CN117642686A

CN117642686A - Liquid crystal display with polarized infrared illumination

Info

Publication number: CN117642686A
Application number: CN202180100725.6A
Authority: CN
Inventors: M·T·普法伊弗
Original assignee: Xinhuikai Co
Current assignee: Xinhuikai Co
Priority date: 2021-08-06
Filing date: 2021-08-06
Publication date: 2024-03-01
Also published as: EP4381344A1; WO2023014377A1; CA3228335A1

Abstract

Systems and methods for IR readable transmissive and reflective displays are disclosed that do not suffer from undesirable darkening of the display or mirror-like appearance due to sequential stacking of polarizers. The disclosed systems and methods use available IR LEDs in addition to or in place of visible LEDs. An illuminator or integrator as a light guide is designed to maintain the polarization state of the light. The display may use a conventional visible front polarizer and thus is not subject to brightness degradation caused by IR-capable polarizers.

Description

Liquid crystal display with polarized infrared illumination

Technical Field

The present invention relates to infrared readable liquid crystal displays and, more particularly, to illuminating a liquid crystal display with polarized infrared light to extend the readability beyond the usable contrast range of the liquid crystal display polarizer to longer wavelengths.

Background

Modern Liquid Crystal Displays (LCDs) may include millions of individual pixels, which are thin multi-layer structures of many components in an LCD. Non-polarized light sources such as Light Emitting Diodes (LEDs) are used to illuminate pixels in LCDs.

Most LCD pixels are based on two functional principles: 1) An electrically controllable liquid crystal layer between the transparent substrates changes the polarization state of light passing through it based on an applied electrical signal, and 2) one or more polarizers and optional additional optical films convert the polarization state differences into visible light and dark contrast regions. The polarizer(s) and the liquid crystal layer together form an electrically controllable light valve that passes a portion of the light depending on the electrical stimulus.

Since most LCDs are flat or slightly curved and can have considerable dimensions, a film polarizer is often required. Such films adhere to one or more surfaces of the display and cover substantially the entire image area. Film polarizers typically comprise two layers of transparent isotropic polymer, such as cellulose Triacetate (TAC), sandwiching a stretched polyvinyl alcohol (PVA) layer containing an anisotropic chromophore that aligns and provides a polarizing effect due to stretching of the PVA.

The chromophore is typically iodine, which acts as L ₃ -and I ₅ PVA composites are present in PVA. Alternatively, the PVA layer may be impregnated with different anisotropic dye molecules that together cover the visible range of the spectrum. The iodine method or the dye method has in common that since PVA-iodine complex and commonly used anisotropic dye are transparent in near infrared, the range of available contrast is limited to the visible range of spectrum. Thus, when ordinary LCDs are viewed using IR sensitive cameras or night vision devices, they do not display images when operated under only invisible IR illumination.

It is sometimes desirable or even necessary to be able to view the liquid crystal display using invisible infrared light. For example, it may be necessary to read the LCD using a night vision goggles without any visible light, or it may be necessary to read the LCD using an infrared camera.

Common to most outdoor optical equipment is that it selectively uses vertically polarized light to eliminate glare from glancing reflections off smooth or moist surfaces. This is achieved by adding a polarizing element in front of the camera lens. Such polarizing elements may be selected such that they reject visible and/or infrared light having an undesired polarization state.

While it is possible to add an additional type of dye molecule to the chromophore layer of the polarizer that absorbs at longer infrared wavelengths to expand the usable contrast range of the polarizer, this is undesirable because such a polarizer necessarily has a lower transmittance in the visible range, resulting in a darker display. This is because the addition of more dye molecules reduces the transmittance and most dyes have higher order absorption bands. For example, dyes that will absorb in the infrared range at 900nm may also absorb in the visible range around 450 nm. Higher absorptivity (or lower transmissivity) is a problem, particularly for reflective displays or battery operated displays where lower visible light transmission cannot be compensated for with a stronger light source.

Alternative types of polarizers, such as wire grid polarizers, cholesteric film polarizers, and multilayer birefringent stacked polarizers, operate on the principle of transmitting one polarization state while reflecting the other. These types of polarizers are an option on the back side of the display if supported by absorbers suitable for transmissive polarization. Such polarizers generally have good contrast in the near infrared because they are not based on dye absorption. For example, a common 3M DBEF polarizer or Nagase WGF works well for 850 nm. Such a device may be used to replace the absorptive rear polarizer in an LCD. For example, an IR wire grid polarizer may be placed behind the display as a rear polarizer. However, the use of such polarizers in front of the display is undesirable because they have a metallic, mirror-like appearance due to specular reflection of about half of the incident light. If they are used in front of a display they will have to be hidden under an additional absorbing polarizer which will remove the reflected part of the light in the visible range, while taking appropriate steps such that such specular reflection of infrared light will not interfere with reading the display with infrared equipment.

It is therefore desirable to extend the usable contrast range of LCDs to longer wavelengths and make them readable by infrared devices without the following side effects: a low brightness in the visible range, a mirror-like appearance or an additional polarizer that prevents the mirror-like appearance is required.

Disclosure of Invention

For purposes of summarizing the invention, certain aspects, advantages, and novel features of the invention have been described herein. It is to be understood that not necessarily all such advantages may be achieved in accordance with any one particular embodiment of the invention. Thus, the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other advantages as may be taught or suggested herein.

Many liquid crystal displays have an integrated light source as a backlight or front light. The front light or backlight is typically composed of the actual light source and an "integrator" which distributes the light evenly across the display. The light source is typically a CCFL tube, an electroluminescent film, or most suitably a light emitting diode, such as a solid state LED or an organic LED. The integrator may be a light cavity or a transparent light guide that distributes light evenly across the display surface.

Since LEDs are available that emit light in the desired infrared wavelength range, such LEDs may be substituted for or added to visible LEDs to form infrared illuminators. The present invention and disclosure includes an illuminator or integrator that is a light guide designed to maintain the polarization state of light. The display may use a conventional visible front polarizer and thus is not subject to brightness degradation caused by IR-capable polarizers.

The polarizer in front of the LCD display has two functions: 1) Polarizing incident light; and 2) it analyzes or compares the polarization state of the light exiting the display. If the observer uses polarized light, for example polarized sunglasses, or more suitably optical means, such as a camera or night vision device, the device already provides the second function. Therefore, in order to extend the usable wavelength range of an LCD display to the IR range, either both functions need to be extended to the IR range, or it is sufficient to extend the polarization of the incident light only to the IR range if any relevant IR device can be ensured to have its own polarizer.

