CA2333989A1

CA2333989A1 - Illumination of a liquid crystal display

Info

Publication number: CA2333989A1
Application number: CA002333989A
Authority: CA
Inventors: Akira Tomita; Mark R. Munch
Original assignee: Tyco Electronics Corp
Current assignee: Individual
Priority date: 1998-06-04
Filing date: 1999-06-04
Publication date: 1999-12-09
Also published as: JP2002517783A; WO1999063399A1; EP1084444A1

Abstract

A high contrast, rapid response liquid crystal imaging system is provided. T he imaging element (901) uses a liquid crystal composite (e.g. made of encapsulated liquid crystal) that is less than 4 micrometers thick, and preferably less than 2.5 micrometers thick. The imaging element is illuminat ed by at least one light source (903) that is positioned at an angle of less th an 30 degrees off of the display normal. The viewing system (1307) is positione d on the same side of the display as the light source and is located approximately normal to the display. In one aspect, the liquid crystal composite is operated in a reverse mode and a virtual image is created. In another aspect, at least three light sources of different colors illuminate a liquid crystal imaging element. The sources are all located within an angle of approximately 30 degrees from the imaging element normal. A processor (1311) is coupled to both the light sources (903) and the imaging element (901), thereby allowing the individual color light sources to be synchronized with the output of the imaging element.

Description

ILLUMINATION OF A LIQUID CRYSTAL DISPLAY
TECHNICAL FIELD OF THE INVE'I~TION
The present invention relates generally to liquid crystal display panels and, more particularly, to a method and apparatus for utilizing thin liquid crystal displays in a high contrast frame sequential color display.
BACKGROUND OF THE INVENTION
Light valves having an electro-optically active element comprising a liquid crystal composite have been used in displays (directly driven, passive matrix, and active matrix addressed), windows, and privacy panels. In a liquid crystal composite, plural volumes or droplets of a liquid crystal material are dispersecE, encapsulated, embedded, or otherwise contained within a matrix material such as a polymer. Exemplary disclosures include Fergason, US 4,435,047; West et al., US 4,685,771; Pearlman, US
4,992,201; and Dainippon Ink, EP 0,313,053, incorporated herein by reference.
The liquid crystal composite is disposed betr~reen electrodes, at least one of the electrodes typically being patterned to form a matrix. Tlae electrodes are supported by substrates. When voltage is applied to a pair of electrodes, an electric field is created and the liquid crystal located between the electrodes will become transmissive. In this optical state incident light is transmitted through the composite. When the voltage to the pair of electrodes is switched off, the electric field no longer exists and the liquid crystal composite between the electrodes changes its optical state to one in which incident light is substantially scattered andJor absorbed. In this state the mal:er~ial will typically be opaque with a frosty appearance if scattering is predominant or dark gray if absorption is predominant. By individually controlling the voltage applied to each pair of electrodes in an electrode matrix, a graphical image may be generated. The electrode matrix can be transparent or reflective and is typically a matrix of thin film transistors (TFT), MOS

