Thin illuminator for reflective displays

A display that includes an array of reflective pixels, a linear light source; and a reflector. The reflector includes a cylindrical surface, the axis of the cylindrical surface being parallel to the linear light source. The linear light source is positioned relative to the reflector such that light from the linear light source is reflected by the reflector onto the array of reflective pixels. The reflector is constructed from a material that is partially reflecting. The linear light source preferably includes a plurality of light emitting diodes and an optical diffuser. In a color display, the light emitting diodes include diodes having different emission spectra. In one embodiment of the invention, the reflector is constructed from a material that reflects light of a first linear polarization while transmitting light having a linear polarization orthogonal to the first linear polarization. In this embodiment, each pixel in the array of reflective pixels preferably includes a polarization rotating cell that rotates the linear polarization vector of light reflected by the pixel in response to the receipt of an electrical signal by the pixel.

FIELD OF THE INVENTION
 The present invention relates to display systems, and more particularly, to
 the illumination of display systems in which a plurality of pixels
 generate an image by reflecting light from one or more light sources.
 BACKGROUND OF THE INVENTION
 simplify the following discussion, the present invention will be discussed
 in terms of displays utilized in head mounted computer displays; however,
 it will be apparent to those skilled in the art from the following
 discussion that the present invention may be applied to other types of
 displays. Head-mounted computer displays may be viewed as "eye glasses"
 that are worn by the user to view images created by a computer or other
 video source. The image seen by each eye is generated on a display screen
 having a two dimensional array of pixels.
 In one type of display, each pixel is a small mirror that is covered by a
 "shutter" that is controlled by the voltage of the mirror. The shutter is
 constructed from a layer of liquid crystal on the mirrors. The voltage
 controls the state of the liquid crystal on top of the pixel so as to
 modulate the reflected light. A light source illuminates the pixels and
 the modulated reflected light from the pixels is imaged into the eye of
 the viewer. The imaging optics typically consist of lenses that magnify
 the pixels and form a virtual image. The light source is typically
 constructed from 3 LEDs that emit different colors.
 For this type of display to function properly, the intensity of light
 reflected by each micro-mirror must be independent of the pixel's location
 in the display. In addition, each pixel must appear to be an independent
 light source. The illumination must be both spatially and angularly
 uniform, with the angular extent given by the acceptance angle (f-number)
 of the imaging optics. In prior art systems these constraints are met by
 converting the three point light sources into a diffuse light beam that
 strikes the display at right angles to the plane of the mirrors. The light
 source utilizes a condenser lens to collimate or slightly diverge the
 light to match the telecentricity of the imaging optic and an array of
 micro-lenses or a diffuser in the collimated light beam to provide the
 required diffusion. Since the light source must be outside the field of
 view of the user so as not to block the image generated by the display, a
 half silvered mirror is used to illuminate the display while allowing
 light reflected by the display to reach the eye of the viewer.
 This prior art solution to the illumination problem has several problems.
 First, the distance between the first imaging optic and the display must
 be at least as great as the shortest dimension of the display to provide
 room for the half-silvered mirror. Second, the illuminator requires a
 condenser lens and diffuser which must be at least as large as the
 display. These constraints lead to a bulky display. Both the size and the
 weight of this type of display are objectionable.
 To collimate the light source, all of the LEDs must be very close to the
 focal point of the collimating lens and limited in size so as to simulate
 a single point source and properly mix the colors of the LEDs. This
 constraint limits the size of the LEDs, and hence, the maximum intensity
 of light from the display. In addition, the half-silvered mirror decreases
 the brightness of the display, since only one fourth of the light in the
 collimated beam actually reaches the viewer's eye.
 Broadly, it is the object of the present invention to provide an improved
 illumination system for a reflective display.
 It is a further object of the present invention to provide a display system
 that does not require the use of a half-silvered mirror to illuminate the
 pixels.
 These and other objects of the present invention will become apparent to
 those skilled in the art from the following detailed description of the
 invention and the accompanying drawings.