The visible light range polarizer is isotropic and transparent in the infrared range used by night vision devices or infrared cameras, in particular in the range of 700 to 1100 nm. Thus, if the display is illuminated with polarized IR light, it will remain readable for IR detection devices that use an IR polarizer to reduce glare.

If the display is provided with a rear polarizer having infrared capability, illumination of the optical device with a polarized IR source will result in contrast even if it is devoid of a polarizer, since light will be reflected off the display, depending on the polarization state after the first pass through the display.

An exemplary embodiment of the present invention and disclosure is an infrared transmissive LCD display with backlight, including a light guide and LED edge illumination. In addition to visible light LEDs, IR-emitting LEDs are added to the edges of the light guide. An infrared polarizer, such as a wire grid polarizer, is placed between the infrared and optional visible light LEDs and the edge of the light guide to provide the proper polarization state. The light guide is designed to maintain this polarization.

In visible light operation, polarized or unpolarized visible light is directed through the light guide into the display and from back to front through the display. When exiting the light guide, the light first encounters a polarizer that polarizes it into a desired state. It then passes through a liquid crystal layer which may change the polarization of the light depending on the desired state of the pixel. Finally, it passes through a front polarizer, which acts as an analyzer and thus produces a brightness contrast that is visible to the naked eye.

In infrared operation, light from an IR LED is polarized as it passes through an IR polarizer. Polarized IR light passes through the light guide and is directed to the LCD. Polarizers in LCDs appear transparent to IR light. The liquid crystal layer changes the polarization state based on the state of the pixel, if desired. The front polarizer appears transparent to the IR light and therefore does not act as an analyzer.

Thus, some regions of the display emit light in one polarization state, while other regions emit light in another polarization state. Infrared sensitive optical devices, such as infrared cameras or night vision goggles with IR glare proof polarizers, can detect images because their polarizers act as analyzers, converting polarization differences into the brightness contrast of the sensor elements. The human eye cannot perceive the difference in polarization state of light, but they can perceive the difference in brightness. The function of the analyzer is to convert the difference in polarization states into a difference in brightness by passing one polarization state while absorbing or reflecting the other. This embodiment may also include further optical films such as retardation films, compensation films or other light control films that optimize device performance.

Another exemplary embodiment of the present invention and disclosure is a reflective LCD display with polarized IR illumination. In visible light operation, ambient light or light from a visible LED in the front light passes through the front light, leaves it unchanged, and then reaches the front polarizer, where the light is polarized. In an alternative embodiment, ambient visible light first encounters a visible (absorptive) polarizer that imparts a particular polarization state that remains unchanged as it passes through the front light guide. The polarized light then enters the liquid crystal layer, where its polarization state may change depending on the applied electrical signal. It may then pass through an optional polarizer before being reflected by the mirror, or it may be reflected by a polarizer (such as a reflective polarizer backed with an absorber).

On the return path, the light passes again through the liquid crystal layer and through the front polarizer, which acts as an analyzer, converting the polarization state difference into brightness contrast. Finally, the light passes through a front light, which appears largely transparent to the light. In an alternative embodiment, the light first encounters the front light guide where its polarization state remains unchanged before being analyzed by the front polarizer.

In IR light operation, IR light from an IR LED is coupled into a front light guide via an IR polarizer. Polarized IR light passes through a light guide, which may be located in front of the front polarizer or between the front polarizer and the LCD, without changing the polarization state. The light guide transmits light through the LCD, wherein the polarization state of the light may be changed according to the applied signal. Light is selectively reflected only one polarization and absorbs the other polarization by a reflective polarizer or polarizer-mirror combination behind the display.

The reflected light returns through the liquid crystal layer where further polarization adjustment may occur. Upon exiting the LCD layer, the light encounters a front polarizer and front light, both of which appear transparent to IR light. Thus, certain areas of the display appear to emit IR light, while other areas do not. Even without an infrared glare-proof polarizing element, an infrared sensitive optical device would detect different brightness levels depending on the liquid crystal state. This embodiment may also include further optical films such as retardation films, compensation films, diffusion films, and other light control films that optimize device performance.

Another exemplary embodiment of the present invention and disclosure is a reflective liquid crystal display for digital license plate applications, including a reflective liquid crystal display with front light, comprising polarized IR illumination. License plate recognition systems operate at specific infrared wavelengths, such as 750nm, 850nm, 870nm, etc. The reflective LCD may be a bi-stable or multi-stable LCD, because the power requirements of such a display are lower than those of a display that needs to be updated continuously. One such bistable LCD type may be a pixel memory LCD, and another may be a bistable nematic LCD called "Binem" or a bistable nematic display called "ZBD".

The LCD may operate on a single polarizer basis, or have a reflective rear polarizer, such as 3M ^TM A multilayer polymer stack, known as DBEF, wire grid polarizer, such as Nagase WGF, or the like, is produced that has a useful contrast of about 380nm to greater than 850 nm. The front light guide may be located in front of or behind the front polarizer. The front light guide is edge lit with optional white light LEDs for night vision, and multiple IR LEDs are used to select one or more desired wavelengths depending on the requirements of the location where such license plates are to be issued. For example, if desired, the display may mount several 750nm and several 850nm LEDs. Other combinations are also possible.

A narrow polarizer strip of polarizer with good polarization efficiency at the desired wavelength is placed between the IR LED and the light guide. Such a polarizer may be a dye-type polarizer, the dye of which is selected only for infrared operation, and which may not have good transmission or polarization efficiency in the visible spectrum, since no visible light is required to pass through it. Another suitable type of polarizer may be a wire grid polarizer or a multilayer stack polarizer, since such a polarizer is simpler and easier to produce at lower cost than a wire-clip polarizer for the visible range.

The polarization-protecting light guide may be made of a transparent polymer, glass or a combination of different transparent materials and may be coated with materials having different refractive indices. Light passes from the light source through the light guide due to total internal reflection. Additional features such as certain shapes of alternating materials or certain surface structures such as pits or prisms cause light to be sent toward the display rather than the opposite surface.

The light passes through the liquid crystal layer, where its polarization may be changed depending on the liquid crystal alignment before encountering selective reflection in the retro-reflective polarizer. In the dark areas, the light has a polarization state that passes through the reflective polarizer and is absorbed by a black layer placed behind the display assembly. In the bright areas, the light is reflected back through the display, where further polarization changes may occur. The front polarizer appears transparent to infrared light.