- transistors, MIM diodes, or crossed patterned electrodes. The graphical image can be viewed directly, projected onto a viewing screen, or viewed as a virtual image in the eye:
By combining red, green, and blue images, either via sequential illumination, for example, using field-sequential color with red, green, and blue light or via dedicated red, green, and blue pixels, a colored image may be formed.
The prior art approaches to color displays typically use three separate imaging elements for imparting an image to each of the ligr~t components (e.g., red, green;
and blue). For example, see Sonehara, US 5,098,183 and K:urematsu et al., US
5,170,194.
Since the imaging elements are generally comprised of liquid crystal composites, the use of three different imaging elements is very expensive. This is especially true for high resolution displays due to the large number of pixels per imaging element plus the means for addressing the individual pixels. Furthermore, many of The prior art color displays utilize twisted nematic type liquid crystals in the display elements that may require a polarizer, thus leading to a loss of brightness. Therefore a color display that only requires a single imaging element that can operate in the absence of a polarizer is desirable.
Other prior art which may be relevant to the present invention includes US
5,398,081, WO 90/05429 and WO 98/00747.
Although a variety of single imaging element color displays have been disclosed in the prior art, a simple frame sequential color display that does not require relatively complex and expensive filtering means is desired.
SUMMARY OF THE INVENTIfON
The present invention provides a rapid response liquid crystal imaging element with high brightness and contrast levels. The invention uses a liquid crystal composite that is less than 4 micrometers thick, and preferably less than 2.5 micrometers thick. The display is illuminated by at least one Iight source that is positioned at an angle of less than 30 degrees off of the display normal, and preferably less than 20 degrees off of the normal. The viewing system is positioned on the sanne side of the display as the light source and is located approximately normal to the display.
-z-~'O 99/63399 PCTJUS99/I2652 In one aspect of the invention, the voltage applied across the electrodes of an individual pixel are varied between a first voltage necess<try to place the selected pixel into the on state and a second voltage necessary to place the selected pixel into the off state. The second voltage may either be a zero voltage or a non-zero minimum voltage.
In the preferred embodiment, a non-zero minimum voltage is used in order to avoid the contrast reversal that occurs in thin liquid crystal composites at small illumination angles.
In one embodiment, at least three light sources of different colors (e.g., primary colors, complementary colors, etc. ) illuminate a liquid crystal imaging element.
The sources are all located within an angle of approximately 30 degrees, and preferably less than 20 degrees from the imaging element normal. The viewing system is located on the same side of the imaging element as the light sources and may be comprised of a virtual image viewing system, a direct viewing system, or a projection viewing system. A
processor is coupled to both the light sources and the imaging element, thereby allowing the individual color light sources to be synchronized with the output of the imaging element. Preferably the controlled switching rate of the light sources and the imaging element are sufficient to avoid frame flicker and color break-up.
A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a cross-sectional view of a liquid crystal light valve in the on state according to the prior art, while Fig. 2 is a cross-sectional view of the liquid crystal light valve of Fig. 1 in the off state.
Fig. 3 is a cross-sectional view of a liquid crystal display panel according to the prior art, while Fig. 4 is a top view of the liquid crystal display panel of Fig. 3.
Fig. S is an illustration of a liquid crystal display operating in the normal mode.
Fig. 6 is an illustration of a liquid crystal display operating in the reverse mode.

