Information
-
Patent Grant
-
6295039
-
Patent Number
6,295,039
-
Date Filed
Tuesday, August 25, 199826 years ago
-
Date Issued
Tuesday, September 25, 200123 years ago
-
Inventors
-
Original Assignees
-
Examiners
- Brier; Jeffery
- Blackman; Anthony J.
-
CPC
-
US Classifications
Field of Search
US
- 353 20
- 353 8
- 353 99
- 345 32
- 345 82
- 345 7
- 345 8
- 359 495
- 359 500
- 359 853
-
International Classifications
- G02B510
- G03B2114
- G09G500
- G09G300
-
Abstract
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.
Description
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.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1
is a cross-sectional view of a prior art display system.
FIG. 2
is a side view of a display system according to the present invention.
FIG. 3
is a top view of the display shown in FIG.
2
.
FIG. 4
is a cross-sectional view of a display system according to the present invention.
FIG. 5
illustrates the manner in which a typical prior art reflective display operates.
FIG. 6
is an expanded view of a reflective pixel according to the present invention illustrating the manner in which the preferred reflector material improves the efficiency of the display.
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°. 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 ⅛
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.
Claims
- 1. A display comprising:an array of reflective pixels; a linear light source comprising a line source that emits light in a pattern defined by an axis; and a reflector comprising a cylindrical surface, the axis of said cylindrical surface being parallel to said axis of said light source, said linear light soucer being position relative to said reflector such that light from said linear light soucer is reflected by said reflector into said array of reflective pixels, said reflector comprising a material that is partially reflecting.
- 2. The display of claim 1, wherein said linear light source comprises a plurality of light emitting diodes and an optical diffuser.
- 3. The display of claim 2 wherein said light emitting diodes comprise diodes having different emission spectra.
- 4. The display of claim 1 wherein said reflector comprises a material that preferentially reflects light of a first linear polarization while preferentially transmitting light having a linear polarization orthogonal to said first linear polarization.
- 5. The display of claim 4 wherein each pixel in said array of reflective pixels comprises a polarization rotating cell that rotates the linear polarization vector of light reflected by said pixel in response to the receipt of an electrical signal by said pixel.
- 6. An illumination system for illuminating a reflective display, said illumination system comprising:a linear light source comprising a line source that emits light in a pattern defined by an axis; and a reflector comprising a cylindrical surface, the axis of said cylindrical surface being parallel to said axis of said linear light source, said linear light source being positioned relative to said reflector such that light from said linear light source is reflected by said reflector onto said reflective display, said reflector comprising a material that preferentially reflects light of a first linear polarization while preferentially transmitting light having a linear polarization orthogonal to said first linear polarization.
US Referenced Citations (10)
Foreign Referenced Citations (1)
Number |
Date |
Country |
06110033 |
Apr 1994 |
JP |