These structures in the front light are designed to allow at least a portion of the light reflected by the display to pass through the front surface. License plate recognition systems with optional glare-proof IR polarizers on the lenses operate at any wavelength provided by the IR LEDs, and different areas of the display will now be seen as bright or dark depending on the local polarization state of the light.

Accordingly, one or more embodiments of the present invention overcome one or more of the disadvantages of the known prior art.

For example, in one embodiment, an infrared light readable liquid crystal display system includes: a liquid crystal display, comprising: a liquid crystal display unit comprising: a liquid crystal layer for controlling polarization states of visible light and infrared light; a front substrate; a rear substrate; and wherein the liquid crystal layer is located between the front substrate and the rear substrate; a visible light front polarizer, wherein the visible light front polarizer is transparent to infrared light; and a reflective rear polarizer for polarizing visible light and infrared light; and a lighting unit including a plurality of light sources, wherein at least one of the plurality of light sources emits infrared light.

In this embodiment, the infrared light readable liquid crystal display system may further include: wherein the liquid crystal display is an infrared transmissive liquid crystal display, and wherein the illumination unit is a backlight; wherein the backlight further comprises an absorber that absorbs additional light; wherein the infrared transmissive liquid crystal display further comprises a visible opaque layer transparent to infrared light; and the backlight further comprises a reflector, wherein the reflector reflects infrared light; wherein the backlight further comprises: a polarization protecting light guide; and an infrared polarizer, wherein the infrared polarizer is positioned between the plurality of light sources and the polarization-preserving light guide; wherein the visible opaque layer is transparent to infrared light, and wherein the visible opaque layer appears black under visible light; wherein the visibly opaque layer is transparent to infrared light, and wherein the visibly opaque layer is non-black under visible light; wherein the plurality of light sources comprises at least one infrared emitting light source and at least one visible light emitting light source; wherein the liquid crystal display is a reflective liquid crystal display, and wherein the illumination unit is a front light; wherein the front light is located between the visible light front polarizer and the liquid crystal display unit; wherein from the perspective of the viewer, the front light is in front of the visible front polarizer; wherein the front light further comprises a polarization protecting light guide and an infrared-capable polarizer, and wherein the infrared-capable polarizer is located between the illumination unit and the polarization protecting light guide; wherein the plurality of light sources comprises at least one infrared emitting light source and at least one visible light emitting light source.

In another exemplary embodiment, an infrared light readable liquid crystal display system for an electronic license plate, comprises: a liquid crystal display, comprising: a liquid crystal display unit comprising: a liquid crystal layer for controlling polarization states of visible light and infrared light; a front substrate; a rear substrate; and wherein the liquid crystal layer is located between the front substrate and the rear substrate; a visible light front polarizer, wherein the visible light front polarizer is transparent to infrared light; and a reflective rear polarizer for polarizing visible light and infrared light; a lighting unit comprising a plurality of light sources, wherein at least one of the plurality of light sources emits infrared light; a plurality of light sensors, wherein at least one of the plurality of light sensors is sensitive to infrared light; and an electronic circuit capable of driving at least one of the plurality of light sources emitting infrared light.

In this embodiment, the infrared light readable liquid crystal display system for an electronic license plate further comprises a microcontroller for receiving input from at least one of the plurality of light sensors that is sensitive to infrared light, and wherein based on the input, the microcontroller controls at least one of the plurality of light sources that emits infrared light; further comprising an independent circuit for receiving an input from at least one of the plurality of light sensors that is sensitive to infrared light, and wherein based on the input, the independent circuit controls at least one of the plurality of light sources that emits infrared light; wherein the plurality of light sources comprises at least one infrared emitting light source and at least one visible light emitting light source; and wherein the liquid crystal display maintains a stable visible image without requiring more than one refresh per second.

In another example embodiment, a method of operating an infrared light readable liquid crystal display system includes: providing a liquid crystal display, the liquid crystal display comprising: a liquid crystal display unit comprising: a liquid crystal layer for controlling polarization states of visible light and infrared light; a front substrate; a rear substrate; and wherein the liquid crystal layer is located between the front substrate and the rear substrate; a visible light front polarizer, wherein the visible light front polarizer is transparent to infrared light; and a reflective rear polarizer for polarizing visible light and infrared light; providing a lighting unit comprising a plurality of light sources, wherein at least one of the plurality of light sources emits infrared light; and controlling the plurality of light sources based on the external stimulus.

In another example embodiment, a method of operating an infrared light readable liquid crystal display system for an electronic license plate includes: providing a liquid crystal display, the liquid crystal display comprising: a liquid crystal display unit comprising: a liquid crystal layer for controlling polarization states of visible light and infrared light; a front substrate; a rear substrate; and wherein the liquid crystal layer is located between the front substrate and the rear substrate; a visible light front polarizer, wherein the visible light front polarizer is transparent to infrared light; and a reflective rear polarizer for polarizing visible light and infrared light; providing a lighting unit comprising a plurality of light sources, wherein at least one of the plurality of light sources emits infrared light; providing a plurality of light sensors, wherein at least one of the plurality of light sensors is sensitive to infrared light; providing an electronic circuit capable of driving at least one light source emitting infrared light; and controlling the plurality of light sources with the electronic circuit based on lighting conditions from the plurality of light sensors.

Other objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description and the accompanying drawings.

Drawings

Fig. 1 shows a typical prior art backlight for a transmissive or transflective display.

Fig. 2 shows a typical prior art front light for use in conjunction with a reflective display.

Fig. 3 shows a backlit liquid crystal display according to U.S. patent No. 9,190,004.

Fig. 4 shows the prior art in accordance with U.S. patent No. 9,229, 268, which is an improvement over U.S. patent No. 9,190, 004 in that it eliminates the undesirable mirror-like appearance.

Fig. 5A shows an exemplary embodiment of the present invention, which is comprised of an infrared transmissive LCD display having a reflective rear polarizer and a backlight supported by an absorber.

Fig. 5B shows an exemplary embodiment of the present invention, which is comprised of an infrared transmissive LCD display having a reflective rear polarizer and a backlight supported by a mirror.

Fig. 6 shows an exemplary embodiment of the present invention, which is comprised of an infrared transmissive LCD display having a polarization protecting backlight and a rear polarizer absorbing visible light.

FIG. 7 shows an exemplary embodiment of the present invention consisting of a reflective LCD display with unpolarized IR front light, a reflective rear polarizer, and an IR analyzer on the camera.