ii Fig. 7 schematically illustrates a color frame sequential projector.
Fig. 8 is an illustration of an alternative frame sequential color display according to the prior art.
Fig. 9 is an illustration of the relationship of a liquid crystal display to an illumination source and a viewer according to the present invention.
Fig. 10 is an illustration of a testing system used in conjunction with the invention.
Fig. 11 its a graph comparing the brightness of a thick liquid crystal composite to a thin liquid crystal composite for three different illumination angles.
Fig. 12 is a graph illustrating the illumination angle dependence of the light scattering characteristics of a thin liquid crystal composite.
Fig. 13 is an illustration of a liquid crystal diisplay suitable for use with the present invention, while Fig. 14 is an illustration of the Iiqu.id crystal display shown in Fig. 13 with a specular reflection controlling reflector Fig. 15 is an illustration of the invention utilizing multiple light sources.
Fig. 16 is an illustration of the invention for use in a frame sequential color display while Fig. 17 illustrates a timing sequence suitable for use with the embodiment of the invention shown in Fig. 16.
DESCRIPTION OF THE SPECIFIC EMBODIMENTS
Before describing the present invention in detail, several different configu-rations of prior art liquid crystal displays will be described. Fig. 1 is a cross-sectional view of a typical liquid crystal display 100 according to the prior art. A
medium 101 (e.g., a polymer) containing a plurality of liquid crystal volumes or droplets 103 is sandwiched between a pair of electrodes 105 made of a transparent conductive material such as indium tin oxide. Droplets 103 may be individually encapsulated in one or more encapsulation layers as taught by Fergason, US 4,435,047; :lZeamey et al., US
5,405,551;
and Havens et al., US 5,585,947, the disclosures of which are incorporated herein. While the display is preferably made of encapsulated liquid crystal material, other types of liquid w0 99163399 PCT/US99/12652 crystal displays, for example smectic A, cholesteric, or dynamic scattering nematic displays, may also be employed. Electrodes 105 are coupl<:d to a voltage source 107.
When voltage source 107 is in an on state, a voltage is applied across electrodes 105 creating an electric field. Due to the positive dielectric anisotropy of liquid crystal droplets 103, the material comprising the droplets aligns parallel to the electric field as shown. In this state light incident along a path 109 will pass through droplets 103. Depending upon the thickness of the composite, the voltage applied to electrodes 105, and the transparency of electrodes 105, medium 101, and aligned droplets 103, transmission rates of 70% or greater may be achieved.
When voltage source 107 is in an off state as illustrated in Fig. 2, the electric field between electrodes 105 is effectively zero. As a result, liquid crystal droplets 103 no longer are uniformly aligned. Due to the random orientation of droplets 103, light incident along path 109 is randomly scattered, both in a forward direction and a backward direction as illustrated by scatter paths 201. The scattering of the incident light causes display 100 to appear opaque or frosty.
Fig. 3 is a cross-sectional view of a liquid crystal display panel 300 that may be used to display graphical information. As in liquid crystal display 100, panel 300 includes both medium 101 and liquid crystal volumes 103. In at least one embodiment, the liquid crystal composite comprising medium 101 and liquid crystal volumes 103 is a .
polymer dispersed liquid crystal (i.e., a PDLC composite).
In marked contrast to display 100, panel 300 includes a plurality of bottom electrodes 301 and a common top electrode 303 to form a I>lurality of electrode pairs. The electrode pairs divide panel 300 into an array of separately controllable display elements or pixels. Pane1300 also includes a top support member 305 preferably made of a conductive transparent material such as indium tin oxide (i.e., ITO) coated polyethylene terephthalate or ITO coated glass. Depending upon the desired application, the display can be designed to be either reflective or transparent. If a reflective display is desired, the reflective coating may either be applied to a surface of a bottom support member 307 or to a surface of pixel electrodes 301. Preferably electrodes :301 are reflective electrodes made of aluminum or silver. While the panel co~guration illustrated in Fig. 3 is common, it is understood that other configurations are well. known by those of skill in the art and that this configuration is intended only to be illustrative, not limiting.
Electrically coupled to each electrode 301 is. a switching element 309 that is used to control the application of a voltage across common electrode 303 and electrodes 301. Typically switching elements 309 are thin film transistors when display 300 is a transparent mode display and MOS transistors (as shown in Fig. 3) when display 300 is a reflective mode display. Switching elements 309 act as switches for each electrode "pair"
thus allowing any combination of pixels to be activated. In general, panel 300 is designed so that the voltage that causes switching elements 309 to operate is the threshold voltage of liquid crystal volumes 103. Although in the illustrated embodiment switching ele-ments 309 are MOS transistors, other switching elements such as thin film transistors, MIMs, diodes, or varistors may be used as an alternative. 'Che application of voltage across electrodes 301 and 303, and therefore the activation of individual pixels, is controlled by a processor. In some configurations, such as the MOS transistor configuration illustrated in Fig. 3, capacitive elements 311 are added to the transistor circuit in order to store charge.
Fig. 4 is a top view of panel 300. In the illustrated embodiment, panel 300 is comprised of a 20 by 20 array of square pixels 401. Panel 300 may be comprised, however, of greater or lesser numbers of pixels. Furthermore, the pixel shape is not limited to squares nor is the pixel shape limited to four sided configurations. Lastly, all of the pixels within the panel need not be of a uniform shape or size.
Liquid crystal display panels may be utilized in a variety of different configurations to create direct view, projection, and virtual images. Examples of direct view include computer monitor screens and instrument panel readouts. Examples of projection systems include front and rear systems projecting to a large screen or to a screen in a microdisplay. A virtual microdisplay typically consists ofone or more light sources, a liquid crystal composite, electrode elements, andl imaging optics that form a virtual image in the eye of the user. Additionally, liquid crystal displays may be designed to function in either a transmissive or a reflective mode.

