumination system, thereby lowering cost, weight and volume. 
     By way of preliminary explanation of the present invention, FIG. 4 illustrates a display system based on using the prior art collimation system shown in FIG.  2 . An illumination system  22  is coupled with a viewing system  23  to provide an image to a viewer. The illumination system comprises red  53 , green  54 , and blue  55  LEDs, an optional diffuser  49 , a Fresnel lens  50 , and a polarization beam splitter  71 . Illumination from the LEDs passes through the diffuser if employed, and is collimated by the Fresnel lens  50  to uniformly illuminate the AMLCD  60 . Light of the reflected polarization is directed to the AMLCD by the beam splitter  71 . The AMLCD rotates the polarization of the light at each of its pixels to an angle in accordance with electrical signals representing the image. Rays pass from the illumination stage  22  to the magnification stage  23 , and are then viewed through the lenses  150  and  160 . Any number of optical surfaces may be used to magnify the image and to correct aberrations; for simplicity we have represented the lens surfaces by the singlet lenses  150  and  160 . In practice, these lenses  150 ,  160  may each be achromatic doublet or triplet lenses, aspheres, or more complex combinations of surfaces. 
     FIGS. 5 a  and  5   b  illustrate the optical principal of this invention. A reflective AMLCD  60  is viewed through a lens system that has two stages of magnification, represented by lens system  150  and lens system  160 . A viewing path is defined from the AMLCD  60  through the lens system  150  and lens system  160  to a user&#39;s eye. Each lens system is characterized by a focal length, f. Referring to FIG. 5 a , in accordance with Newton&#39;s lens equation (N·P=f 1 ·f 2 ), lens system  150  forms an intermediate image plane as shown between  150  and  160 ; the position of this plane is given by Newton&#39;s lens equation or its equivalents, which are well known in the art. If lens system  160  is placed at a distance from the image plane equal to its focal length, the user perceives an image at infinity. The position of lens  160  may be varied to change the distance of the virtual image. The magnification of the system is given by the product of the magnification of the two stages, as is well known in the art. 
     FIGS. 5 a  and  5   b  also show the insertion of a reflective and transmissive element, for example, a beam splitter  200  in between lens systems  150  and  160 . The beam splitter may be a polarization beam splitter made by vacuum deposition of thin film multilayers as is known in the art, or made by polymer techniques (such products are offered by 3M for example), or it may be a vacuum-deposited thin metal film with approximately 50% transmission (a half silvered mirror). Alternatively, a polarization beam splitting cube may be used. Referring to FIG. 5 b , it can be seen that the purpose of the beam splitter  200  is to reflect light from an illumination source, such as LED lamp  100 , which may be a multi-color lamp comprising red, green and blue LEDs used in field sequential color illuminators, into the optical path. The path between the LED lamp  100  and the AMLCD  60  defines an illumination path. It can be seen that the illumination path coincides with a portion of the viewing path. If the LED is placed at a distance from lens system  150  equal to the focal length f 2  of lens system  150  (shown as the distance a+b in FIG. 5 b ), then  150  acts to collimate the light and thus improves the uniformity of the illumination on the AMLCD  60 . Diffusers and other optical elements may be used between the LED and the beam splitter to homogenize the light incident on the AMLCD or to develop an extended light source in accordance with the viewing requirements of the complete system. 
     FIG. 6 illustrates an example of an optical design based on this principal. The lens surfaces  150 ,  160  are aspheres known in the art and represented by the equation:          z        :       =         cr   2       1   +       1   -       (     1   +   k     )     ·     c   2     ·     r   2               +       α   2     ·     r   4       +       α   3     ·     r   6       +       α   4     ·     r   8                                
     with coefficients as given in Table 1. Table 2 summarizes the optical prescription for the imaging path (between display  60  and the eye pupil), and Table 3 summarizes the prescription for the illumination path (between lamp  100  and the display  60 ). 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Asphere Coefficients 
               
             
          
           
               
                   
                 Coefficient 
                 Value 
               
               
                   
                   
               
               
                   
                 κ 
                 0 
               
               
                   
                 α 2   
                 −0.1304 × 10 −2  mm −3   
               
               
                   
                 α 3   
                   7.2502 × 10 −6  mm −5   
               
               
                   
                 α 4   
                 −8.3167 × 10 −7  mm −7   
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Optical Prescription for the Imaging Path 
               
             
          
           
               
                   
                 Radius 
                   
                 Thickness 
                   
               
               
                 Description 
                 (mm) 
                 Material 
                 (mm) 
                 Notes 
               
               
                   
               
             
          
           