 SUMMARY OF THE INVENTION
 The present invention is a display that includes an array of reflective
 pixels, a linear light source; and a reflector. The reflector includes a
 cylindrical surface preferably having a parabolic cross-section, the axis
 of the cylindrical surface being parallel to the linear light source. The
 linear light source is positioned relative to the reflector such that
 light from the linear light source is collimated by the reflector onto the
 array of reflective pixels. The reflector is constructed from a material
 that is partially reflecting. The linear light source preferably includes
 a plurality of light emitting diodes and an optical diffuser. In a color
 display, the light emitting diodes comprise diodes having different
 emission spectra. In one embodiment of the invention, the reflector is
 constructed from a material that reflects light of a first linear
 polarization while transmitting light having a linear polarization
 orthogonal to the first linear polarization. In this embodiment, each
 pixel in the array of reflective pixels preferably includes a polarization
 rotating cell that rotates the linear polarization vector of light
 reflected by the pixel in response to the receipt of an electrical signal
 by the pixel.

DETAILED DESCRIPTION ON THE INVENTION
 The present invention may be more easily understood with reference to FIG.
 1 which is a cross-sectional view of the prior art display system 10
 discussed above. A display screen 12 is illuminated by a light source
 consisting of a LED 15 close to the focal point of a Fresnel lens 14.
 Fresnel lens 14 provides either a collimated light source or a slightly
 diverging light source that matches the telecentricity of the imaging
 optic. The light leaving Fresnel lens 14 is diffused by a diffuser or
 micro-lens array 13 as shown at 18. The light from the source is reflected
 from a half-silvered mirror 16 onto display 12. The light reflected back
 by display 12 is imaged by lens 17 into the eye 11 of the user. It should
 be noted that, at most, half of the light leaving diffuser 13 reaches
 display 12, since mirror 16 allows half of the light to pass through the
 mirror. Similarly, only half of the light leaving display 12 reaches lens
 17 for the same reason. It should also be noted that the minimum values
 for the width and height of the display system are set by the illumination
 optics. As noted above, such systems are bulky and have limitations on the
 maximum light intensity that can be delivered to the eye of the viewer.
 Refer now to FIGS. 2 and 3, which are side and top views of a display
 system 100 according to the present invention. In display system 100, the
 half-silvered mirror utilized in prior art systems is replaced by
 cylindrical parabolic reflector 102. FIG. 2 is a side view of display
 system 100 in a direction parallel to the axis of reflector 102. FIG. 3 is
 a top view of display system 100. Reflector 102 provides both the
 functions of the condenser and the partially reflecting mirror. Reflector
 102 is illuminated with a diffuse line source 104, which is preferably
 constructed from a diffuser 105 and a plurality of LEDs 106.
 Refer now to FIG. 4 which is a cross-sectional view of a display system
 according to the present invention. Display system 300 includes a display
 307, a reflector 301, and a diffuse light source 302. To provide the
 angular spread of the illumination required to fill the acceptance angle
 of the imaging optics, the light source needs to have vertical spatial
 extent. In the case of a telecentric system, reflector 301 is parabolic.
 In non-telecentric systems, reflector 301 is typically a hyperbolic or
 ellipsoidal surface. The parabolic surface converts this spatially
 extended source 302 to an angular cone of light having an opening angle
 306 and angle 305 with respect to the display surface. In telecentric
 systems, angle 305 is 90.degree.. The focal point 303 of reflector 301 is
 in the middle of source 302. The cone angle in the orthogonal direction is
 provided by the diffuser on the source, in a manner analogous to the
 micro-lenses discussed with reference to the prior art system shown in
 FIG. 1. If the imaging optics are not telecentric, the cross-section of
 the cylindrical surface can be made elliptical or hyperbolic, so that the
 chief rays match those of the imaging optics. The telecentricity in the
 other direction can not be matched geometrically, but the diffusion of the
 source in this direction provides the necessary rays.
 It should be noted that the distance, D, required to accommodate reflector
 102 is approximately half the distance required for the partially
 reflecting mirror utilized in the above-described prior art display
 systems. Hence, the present invention has substantially less bulk and
 weight than prior art displays. Further, the present invention utilizes a
 plurality of LEDs. Hence, the present invention provides substantially
 higher illumination of the display.