w'O 99/63399 PCT/US99/12652 Basically there are two configurations in which a high contrast image can be formed; normal mode and reverse mode. In a normal mode configuration, the image is formed from the reflected, or transmitted, non-scattered light while the scattered light is blocked. Fig. S is an illustration of a reflective liquid crystal display operating in the normal mode. In this mode both the Light source SO1 and the viewer 503 are on the same side of the panel. As the illustrated panel is a reflective display, either bottom support member 505 is reflective or pixels 507 and 509 are reflective. The construction of reflective member SOS is well known in the art, see, for example, Rowland, US
3,935,359; Kuney, Jr., US 4,957,335; Nelson et al., US 4,938,563; Belisle et al., US
4,725,494; Appledorn et al., US 4,775,219; Tung et al., US 4,712,219; Malek, US
4,712,867; Benson, US 4,703,999; Sick et al., US 4,464,014; Nelson et al., US
4,895,428;
Hedblom, US 4,988,541; Schultz, US 3,922,065; and Linden, US 3,918,795; the disclosures of which are incorporated herein by reference.
As shown in Fig. 5, the pixels defined by electrodes 507 are in an on state, thereby causing the liquid crystal volumes in the pixels defined by these electrodes to become transparent. Due to the transparency of these pixels, light from source 501 (e.g., ambient Light, directed light, etc.) will pass through the pixels and be reflected by sub-strate reflector 505 or, in an alternate configuration, by the reflective electrode. The reflected specular Light forms a bright image at Location 503, typically after first passing ZO through imaging optics 511 and an aperture stop 513. The liquid crystal volumes in those pixels defined by non-activated electrodes 509 are scattered in multiple directions 515, only a fraction of which will pass through optics 51 l and aperture stop 513 to reach viewing location 503.
In an alternate configuration of a normal mode display (not shown), neither the pixel electrodes nor the bottom substrate are reflective, and the image is formed by the light transmitted through those pixels in the on state, i.e., pixels 507.
Fig. 6 is an illustration of a display panel operating in the reverse mode.
As noted above, a reverse mode panel may be used either in a reflective configuration as illustrated, or in a transparent configuration. This panel is basically the same as that illustrated in Fig. 5. However in this configuration it is the scattered light 515 that is _7_ 'WO 99!63399 PCT/US99/12652 collected by imaging optics 51 I to form an image at locatiion 503. The specular light, either reflected as shown by an exemplary light ray 601 or passing through the display in the case of a transparent configuration, is blocked with stop 513. A dark image is formed by those pixels 507 in an on state.
Besides the prior art display panel configurations illustrated above, there are numerous other configurations that are well known by those of skill in the art.
Furthermore, liquid crystal display panels may also be used to produce color images. For example, either pleochroic or isotropic dyes may be included within the liquid crystal material, thereby achieving a colored visual effect. Alternatively, colored filters or colored source light may be used in conjunction with the liquid crystal displays to provide a colored image. By sequentially combining multiple colored images, for example, red, green, and blue images, an image of good color purity may be produced.
One type of frame sequential display is schematically illustrated in Fig. 7.
This system is described in detail in Jones, US 5,398,081, the complete disclosure of which is incorporated herein. The light from a white light source 701 is directed at a color modulator 703, for example, a dichroic cube color separator. The dichroic cube has three color selective reflective surfaces positioned behind three sets of light valves. The three color selective surfaces may be the three primary colors (e.g., red, green, and blue) or some other combination such as three complementary colors (e.g., cyan, yellow, and magenta). The light valves are controlled by a computer that switches the valves between a substantially transparent state and a substantially non-transparent state.
Thus three distinct colors may be obtained from the incident white light of source 701.
The color modulated light from modulator ',~03 is directed at an imaging element 705, also computer-controlled. Element 705 imparts an image onto the incident light that corresponds to the particular color of light reaching it.
Projection system 707 sequentially projects the colored images onto a screen, thus creating a colored image.
Since the computer controls both the switching of color modulator 703 and the image presented on imaging element 705, the color output and thE; image may be synchronized.
Assuming the switching speed is at a high enough rate, the alternating images are not resolvable as distinct colors. Thus the viewer only perceives a composite colored image.
_g_ Fig. 8 is an illustration of an alternative frame sequential color display according to the prior art. This system is described in detail in Williams et al., WO
90/05429, the complete disclosure of which is incorporated herein. The light from a white light source 801 is incident on a color filtering means 803. Color filtering means 803 includes a plurality of liquid crystal light valves 805 aligned with a plurality of color filters 807. As in the prior approach, in order to achieve an accurate color composite three color filters are required (e.g., red, green, and blue). Therefore in order to transmit only one color through filtering means 803, for example blue, only light valves 805 corresponding to blue filters 807 are switched to an on or tr~msparent state while all remaining light valves 805 are switched to an off or scattering state. As a result of this switching, in this example only blue light would pass through filtering means 803.
The color modulated light exiting filtering means 803 is spread by a light spreading means 809, such as a lens or diffusion plate. Element 809 insures that the light from filtering means 803 is relatively uniform, eliminating the effects of discrete color filters 807. The color modulated light then passes through a. liquid crystal array 811, array 811 forming the desired image through individual control of the array pixels. As in the prior approach, a processor 8i3 synchronizes the image to the image color.
Regardless of the type of frame sequential color display utilized, rapid switching speeds are required in order to avoid frame flicker and to insure that the viewer sees a composite color image rather than a series of discrete single color images. As a result, typically switching speeds on the order of at least 90 Hz and preferably at least 180 Hz are necessary (i.e., 30 to 60 sequences of three colors per second). These rates translate to liquid crystal response times of less than 10 milliseconds, and preferably on the order of 3 to 5 nuiliseconds.
There are other parameters that are important in selecting liquid crystal materials besides switching speeds. For example, a high contrast ratio is desirable since it leads to improved image quality. Image contrast is defined as the ratio of light from the pixels in the on state to that from the pixels in the off state. Another quality that is desirable is a low operating voltage, i.e., the voltage required to place a pixel in an on state. As the operating voltage increases, the overall power consumption of the device similarly increases. In addition, high voltage transistors are expensive primarily because common transistor technology is low voltage. Besides requiring more power, a high operating voltage generates more heat that must be dissipated. The higher temperatures may also lead to local temperature fluctuations that may result in thermally induced mechanical stresses. Typically a low field E9o is preferred for direct view and projection applications. A low operating field allows the use of a lov~rer applied voltage or, for a given applied voltage, allows for a thicker layer of liquid crystal composite to be used thereby yielding a lower transmissivity when the pixel is in the off state (i.e., Toff), thus creating a higher contrast image.
The present invention will now be described in more detail. The present invention utilizes thin liquid crystals, typically with a thickness of less than 4 micrometers, and preferably with a thickness of less than 2:5 micrometers. An advantage of thin liquid crystals is that improved switching speeds m;ay be obtained, thereby allowing a colored virtual image to be created using field sequential color schemes.
Furthermore, with appropriate illumination and viewing angles, improved image brightness and contrast may also be achieved.
Fig. 9 is an illustration of the relationship oiE a liquid crystal display 901 to the illumination source and the viewer according to the present invention. The liquid crystal composite in display 901 has a thickness of less than 4 micrometers, and preferably less than 2.5 micrometers. It is illuminated by at least one light source 903 that is positioned at an angle, 8, of less than 30 degrees off of the normal from display 901, and preferably less than 20 degrees off of the normal. A viewing system 905 is located approximately at the normal to display 901. This configuration provides superior image quality in an extremely rapid response time display.
Fig. 10 is an illustration of a testing system 'used to show the improvements offered by the present invention. A sample 1001 was illuminated by an argon laser 1003 operating at a wavelength of 514 nanomel:ers. Three different illumination angles (i.e., 8) were used during the test; 15, 30, and 45 degrees. A detector 1005 was placed normal to sample 1001 and at a distance o~f 5.25 inches from the sample.
The detector was 0.25 by 0.25 inches in size.