               
                 Eye Pupil 
                 Infinity 
                 Air 
                 25 
                 Eye Relief 
               
               
                 Eye Lens 1st Surf 
                 13.2 
                 BK7 
                 5 
                 Planoconvex Lens 
               
               
                 Eye Lens 2nd 
                 Infinity 
                 BK7 
                 6 
                 Bonded to Prism 
               
               
                 Surf 
               
               
                 Mirror Surf 
                 Infinity 
                 BK7 
                 25.44 
               
               
                 Internal Image 
                 Infinity 
                 BK7 
                 25 
                 End of Prism 
               
               
                 Lens Air Space 
                 Infinity 
                 Air 
                 2 
                 Air Space 
               
               
                 Relay Lens 1st 
                 5.9 
                 BK7 
                 4 
                 CV, Lens could be 
               
               
                 Surface (Dual 
                   
                   
                   
                 Plastic 
               
               
                 Use see Table 3) 
               
               
                 Relay Lens 2nd 
                 147.0562 
                 Air 
                 25.98 
                 CC, Asphere as per 
               
               
                 Surface 
                 at Vertex 
                   
                   
                 Table 1 
               
               
                   
                 c=1/R 
               
               
                 Display 
                 Infinity 
                   
                   
                 Cover glass omitted 
               
               
                   
               
               
                 Note: User image at 609 mm.  
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Optical Prescription for the Illumination Path 
               
             
          
           
               
                   
                 Radius 
                   
                 Thickness 
                   
               
               
                 Description 
                 (mm) 
                 Material 
                 (mm) 
                 Notes 
               
               
                   
               
             
          
           
               
                 Illuminator and 
                 Infinity 
                 BK7 
                 0.6 
                 Mount to Prism 
               
               
                 Air Space 
               
               
                 Partial 
                 Infinity 
                 BK7 
                 6 
                 Polarization Beam 
               
               
                 Reflection Surf 
                   
                   
                   
                 Splitter 
               
               
                 Internal 
                 Infinity 
                 BK7 
                 6 
                 To End of Prism 
               
               
                 Reflection 
               
               
                 Lens Air Space 
                 Infinity 
                 Air 
                 2 
                 Air Space 
               
               
                 Collimator Lens 
                 5.9 
                 BK7 
                 4 
                 CV, Lens could be 
               
               
                 1st Surf. (Dual 
                   
                   
                   
                 Plastic 
               
               
                 Use see Table 2) 
               
               
                 Collimator Lens 
                 147.0562 
                 Air 
                 25.98 
                 CC, Asphere, Table 
               
               
                 2nd Surf. 
                 at Vertex 
                   
                   
                 1 
               
               
                   
                 c=1/R 
               
               
                 Display 
                 Inf 
                   
                   
                 Cover glass omitted 
               
               
                   
               
             
          
         
       
     