Fig. 8 shows an exemplary embodiment of the present invention, which is comprised of a reflective LCD display having polarization-preserving front light and a reflective rear polarizer.

Fig. 9 shows an exemplary embodiment of the present invention, which is comprised of a digital license plate display having a reflective LCD, front light, and combined IR/visible light illumination.

Fig. 10 shows in block diagram form an exemplary embodiment of the present invention consisting of a display system.

FIG. 11 illustrates, in block diagram form, one example embodiment of the present invention consisting of an alternative layout of a display system.

Fig. 12 shows an example circuit consisting of an IR sensing board and an LED driver board that can be used to detect light and drive an IR LED without the need to introduce a microcontroller.

Detailed Description

The following is a detailed description of embodiments, which are presented to illustrate the principles of the invention. The examples are provided to illustrate aspects of the invention, but the invention is not limited to any examples. The scope of the invention includes many alternatives, modifications, and equivalents. The scope of the invention is limited only by the claims.

Although numerous specific details are set forth in the following description in order to provide a thorough understanding of the invention, the invention may be practiced according to the claims without some or all of these specific details.

Various embodiments will be described in detail with reference to the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. References to specific examples and implementations are for illustrative purposes, and are not intended to limit the scope of the claims.

Background and prior art

Fig. 1 shows a typical prior art backlight 100 for a transmissive or transflective display 110. From the perspective of the viewer, the backlight 100 is placed behind the display 110. Although other backlight designs and principles exist, the description should be given here using examples of edge-lit light guide based backlights. The backlight 100 comprises a light guide 140, the light guide 140 typically being made of a transparent polymer such as PMMA, PC or glass.

The light guide 140 is similar to the display 110 with respect to length and width dimensions and serves the purpose of propagating light uniformly across the display 110. The light guide 140 has features that may be in the body or on the surface. Such features on the surface may be pits or prisms, which may be random or regularly distributed alternating materials in the body. The features are designed such that the light 150 exits the light guide 140 in a defined direction and has a uniform intensity distribution, as shown in fig. 1.

The features may send light 150 directly to display 110 or from the perspective of observer 160 first to a reflector behind light guide 140 from which light 150 is reflected toward display 110. The light guide 140 may incorporate other optical functions such as diffusion or light shaping and guiding, or these functions may be added to a separate component (typically a film) placed in close proximity to the light guide 140 or adhered to the light guide 140.

The illuminator 130 is positioned along one or more edges of the light guide 140. In one embodiment, illuminator 130 may be a side-evoked LED. Importantly, the light 150 is efficiently coupled into the light guide 140 without excessive waste. This is achieved by the design of the interface between the illuminator 130 and the light guide 140, as well as the design of the light source, such that the emitted light 150 leaves the light source in a useful angular range. Typically, the flexible printed circuit 120 provides current to operate the luminaires, which may be electrically arranged in series, parallel, or in smaller series parallel groups.

In one embodiment, the LEDs comprising illuminator 130 may be selected to emit white, red, green, blue, or Infrared (IR) light, or any combination thereof. One backlight may have a plurality of LEDs of different emission wavelengths.

Fig. 2 shows a typical front light 200 for use in conjunction with a reflective display, such as reflective display 210. The elements of the front light 200 are similar to those of the backlight 100 and include a flexible printed circuit board 220, an illuminator 230, and a light guide 240. However, in contrast to backlight 100, front light 200 is positioned in front of display 210 from the perspective of viewer 260. An additional requirement for the front light 200 is that the lamp 150 must be directed exclusively towards the display 210 and that the front light 200 must allow a clear, sharp and accurate color display image.

Fig. 3 shows a liquid crystal display 300 with a backlight 310 according to U.S. patent No. 9,190,004. The backlight 310 is disposed behind the display 300. The backlight 310 includes an LED light source 320, a light guide plate 340, a reflector 350 behind the light guide plate 340, and an optional light shaping film 330, the light shaping film 330 being located between the light guide plate 340 and the display 300. The display 300 includes a front polarizer 360, an LCD unit 370, and a rear polarizer 380. The LCD unit 370 further includes a liquid crystal layer 390 dispersed between the front substrate 392 and the rear substrate 394.

U.S. patent No. 9,190,004 teaches that the LED light source 320 can emit visible and/or IR light, and that the front polarizer 360 and the rear polarizer 380 must be capable of polarizing both visible and infrared light in order for the display 300 to have contrast in the visible and infrared regions of the electromagnetic spectrum. Typical lcd polarizers 360 and 380 operate only in the visible range, where they absorb one polarization while transmitting the other. Furthermore, it would be necessary to add an anisotropic IR absorbing chromophore to the visible polarizer, and there is no disclosure of how this can be achieved, nor is such additional chromophore disclosed that would reduce the visible light transmission and thus result in a much darker display.

U.S. patent No. 9,191,004 teaches that certain reflective polarizers may be used, such as wire grid polarizers, birefringent polarizers, or cholesteric liquid crystal polarizers, which operate in both the visible and IR regions. A disadvantage of such polarizers is that they reflect rather than absorb unwanted polarization. While this may be addressed in the rear polarizer 380, it is undesirable in the front polarizer 360 because it gives the display a mirror-like appearance. The bright image areas in such a display appear very bright, while the dark image areas appear as metal mirrors. The observer will see both the display image and the reflected scene superimposed on each other.

The backlight 310 also includes an LED light source 320, a light guide plate 340, a reflector 350 behind the light guide plate 340, and an optional light shaping film 330 between the light guide plate 340 and the display 400.

The backlight 310 is disposed behind the liquid crystal display 400, and the liquid crystal display 400 includes an LCD unit 370, a sequential stack of two front polarizers including a visible front polarizer 460 and an IR front polarizer 465, and a sequential stack of two rear polarizers including a visible rear polarizer 480 and an IR rear polarizer 485. The LCD unit 370 further includes a liquid crystal layer 390 dispersed between the front substrate 392 and the rear substrate 394.

In one embodiment, each of the front polarizers 460 and 465 and the rear polarizers 480 and 485 are reflective, such as wire grid polarizers, birefringent polarizers, or cholesteric liquid crystal polarizers that operate in both the visible and IR regions. The other of the front polarizers 460 and 465 and the rear polarizers 480 and 485 are standard lcd polarizers that operate only in the visible range by absorbing one polarization while transmitting the other polarization. This is possible because standard absorbing polarizers are transparent to both polarization states in the IR region.