Fig. 11 is a graph of the brightness of sample 1001 as a function of the applied voltage. Two different samples were tested; a 1.83 micrometer thick liquid crystal and a 4.9 micrometer thick liquid crystal. At 45 degrees illumination, the thick sample (test run # 11 O 1 ) produced approximately twice the brightness of the thin sample (test run #I I03). At 30 degrees illumination, the thick sample (test run #1105) and the thin sample (test run # 1107) produced approximately the same brightness.
However, at degrees the thin sample (test run # 1109) produced significantly higher scatter levels;
and therefore brightness, than the thick sample (test run #1 I 11). This is an unexpected result since thick liquid crystal composites were thought to produce more scatter, and thus 10 more brightness, than thin composites due to the number of available scattering sites.
Fig. 12 is a graph illustrating the illumination angle dependence of the light scattering characteristics of a I.83 micrometer thick liquid crystal composite using the testing apparatus shown in Fig. 10. The illumination angle, 9, was varied from 10 to 50 degrees in 2 degree increments. The viewing angle remained constant at the sample 15 normal. As in Fig. 11, the brightness is shown as a function of the voltage applied to the composite.
Regardless of the applied voltage, as the illwnination angle decreases, the - sample scatter, and therefore brightness; increases: For angles greater than 40 degrees, there is very little difference in the scatter level as the applied voltage is varied, resulting in the creation of low contrast images. For angles greater than 20 degrees, the brightness decreases as the voltage increases from 0 to 10 volts. For small illumination angles, i.e., less than 20 degrees, the brightness reaches a maximum value at a non-zero voltage in the range of 2 to 3 volts. As a consequence of this unexpected result, the optimwn contrast is achieved between the operating voltage and some non-zero voltage. For example, at a 10 degree illumination angle, the optimum contrast is achieved by setting the minimum voltage to 3 volts.
Fig. 13 is an illustration of one embodiment ~of the invention utilizing a reverse mode configuration. In this embodiment sample 901 is comprised of a thin liquid crystal composite 1301, upper electrode 1303, lower pixel electrodes 1305, and upper and lower support members 1307 and 1309, respectively. Preferably upper electrode 1303 is - made of an optically transparent conductive coating (e.g., ITO) and lower electrodes 1305 are made of optically reflective and electrically conductive materials (e.g., aluminum, silver, etc.). Alternatively, lower electrodes 1305 may be transparent and lower support member 1309 reflective. As previously described, the electrode pairs controlling the state of each pixel are controlled by a processor 1311. The lower voltage extreme may be either 0 volts or, alternatively, a non-zero voltage. A non-zE:ro voltage is used in order to avoid contrast reversal from occurring, as noted above. If a non-zero voltage is used, it is typically in the range of 2 to 3 volts.
Source 903 is positioned at an angle of less tlhan 30 degrees off of the normal from display 901, and preferably less than 20 degreea off of the normal. As Fig.
13 illustrates a reverse mode configuration, the image is formed by collecting scattered rays 1313. ~ Assuming a reflective display as illustrated, the :light rays passing through those pixels that are in an on state and reflected back as a specular reflection are preferably eliminated with the use of an aperture stop. If both electrodes 1305 and substrate 1309 are transparent, light rays passing through activated pixels are either eliminated with a light trap (e.g., absorbing coating) or reflected away from the display and the viewing system.
_ In this embodiment a virtual image is created. for viewing at a location 1315. The virtual image is formed by imaging optics 1307 which collects a portion of scattered rays 1313. In one configuration, imaging optics 1:307 are similar to those used in a microscope.
In an alternate embodiment, imaging optics 1307 are comprised of a projection optical system. The projection optical system can be used to project an image onto a screen.
Given the relatively small angle between source 903 and the display normal, it is important to prevent specular reflections from entering the viewing system.
In an alternate embodiment of display 901 illustrated in Fig. 14, lower member includes a tilting reflective: member 1401. Reflective member 1401' is used to direct the specularly reflected light rays 1403 of source 903 sufficiently away from viewer 1315 so that they are not collected by optics I 307. Reflective member 1401 may either reflect the WO 99!63399 PCT/US99/12652 - light rays fiuther away from viewer 1315 as illustrated, or reflect the light rays back at source 903. A reflecting member is disclosed by Kamath et al., US 5,132,823, the disclosure of which is incorporated herein. In one embodiment, reflective member 1401 is comprised of a plurality of individual tilted reflective surfaces, each of said individual S reflective surfaces corresponding to a display pixel. The individual tilted surfaces may be simple planar surfaces or more complex surfaces such as cones, the latter configuration suitable for use with multiple light sources.
In an alternate embodiment of the invention, multiple sources 903 are used to increase the display brightness and uniformity. Fig. 15 is a top view of display 901.
i 0 As disclosed above, viewing system 905 is positioned norcr~al to the display. In this embodiment multiple sources 903 are positioned around viewing system 905 and within an angle of approximately 30 degrees, and preferably less than 20 degrees from the display normal. Although four sources 903 are shown in Fi.g. 15, both fewer and greater numbers of sources may be used with the invention. Additional increases in contrast 15 levels may be achieved by positioning sources 903 to minimize their alignment with the pixel edges as shown in Fig. 15, and as disclosed in co-pending U.S. Patent Application Serial No. 09/090,749, filed 4 3une 1998, entitled High Contrast Micro Display With Off Axis Illumination.
Fig. 16 is an alternate embodiment of the invention providing a frame 20 sequential color display system. This embodiment is subsW ntially the same as that previously illustrated except that at least three different color sources 903 are used to sequentially illuminate sample 901. For example, sources 903 may be comprised of a red source 1601, a blue source 1602, and a green source 1603. If improved brightness and uniformity are desired as illustrated in Fig. 15, each color light may be provided by 25 multiple sources (e.g., sources 1601', 1602' and 1603').
In this embodiment the processor that controls the state of the individual pixels of display 901 also controls the activation of the individual color light sources 1601-1603. As disclosed above, the individual colors are synchronized with the images displayed on liquid crystal panel 901. Assuming a fast enough switching speed, only a 30 composite colored image is perceived by the viewer.