     In these designs, reduced weight is attained by eliminating the need for a separate lens for collimating light from the LEDs. Referring to FIG. 4, it can be seen that by eliminating the collimating lens  50  (shown in FIG.  4 ), and the required distances associated with its focal length, a reduction in size and weight is attained. 
     FIG. 7 illustrates how the invention can be used in an eyeglass display of the type described in U.S. Pat. No. 5,886,822. A housing  170  for a reflective display  60  and magnifying lens system  150  is placed in proximity to an eyeglass lens  210 . The display  60  is illuminated by a light source  100 . Lens system  150  projects an image plane at the entrance pupil of the optical system in lens  210 . Lens  210 , with a magnifying internal optical relay formed by lens  220 , polarization beam splitter  240 , quarter wave plate  260 , and concave mirror  250  provide an image to the eye of the user which is perceived as originating in front of the lens  210  at a comfortable distance. 
     In the foregoing discussion, we have referred to the image source as an LED or array of LEDs, since LEDs are a common light source in these displays. Many alternative illumination sources may be used, including lasers, an optical fiber delivering light from a remote source, or other lamps. 
     The use of sequential red, green, blue illumination in an eyeglass system of the type shown in FIG. 7 provides an image of excellent color. Sequential lighting of the LEDs however, reduces the duty cycle of the LEDs and therefore reduces the total amount of light provided to the eye. In systems of the type shown in FIG. 7, in which a beam splitter  240  is used as a combiner to fuse the ambient scene and the rays from the display, the viewer may require an unusually bright image, such as when in bright ambient sunlight. 
     Many applications such as the eyewear display previously cited require a color display for certain images, and a high brightness display for other images viewed in high ambient lighting, which may not need color. To make the sequential color display very bright, this invention also includes a method for implementing a high brightness monochrome mode, which involves powering the red, green, and blue LEDs simultaneously at up to 100% duty cycle. 
     In one embodiment of this invention shown in FIG. 8, the LEDs  301 ,  302 ,  303  are turned on continuously without any change to the frame rate of the AMLCD using switch  300 . Continuous and simultaneous illumination of all three LEDs results in the portrayal of the image as black and white, and with the white being much brighter than any color that would be developed from sequential LED flashes of red, green, and blue. 
     Simultaneous illumination can be attained by employing a switch  300 , as shown in FIG. 8 applied to the illuminator power circuit. In one position of the switch, the LEDs are connected to the sequential color drive circuit in the backlight controller. In the other position, the LEDs are connected to a current source. FIG. 8 shows the current source comprising VDD, the LEDs  301 ,  302 ,  303 , current-limiting series resistors  311 ,  312 ,  313 , and ground; however, other circuits may be used to provide current to the LEDs. The switch  300  provides current to the LEDs continuously and simultaneously, so that at least triple the duty cycle for each LED is obtained. The technique may be applied to transmissive AMLCD illuminators (FIG. 1) or to reflective AMLCD illuminators (FIG.  2 ). 
     An alternative embodiment is shown in FIG. 9. A switch  350  is used to provide a logic input to the display controller  330  to control the LEDs. The display controller  330  supplies illumination signals along paths  319  to the LEDs through OR gates  304 ,  305  and  306 . Logic output from the OR gates is applied to the gates of the control transistors  307 ,  308 ,  309  which enables current to flow through the LEDs in correspondence with the logic signal supplied on the paths  319 . The switch  350  is used to select the operating mode of the illuminator. In the left position (ground), the inputs of the OR gates are held to ground, allowing the control signals on paths  319  to have full authority over the control of the LEDs. In the center position, the OR gate inputs  321 ,  322 , and  323  are held at VDD meaning that the output of the OR gates is held high, and the LEDs are consequently continuously illuminated, regardless of the signals on paths  319 . If switch  350  is in the far right position, the lines  321 ,  322 ,  323  are held at a value established by the logic within the display controller, or by an alternative logic path (not shown) from another control circuit. 
     A third embodiment (FIG. 10) comprises a switch  310  that also provides a logic signal path  320  to the AMLCD display controller. The logic signal on line  320  signifies the selection by the user of the high brightness monochrome mode. This signal enables the controller logic to reduce power consumption in memory and elsewhere and to adjust the signal to use the optimal balance of red, green, and blue information from which to construct a monochrome image. 
     A further improvement to this invention comprises a circuit that allows the microprocessor to select the background color. Ordinarily, by running red, green, and blue LEDs at full brightness, one obtains a high brightness black and white display, as previously described. By controlling the balance of current among the three LEDs, any backlight color may be attained. Black and white may be used as the primary high brightness illumination, but if the application running on the computer so selects, the background color can be switched to, for example, red to indicate a warning in black and red. This is obtained by powering only the red LED. 
     Note that the LEDs do not have to be illuminated continuously in monochrome mode, or for their full duty cycle in color mode. The logic devices described above can be used to provide reduced duty cycles to affect reduced brightness in either monochrome or color mode. FIG. 12 shows a diagram of how such brightness control can be attained. The signal to illuminate one of the LEDs originates at the display controller  330 . A pulse is provided simultaneously to a one shot multivibrator  400  by path  451 . The signal to illuminate LED  303 , for example, passes from the display controller  330  through the OR gate  306  and is passed to an AND gate  403 . The AND gate passes the illumination signal only for the time that the pulse  420  is present. This pulse  420  is initiated by multivibrator  400  upon receipt of the initiating pulse from line  451 . The duration of the pulse is controlled by the setting of the potentiometer  410 , under the control of the user of the system. If the pulse width time is t, the corresponding AND gate is held open for a corresponding time, t, and the LED is illuminated for the time t. Thus, the width of the pulse from multivibrator  400  exerts control over the duty cycle of the LEDs and hence brightness. FIG. 11 shows that the signal from the AND gate passes through a series resistor  413  which controls the current through the matched pair of field effect transistors  460 . Note that the pulse width, t, may also be controlled by logic signals that can be applied by the display controller, or that can be applied to the one-shot multivibrator through an additional logic path (not shown). 
     The illumination circuits shown in FIGS. 8 through 12 may be implemented in discrete logic devices, in a programmable logic device, or in a custom integrated circuit. Alternatively, the circuits may also be integrated within the display controller. The circuits may be configured to control alternative illumination sources such as laser diodes. 
     The invention is not to be limited by what has been particularly shown and described, except as indicated by the appended claims.