Thus, in the infrared region, the display 400 remains a mirror-like display, while in the visible region, the display appears to have black and bright image areas as intended. The reflected infrared light will not be visible to a human observer, whereas the infrared devices used to view the display must be arranged and designed such that the reflected unwanted polarization does not impair the image quality or functionality of the IR system, such as by using an optical pattern or character recognition system. The use of two front polarizers 460 and 465 and two rear polarizers 480 and 485, instead of one for each, adds two expensive components to the display 400 and, since neither type of polarizer is 100% transmissive in the visible region of the electromagnetic spectrum, the brightness of the display is reduced by the second polarizer, which is unnecessary for visible light.

Furthermore, neither U.S. patent No. 9,190,004 nor U.S. patent No. 9,229,268 allow the use of reflective displays, which cannot operate in a backlight. Neither patent teaches the use of front light, but if the proposed structure is illuminated with front light and a reflector is necessary for the back, the display will be very dark because the light will pass through a series of eight polarizer layers, each absorbing a substantial portion of the light.

Accordingly, the present disclosure describes systems and methods for IR readable transmissive and reflective displays that do not have a mirror-like appearance and that do not cause unwanted darkening of the display due to the continuously stacked polarizers.

Infrared transmissive LCD displays 500A and 500B with reflective rear polarizer 510 and unpolarized infrared backlights 505A and 505B

Fig. 5A shows an exemplary embodiment of the invention, consisting of an infrared transmissive LCD display 500A with a backlight 505A. As an infrared transmissive display, the LCD display 500A generally transmits infrared light and reflects surrounding visible light.

The backlight 505A includes a light source such as an LED 515 that emits visible and infrared light, a light guide 520, an absorber 525 positioned behind the light guide 520 from the vantage point of the viewer 530, and in one embodiment, a light directing and diffusing film 535 positioned in front of the light guide 520 and behind the LCD display 500A. The optional light directing and diffusing film 535 may include optical films such as retarder films, compensation films, and other light management films that optimize device performance.

LCD display 500A also includes an LCD cell 540 positioned between a visible front polarizer 545 and an IR-capable reflective rear polarizer 510. The LCD unit 540 includes a liquid crystal layer 550 dispersed between a front substrate 555 and a rear substrate 560.

The front polarizer 545 is an absorbing type that absorbs visible light of unwanted polarization while transmitting visible light of the desired polarization and all infrared light that is independent of polarization. The rear polarizer 510 is reflective, such as a wire grid polarizer, a birefringent polarizer, or a cholesteric liquid crystal polarizer, which operate in both the visible and IR regions.

An infrared sensitive image capturing or recording device, such as a camera 570, is directed toward the LCD display 500A. In one embodiment, the camera 570 comprises an infrared camera. The camera 570 includes a lens 575 for focusing the display image onto a sensor element (not shown) inside the camera 570. It also includes an IR analyzer 580 to avoid glare from reflective surfaces, such as polarizers used in photographic equipment, to reduce glare, the function of which is optimized for IR wavelengths only.

Also facing the LCD display 500A is a viewer 530 that views the LCD display 500A via light reflection from the visible ambient light source 590. The ambient light source 590 may be diffuse sunlight, direct sunlight, indoor light, light from a dedicated illumination source, and the like.

In fig. 5-11, unpolarized light 592 is represented by solid arrows, dotted lines represent a first polarization 594, such as a linear s-polarization or a circular l-polarization, and dashed lines represent a second polarization orthogonal to the first polarization 596, such as a linear p-polarization or a circular r-polarization.

In the visible light view of the LCD display 500A, unpolarized light 592 from the ambient light source 590 enters the LCD display 500A. The portion of unpolarized light 592 having the undesired polarization is absorbed by the front polarizer 545. Light of a desired polarization is transmitted through front polarizer 545 into LCD display 500A, where it either maintains its polarization 596 or is deformed or becomes orthogonal 594 in liquid crystal layer 550, depending on the state of liquid crystal layer 550.

Unchanged light is transmitted through the reflective rear polarizer 510 and backlight 505A until it is absorbed by the absorber 525. The corresponding display area appears black or dark to the viewer 530. Light having the altered polarization state is reflected by the rear polarizer 510 and changes back to its original polarization state in the liquid crystal layer 550 and thus has the correct polarization state to pass through the front polarizer 545 and then propagate to the viewer 530. The corresponding area of LCD display 500A appears bright to viewer 530. For visual observation of light from ambient light source 590, backlight 505A is not required, however, absorber 525 must be provided.

For infrared viewing, the IR LED 515 is activated. Unpolarized IR light from IR LED 515 propagates through light guide 520 and uniformly illuminates LCD display 500A from behind. After passing through the light shaping and diffusing film 535, the light is polarized in the reflective rear polarizer 510 because only one polarization state is transmitted and the other polarization state is reflected. The reflected portion of the light returns into the light guide 520 and may be absorbed in the absorber 525.

Polarized light passes through the liquid crystal layer 550 and, depending on the arrangement of liquid crystals in the liquid crystal layer 550, the light remains polarized or changes to another polarization state. The "bright" and "dark" areas of the image emit equal amounts of light, but have different polarization states. One of these states may pass through the IR analyzer 580 of the camera 570 while the other polarization is rejected.

Thus, the bright and dark regions are projected by lens 575 onto the image sensor within camera 570, corresponding to the polarization state emitted from the corresponding region of the display. If desired, the contrast of the image may be electronically reversed prior to image analysis or prior to displaying the image on LCD display 500A. Because only one polarizer is used on either side of LCD display 500A, there is no additional cost, thickness, and excessive darkening of the brightness of LCD display 500A. Because the front polarizer 545 is absorptive, there is no mirror-like image display surface.

In alternative embodiments, the backlight 505A may be used in combination with visible light from the LEDs 515. In this embodiment, the visible light image using the backlight 505A has an inverted contrast. Such a display device may use a visible light sensor (not shown) and activate the backlight 505A, thereby simultaneously electronically reversing the contrast of the image so that an observer sees the appropriate contrast (two reversals).

It will be apparent to those skilled in the art that the LCD display 500A may operate with the transmission axes of the front and rear polarizers 545, 510 orthogonal (crossed) or parallel, and the liquid crystal layer 550 is arranged to remain polarized when powered on or off or in one or the other stable states. This results in several possible alternative embodiments, commonly referred to as normally white and normally black with direct or opposite contrast.