't~VO 99/63399 PCT/US99l12652 Fig. 17 illustrates a timing sequence suitable; for use with the embodiment of the invention shown in Fig. 16. Three colors, red, green, and blue, are shown although other combinations may be used with the invention as discussed above. As previously disclosed, in order to prevent frame flicker and color break-up, a frame rate of at least 180 Hz is desired. This requires a combination of the liquid crystal response time and the active matrix array setup time to be shorter than 5.6 milliseconds. Therefore if it takes 2 milliseconds to refresh the entire display, the liquid crystal response time must be shorter than 3.6 milliseconds as illustrated in Fig. 17. Light sources 903, for example LEDs, are preferably not turned on during the array setup time nor du.~~ing the liquid crystal response time as illustrated. If the sources are tuned on during either of these times, there may be a color shift in the middle of the display.
Typically the active matrix is addressed from the top to the bottom of the display using a line at a time scheme. Alternatively, the array is divided into multiple zones with dedicated light sources per zone, thereby reducing the array setup time by the number of zones. Thus by using two zones, the liquid crystal response time requirement is reduced from 3.6 milliseconds to 4.6 milliseconds. This .embodiment also allows the pulse width of the light sources to be extended, thus increasing the display brightness.
As will be understood by those familiar with the art, the present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. For example; the invention may be used with a variety of different liquid crystal materials, display panel designs, pixel sizes, pixel shapes, and electrode configurations. Accordingly, the disclosures and descriptions herein are intended to be illustrative, but not limiting, of the scope of the invention which is set forth in the following claims.