 In a color display according to the present invention, the light source
 includes a plurality of LEDs for each color of light. Typically, three
 different colors are utilized to construct the color image. The color
 image is constructed by sequentially displaying the red, blue, and green
 images in a time-span that is shorter than the time interval in which the
 eye can resolve separate images. The various color LEDs are positioned
 along the axis of the light source such that the light source is
 effectively three linear light sources that are superimposed on one
 another.
 As noted above, one problem with prior art displays results from the use of
 a partially reflecting mirror, which reduces the effective illumination by
 75%. The preferred embodiment of the present invention utilizes a material
 for the construction of the parabolic reflector that overcomes this
 problem when utilized with a display that operates by rotating the
 polarization of the incident light. The manner in which this aspect of the
 present invention operates may be more easily understood with reference to
 FIG. 5 which illustrates the manner in which a typical prior art
 reflective display operates. To simplify the drawing, only one pixel of
 the display is shown. Pixel 200 consists of a polarization filter 201
 which selects one linear polarization component of the incident light
 which may be viewed as consisting of two equal intensity linearly
 polarized components as shown at 210. In the case shown in FIG. 5, it is
 assumed that the vertical component is passed by filter 201. The light
 passing through filter 201 is reflected by a reflective coating 203 on the
 back of a liquid crystal element 202. This coating also acts as an
 electrode for applying a voltage across the liquid crystal element. The
 light exiting the liquid element will have a polarization that is either
 vertical or horizontal depending on the potential across the liquid
 crystal element. If the exiting light has a polarization that has been
 rotated to the horizontal direction as shown at 211, the light will be
 blocked by the polarization filter, and hence, the pixel will appear
 black. If the direction of polarization remains vertical, the light will
 pass through filter 201, and the pixel will be bright.
 The reflected light must still pass back through the half-silvered mirror
 216 in prior art displays. Hence, the maximum light intensity relative to
 the source intensity is 1/8.sup.th, since one half of the light is lost in
 the first reflection that directs the light onto the display. Another 50%
 of the light intensity is lost in polarization filter 201. Finally, yet
 another 50% of the remaining light is lost passing back through half
 silvered mirror 216.
 The present invention combines the polarization function of filter 201
 utilized in prior art displays with the parabolic condenser lens. As a
 result, the effective light intensity reaching the viewer is one half of
 the source intensity. The manner in which this is accomplished may be more
 easily understood with reference to FIG. 6 which is an expanded view of a
 pixel according to the present invention. Light from source 306 is
 directed toward parabolic reflector 322. The light is assumed to be
 unpolarized, and hence, consists of equal intensities of vertical and
 horizontally polarized light as shown at 310. Reflector 322 is constructed
 from a material that reflects light of one polarization while transmitting
 light of the orthogonal polarization. Such materials are known to the art.
 For example 3M markets such a material under the trade name DUAL
 BRIGHTNESS ENHANCEMENT FILM (DBEF). For the purposes of this discussion,
 it will be assumed that reflector 322 has been constructed such that
 vertically polarized light is reflected and horizontally polarized light
 is reflected. Hence, the light from source 306 that is vertically
 polarized is reflected toward the pixel as shown at 323 while the
 horizontally polarized component passes through reflector 322 as shown at
 324.
 The vertically polarized light goes on to strike the reflective surface 203
 of the pixel after passing through the liquid crystal element 202. If the
 potential across the liquid crystal element is set such that the direction
 of polarization is rotated through 90degrees as shown at 325, the
 reflected light will pass through reflector 322 and reach the eye of the
 viewer. In this case, the pixel will appear bright. If, however, the
 voltage across the liquid crystal element is such that the direction of
 polarization is not rotated, the light reflected by the pixel will also be
 reflected by reflector 322 back toward the light source 306. In this case,
 the pixel will appear dark.
 It should be noted that the light passing through the reflector upon
 reflection by the pixel does not suffer any attenuation. That is, the
 reflector appears transparent to that light. Accordingly, the only light
 loss due to reflector 322 is the initial 50 percent loss associated with
 the separation of the unpolarized light from source 306 into vertical and
 horizontal components, i.e., the loss of the light shown at 324. Hence,
 the present invention has 4 times the efficiency of prior art displays.
 Various modifications to the present invention will become apparent to
 those skilled in the art from the foregoing description and accompanying
 drawings. Accordingly, the present invention is to be limited solely by
 the scope of the following claims.