Fig. 5B shows an alternative embodiment of the LCD display 500A shown in fig. 5A. In fig. 5B, the backlight 505B has a reflector 527 as the element furthest from the observer 530, rather than the absorber 525 in the backlight 505A. In this embodiment, the LCD display 500B has a visible opaque (black) layer 526 added to the LCD display 500B near the backlight 505B. The visible opaque (black) layer 526 is transparent to infrared light, but absorbs visible light. The visibly opaque (black) layer 526 may be printed with a special dye, such as an epoight dye, or it may be formed of a visible light absorbing polarizer with its transmission axis orthogonal (crossed) to the transmission axis of the reflective rear polarizer 510. In other embodiments, the visible opaque (black) layer 526 may include other colors, such as an IR transmissive blue opaque layer, which results in embodiments of the LCD display 500B having blue and white contrast instead of black and white contrast.

Since the absorber 525 is closer to the liquid crystal layer 550, the lcd display 500B has an advantage of reducing parallax shadows in visible light operation. In infrared operation, the reflected polarization of the infrared light at the rear polarizer 510 is recycled in the backlight 505B. This increases the efficiency of the backlight 505B.

An infrared transmissive LCD display 600 having a rear polarizer 610 that absorbs visible light and a polarized infrared backlight 605

Fig. 6 shows another embodiment of the present invention. The infrared transmissive LCD display 600 is similar to the LCD display 500A except that the reflective rear polarizer 510 has been replaced with a rear polarizer 610 that absorbs visible light. Backlight 605 is similar to backlight 505B except that an IR polarizer 612 is added between IR LED 515 and light guide 620. In this embodiment, light guide 620 is polarization-protected.

For visible light viewing, unpolarized light 592 from ambient light source 590 enters LCD display 600. The portion of unpolarized light 592 having the undesired polarization is absorbed by the front polarizer 645. Light of a desired polarization is transmitted into LCD display 600, wherein in liquid crystal layer 550, the light either maintains its polarization 596 or has its polarization distorted or becomes orthogonal 594, depending on the alignment of the liquid crystals in liquid crystal layer 550. The unaltered light is absorbed by the rear polarizer 610. The corresponding area of LCD display 600 appears black to viewer 530.

Light having the altered polarization state is transmitted through the rear polarizer 610 and the backlight 605 and is reflected by the reflector 527. The reflected light passes through the rear polarizer 610 and changes back to its original polarization state in the liquid crystal layer 550 and thus has the correct polarization state to pass through the front polarizer 545 and then propagate to the viewer 530. The corresponding area of the LCD display 600 appears bright to the viewer 530. For visual observation with light from ambient light source 590, backlight 605 is not required, but reflector 527 must be provided.

In an alternative embodiment, the backlight 605 may be used with an optional visible light source (not shown) if the ambient light is insufficient. In this case, unpolarized light 592 exiting light guide 620 is polarized by rear polarizer 610 and remains unchanged or changes its polarization state in liquid crystal layer 550, depending on the orientation of the liquid crystal. Light having the changed polarization state passes through the front polarizer 545 and reaches the viewer 530. The corresponding image area of the LCD display 600 appears bright to the viewer 530. Light having an unchanged polarization state is absorbed in the front polarizer 545. The corresponding image area of LCD display 600 appears dark to viewer 530. This arrangement does not result in contrast inversion.

For infrared viewing, the IR LED 515 is activated. Unpolarized IR light from IR LED 515 is polarized with IR polarizer 612 before entering light guide 620. Polarized IR light propagates through light guide 620 and uniformly illuminates LCD display 600 from behind.

After passing through the optional light shaping and diffusing film 535, the light passes unchanged through the rear polarizer 610, as this polarizer type is transparent to IR light. Polarized light passes through the liquid crystal layer 550, and in the liquid crystal layer 550, the light maintains its polarization or changes to another polarization state depending on the arrangement of the liquid crystals in the liquid crystal layer 550. The "bright" and "dark" areas of the image emit equal amounts of light, but have different polarization states. One of these states may pass through the IR analyzer 580 of the camera 570 while the other polarization is rejected.

Thus, the bright and dark regions are projected by lens 575 onto the image sensor within camera 570, corresponding to the polarization states emanating from the corresponding regions of LCD display 600. If desired, the contrast of the image may be electronically reversed prior to image analysis or prior to displaying the image on LCD display 600. Since only one polarizer is used on either side of the LCD display 600, there is no additional cost, thickness, and excessive darkening of the display brightness. Because the front polarizer 545 is absorptive, the LCD display 600 does not have a mirror-like image display surface.

Those skilled in the art will appreciate that alternative embodiments have the equivalent of parallel and crossed polarizers, as well as different liquid crystal director configurations, some of which may be more or less advantageous.

Reflective LCD display 700 having reflective rear polarizer 510 and unpolarized IR front light 705

Fig. 7 shows another embodiment of the invention, based on a reflective liquid crystal display 700. A front light 705 comprising an LED illuminator 515 and a light guide 720 is placed between the front polarizer 545 and the LCD unit 740. The front polarizer 545 is absorptive, acting on visible light, but appears transparent to infrared light. The rear polarizer 510 is reflective and acts on both visible and infrared light. Absorber 525 is located behind rear polarizer 510.

For visible light viewing, unpolarized light 592 from ambient light source 590 enters LCD display 700. A portion of the light having undesired polarization is absorbed by the front polarizer 545. Light of a desired polarization 596 is transmitted through the front polarizer 545 and the front light guide 720 and into the LCD display 700, where it either maintains its polarization or deforms its polarization or becomes orthogonal 594, depending on the alignment of the liquid crystals in the liquid crystal layer 750.

Unchanged light 596 is transmitted through reflective rear polarizer 510 and is absorbed in absorber 525. The corresponding area of the LCD display 700 appears dark to the viewer 530. The viewer 530 "sees" the black absorber 525. Light having the altered polarization state 594 is reflected by the rear polarizer 510, changes back to its original polarization state 596 in the liquid crystal layer 750, and thus has the correct polarization state to pass through the front polarizer 545, and then propagates to the viewer 530. The corresponding area of the LCD display 700 appears bright to the viewer 530. No front light 705 is needed for visual observation with light from the ambient light source 590.

In an alternative embodiment, if there is insufficient light from the ambient light source 590, front light 705 may be used between the visible light source and the light guide 720, along with an optional visible light source (not shown) and a visible light polarizer (not shown). In this case, polarized light exiting the light guide 720 takes the same path as ambient light after passing through the front polarizer 745.

In another alternative embodiment, front light 705 may be located before front polarizer 745. In this case, a visible light polarizer (not shown) between the visible light source (not shown) and the light guide 720 is not necessary.