Claims

WHAT IS CLAIMED IS:

1. An apparatus for forming an image for viewing, comprising:
an imaging element comprising a liquid crystal composite less than 4 micrometers thick;
a light source for directing light at a first side of said imaging element, said light source positioned less than 30 degrees away from a normal to said imaging element;
a processor coupled to said imaging element for controlling said image generated by said imaging element; and an optical imaging system located substantially along said imaging element normal on said first side of said imaging element, said optical imaging system forming said image.

2. The apparatus of claim 1, further comprising a second light source for directing light at said first side of said imaging element, said second light source positioned less than 30 degrees away from said imaging element normal.

3. The apparatus of claim 1, wherein said light source is positioned less than 20 degrees away from said imaging element normal.

4. The apparatus of claim 1, wherein said liquid crystal composite is less than 2.5 micrometers thick.

5. The apparatus of claim 1, wherein said formed image is a virtual image.

6. The apparatus of claim 1, said apparatus further comprising a viewing screen, wherein said optical imaging system projects said image onto said viewing screen.

7. The apparatus of claim 1, wherein said imaging element utilizes said liquid crystal composite in a reverse mode.

8. An apparatus for generating a colored image for viewing, comprising:
an imaging element comprising a liquid crystal composite less than 4 micrometers thick, wherein said colored image is generated at a position located substantially along a normal to said imaging element on a first side of said imaging element;
a first light source for directing a first wavelength of light at said first side of said imaging element, said first light source positioned less than 30 degrees away from said imaging element normal;
a second light source for directing a second wavelength of light at said first side of said imaging element, said second light source positioned less than 30 degrees away from said imaging element;
a third light source for directing a third wavelength of light at said first side of said imaging element, said third light source positioned less than 30 degrees away from said imaging element normal; and a processor coupled to said imaging element and said first, second, and third light sources for synchronizing said light sources with an image output of said imaging element to generate said colored image.

9. The apparatus of claim 8, further comprising an optical imaging system located substantially along said imaging element normal on said first side of said imaging element, said optical imaging system forming said colored image.

10. The apparatus of claim 9, wherein said colored image is a virtual image.

11. The apparatus of claim 9, said apparatus further comprising a viewing screen, wherein said optical imaging system projects said colored image onto said viewing screen.

12. The apparatus of claim 8, wherein said imaging element utilizes said liquid crystal composite in a reverse mode.

13. The apparatus of claim 8, wherein said first wavelength of light corresponds to a red color, said second wavelength of light corresponds to a green color, and said third wavelength of light corresponds to a blue color.

14. The apparatus of claim 8, wherein said first wavelength of light corresponds to a cyan color, said second wavelength of light corresponds to a yellow color, and said third wavelength of light corresponds to a magenta color.

15. The apparatus of claim 8, wherein said imaging element is comprised of a plurality of pixels, wherein each pixel of said plurality of pixels includes multiple edges, wherein said multiple edges of said plurality of pixels are substantially aligned to form multiple pluralities of aligned edges, and wherein said first, second, and third light sources are not aligned with any of said multiple pluralities of pixel edges.

16. The apparatus of claim 8, further comprising:
a fourth light source for directing said first wavelength of light at said first side of said imaging element, said fourth light source positioned less than 30 degrees away from said imaging element normal;
a fifth light source for directing said second wavelength of light at said first side of said imaging element, said fifth light source positioned less than 30 degrees away from said imaging element;
a sixth light source for directing said third wavelength of light at said first side of said imaging element, said sixth light source positioned less than 30 degrees away from said imaging element normal; and wherein said fourth, fifth, and sixth light sources are coupled to said processor.

17. The apparatus of claim 16, wherein said processor operates said first and fourth light sources simultaneously, said second and fifth light sources simultaneously, and said third and sixth light sources simultaneously.

18. The apparatus of claim 8, wherein said processor operates at a frame rate of at least 180 Hz.

19. The apparatus of claim 8, wherein said first light source is positioned less than 20 degrees away from said imaging element normal, said second light source is positioned less than 20 degrees away from said imaging element normal, and said third light source is positioned less than 20 degrees away from said imaging element normal.

20. The apparatus of claim 8, said imaging element further comprising a reflective member proximate to a second side of said imaging element.

21. The apparatus of claim 20, wherein said reflective member directs a plurality of specular reflections associated with said first, second, and third light sources away from said viewing position.

22. The apparatus of claim 8, wherein said liquid crystal composite is less than 2.5 micrometers thick.

23. The apparatus of claim 8, wherein a minimum voltage applied to said imaging element is approximately 2 volts.

24. A method of generating a colored image at a viewing position, the method comprising the steps of:
locating said viewing position substantially along a normal to an imaging element on a first side of said imaging element, said imaging element comprising a liquid crystal composite less than 4 micrometers thick;
positioning a first light source less than 30 degrees away from said imaging element normal on said first side of said imaging element, wherein said first light source provides light of a first wavelength;

positioning a second light source less than 30 degrees away from said imaging element normal on said first side of said imaging element, wherein said second light source provides light of a second wavelength;
positioning a third light source less than 30 degrees away from said imaging element normal on said first side of said imaging element, wherein said third light source provides light of a third wavelength; and synchronizing an image output from said imaging element with said first, second, and third light sources to generate said colored image at said viewing position.

25. The method of claim 24, wherein said colored image generated at said viewing position is a virtual image.

26. The method of claim 24, wherein said synchronizing step is at a rate of at least 180 Hz.

27. The method of claim 24, further comprising the step of reflecting specular reflections from said first, second, and third light sources away from said viewing position.

28. The method of claim 25, further comprising the steps of:
positioning a fourth light source less than 30 degrees away from said imaging element normal on said first side of said imaging element, wherein said fourth light source provides light of said first wavelength;
synchronizing said fourth light source with said first light source;
positioning a fifth light source less than 30 degrees away from said imaging element normal on said first side of said imaging element, wherein said fifth light source provides light of said second wavelength;
synchronizing said fifth light source with said second light source;
positioning a sixth light source less than 30 degrees away from said imaging element normal on said first side of said imaging element, wherein said sixth light source provides light of said third wavelength; and synchronizing said sixth light source with said third light source.