For infrared viewing, the IR LED 515 is activated. Unpolarized IR light from the light source 590 propagates through the light guide 720 and uniformly illuminates the LCD display 700 from the front. Because the front polarizer 545 is transparent to IR light, in a modification of this embodiment, the front light 705 may also be in front of the front polarizer 545 from the vantage point of the viewer 530. Light exiting the light guide 720 toward the LCD display 700 passes unchanged through the liquid crystal layer 750 to the rear polarizer 510, where light of one polarization is reflected into the LCD display 700, while light of the other polarization is transmitted through the reflective rear polarizer 510 and absorbed in the absorber 525.

Polarized light reflected into LCD display 700 passes through liquid crystal layer 750, where the light remains polarized or changes to another polarization state depending on the arrangement of liquid crystals in layer 750. The front light 705 and front polarizer 545 are transparent to the IR light. The "bright" and "dark" areas of the image emit equal amounts of light, but have different polarization states. One of these states may pass through the IR analyzer 580 of the camera 570 while the other polarization is rejected.

Thus, the bright and dark regions are projected by lens 575 onto the image sensor within camera 570, corresponding to the polarization states emitted from the respective regions of display 700. If desired, the contrast of the image may be electronically reversed prior to image analysis or prior to displaying the image on LCD display 700. Since only one polarizer is used on either side of the LCD display 600, there is no additional cost, thickness, and excessive darkening of the display brightness. Because the front polarizer 545 is absorptive, the LCD display 600 does not have a mirror-like image display surface.

Reflective LCD display 800 having reflective rear polarizer 510 and polarized infrared front light 805

Fig. 8 shows another embodiment of the present invention. The reflective LCD display 800 is similar to the LCD display 700 except that an IR polarizer 812 is placed between the IR LED 515 and the front light guide 820. The front light guide 820 must be polarization maintaining.

The optical path and functional principle of the visible light observation of the LCD display 800 are the same as those of the LCD display 700. For infrared viewing, the IR LED 515 is activated. Unpolarized IR light from IR LED 515 is polarized with IR polarizer 812 at the light source. Polarized IR light propagates through polarization maintaining light guide 820 and uniformly illuminates LCD display 800 from the front. In another modification of this embodiment, the front light 805 may also be in front of the front polarizer 545 because the front polarizer 545 is transparent to IR light.

Polarized light exiting the light guide 820 into the LCD display 800 propagates unchanged to the liquid crystal layer 750, where the light either maintains its polarization or changes to another polarization state depending on the orientation of the liquid crystals in the liquid crystal layer 750. Unchanged light 596 is transmitted through reflective rear polarizer 510 and is absorbed in absorber 525. The corresponding area of the LCD display 800 is imaged via the lens 575 as a black area on the sensor inside the camera 570, as no light propagates to the camera.

Light having altered polarization state 594 is reflected by rear polarizer 510 and changes back to its original polarization state 596 on a second pass through liquid crystal layer 750. The front polarizer 545 is transparent to the IR light, so the light continues to the camera 570. The corresponding area of the LCD display 800 is imaged via the lens 575 as a bright area on the sensor inside the camera 570. In this configuration, the IR analyzer 580 is not required. If the camera system has an anti-glare IR polarizer 580, the polarization direction of the LCD display 800 must be configured such that the light 596 propagating to the camera 570 is substantially vertically polarized. This ensures that light 596 can pass through glare proof IR polarizer 580, which blocks horizontally polarized light.

Digital license plate with reflective LCD, front light and combined infrared and visible light illumination

Fig. 9 illustrates a display system 900, which in one embodiment is a display system for a digital license plate that requires visibility and IR optical pattern or character recognition. In this example, for display system 900, LCD display 800 is placed in housing 905. However, in other embodiments, LCD display 500, LCD display 600, or LCD display 700 may also be used in place of LCD display 800 in display system 900. The housing 905 includes a front lens 950, an IR light sensor 955 that is sensitive only to IR light, and a daylight sensor 960 that is sensitive only to visible light wavelengths. Further, in one embodiment, additional optional visible light LEDs 915 may be added.

A digital license plate (not shown) must be readable by an Automatic License Plate Recognition (ALPR) camera system 975. The ALRP camera system 975 includes one or more ALRP cameras 970 that work with visible and IR light and an infrared illuminator 980 that is adjacent to the ALRP cameras 970. The ALRP camera system 975 is optimized to read retroreflective license plates or license plates with diffuse reflection Lambertian and must be placed above or beside the roadway or on another vehicle.

Illumination from the IR illuminator 980 is coaxial with the ALPR camera 970. However, this light is reflected by the digital license plate substantially away from the ALPR camera 970, rather than back toward the ALPR camera 970. This requires the use of internal IR illumination of the license plate.

The display system 900 has suitable electronic circuitry (as shown in fig. 10) that activates the IR LED 515 when the IR light sensor 955 senses a rapid change in IR intensity, for example, if the LCD display 800 is flashed with an ALPR camera 970, or if a vehicle is driving into an IR flood illumination zone, a rapid change in IR intensity occurs. Such electronic circuitry may determine when to turn on the IR LED 515 based on the microcontroller unit and firmware.

Fig. 10 shows a display system 900 in the form of a block diagram 1000, which consists of: a microcontroller unit (MCU) 1040 connected to the battery 1010, a daylight sensor 960, an IR sensor 955, an optional peripheral 1050, an LCD display 800 and a lighting unit 1070, the lighting unit 1070 consisting of: a visible light LED 915, a second light source 1082, an IR LED 515, and a fourth light source 1086. The illuminator unit 1070 is used to illuminate the display 800. In alternative embodiments, additional light sources may also be used.

Alternatively, to reduce power consumption and speed up the response, such circuitry may connect the IR light sensor 955 to an operational amplifier that drives the current source of the IR LED 515, flashing the IR LED 515 back into synchronization with the flashing of light passing through the IR, without waking up the digital license plate and its microcontroller system from a low power state.

Fig. 11 shows such an alternative layout of a display system 900 in the form of a block diagram 1100, consisting of: an MCU 1040 connected only to the battery 1010, an optional peripheral 1050, and an LCD display 800. The light sensor circuit 1110 is directly connected to the battery and controls the illuminator unit 1070, the visible light LED 915, and the IR LED 515. The illuminator unit 1070 is used to illuminate the display 800.

Fig. 12 shows an example of a light sensor circuit 1110 that can be used to detect light and drive the IR LED 515, without involving the MCU 1040, which consists of an IR sensing plate 1210 and an LED drive plate 1220.

Returning to fig. 9, the internal IR illumination of the display system 900 represents an active response to an interrogation, toward the ALPR camera 970, producing a brighter image, lower signal-to-noise ratio, and thus greater accuracy of optical pattern or character recognition. The IR wavelength of the interrogation system and the IR wavelength of the active response are independent. The IR wavelength of the internal IR LED 515 illuminating the LCD display 800 may be selected to achieve the best possible contrast and accuracy in the IR image capture and recording system.

Reflective LCD display 800 may be a bi-stable or multi-stable LCD because of the lower power requirements of such displays as compared to displays that require constant updating. In one embodiment, the liquid crystal display maintains a stable visible image without requiring more than one refresh per second. One such bistable LCD type may be a pixel memory LCD, another bistable nematic LCD known as Binem, or a bistable nematic display known as ZBD.

The LCD display 800 may work with a reflective rear polarizer 510, such as 3M ^TM A multilayer polymer stack produced known as DBEF, a wire grid polarizer such as WGF produced by Nagase, or the like. Such reflective polarizers have useful contrasts from about 380nm to greater than 850 nm.

The front light guide 820 may be located on top of or below the front polarizer 845. An optional white light LED 915 for night-time visualization, controlled by a daylight sensor 960, is utilized, and a plurality of IR LEDs 515, which are required to be selected for one or more desired wavelengths depending on the location where such license plates are issued, are utilized to illuminate it from the edge. The automatic license plate recognition system operates at specific infrared wavelengths, such as 740nm, 850nm, 940nm, etc. For example, if satisfactory, the IR LEDs may include several 740nm and several 850nm LEDs. Those skilled in the art will appreciate that other combinations are possible.

Polarizer 812 may be a dye-type polarizer whose dye is selected for infrared operation and which is not required to have good transmission or polarization efficiency in the visible spectrum because it is not required for visible light to pass through it. Another suitable type of polarizer may be a wire grid polarizer or a multilayer stacked polarizer, as such a polarizer is simpler and easier to produce at lower cost than a wire-clip polarizer for the visible range.

The polarization-preserving light guide 820 may be made of a transparent polymer, glass, or a combination of different transparent materials, and may be coated with materials of different refractive indices.

Also shown in fig. 9 is the optical path 990 of observer 530 in night mode. If the daylight sensor 960 detects a dark environment, it may activate the visible light LED 915. Unpolarized light from the visible light LED 915 passes through the light guide 820, from where it is uniformly directed to the LCD display 800. Because the light is unpolarized, it passes unchanged through the display.

Some of the light is reflected at the rear polarizer 510, while light of undesired polarization passes through the rear polarizer 510 and is absorbed by the absorber 525. The reflected polarized light will remain unchanged or its polarization will be changed by the liquid crystal layer 850, depending on the orientation of the liquid crystals in the liquid crystal layer 750.

Light having an unchanged polarization state will be absorbed by the front polarizer 545. The corresponding image area of the LCD display 800 appears dark to the viewer 530. Light having the changed polarization state passes through the front polarizer 545 and reaches the viewer 530. The corresponding display area of the LCD display 800 appears bright. In alternative embodiments, front light 805 may also be placed in front of front polarizer 845. In this case, the visible light exiting the light guide 820 will first be polarized by the absorptive front polarizer 845 before being passed to the display in a similar manner. In yet another alternative embodiment, a visible light polarizer may be placed between the visible light LED 915 and the light guide 820.

While the invention has been specifically described in connection with certain embodiments thereof, it is to be understood that this is by way of illustration and not of limitation. Reasonable variations and modifications are possible in the scope of the foregoing disclosure and the drawings without departing from the spirit of the invention.

Claims

1. An infrared light readable liquid crystal display system comprising:

a liquid crystal display, comprising:

a liquid crystal display unit comprising:

a liquid crystal layer for controlling polarization states of visible light and infrared light;

A front substrate;

a rear substrate; and

wherein the liquid crystal layer is located between the front substrate and the rear substrate;

a visible light front polarizer, wherein the visible light front polarizer is transparent to infrared light; and

a reflective rear polarizer for polarizing visible light and infrared light; and

a lighting unit comprising a plurality of light sources, wherein at least one of the plurality of light sources emits infrared light.

2. The infrared light readable liquid crystal display system of claim 1, wherein the liquid crystal display is an infrared transmissive liquid crystal display, and wherein the illumination unit is a backlight.

3. The infrared light readable liquid crystal display system of claim 2, wherein the backlight further comprises an absorber that absorbs additional light.

4. The infrared light readable liquid crystal display system of claim 2, wherein:

the infrared transmissive liquid crystal display further includes a visible opaque layer transparent to infrared light; and

the backlight further comprises a reflector, wherein the reflector reflects infrared light.

5. The infrared light readable liquid crystal display system of claim 2, wherein the backlight further comprises:

a polarization protecting light guide; and

an infrared polarizer, wherein the infrared polarizer is positioned between the plurality of light sources and the polarization-preserving light guide.

6. The infrared light readable liquid crystal display system of claim 4, wherein the visible opaque layer is transparent to infrared light, and wherein the visible opaque layer appears black under visible light.

7. The infrared light readable liquid crystal display system of claim 4, wherein the visible opaque layer is transparent to infrared light, and wherein the visible opaque layer is non-black under visible light.

8. The infrared light readable liquid crystal display system of claim 2, wherein the plurality of light sources comprises at least one infrared emitting light source and at least one visible light emitting light source.

9. The infrared light readable liquid crystal display system of claim 1, wherein the liquid crystal display is a reflective liquid crystal display, and wherein the illumination unit is a front light.

10. The infrared light readable liquid crystal display system of claim 9, wherein the front light is located between the visible light front polarizer and the liquid crystal display unit.

11. The infrared light readable liquid crystal display system of claim 9, wherein the front light is in front of the visible light front polarizer from the perspective of a viewer.

12. The infrared light readable liquid crystal display system of claim 9, wherein the front light further comprises a polarization protecting light guide and an infrared-capable polarizer, and wherein the infrared-capable polarizer is located between the illumination unit and the polarization protecting light guide.

13. The infrared light readable liquid crystal display system of claim 9, wherein the plurality of light sources comprises at least one infrared emitting light source and at least one visible light emitting light source.

14. An infrared light readable liquid crystal display system for an electronic license plate, comprising:

a liquid crystal display, comprising:

a liquid crystal display unit comprising:

a front substrate;

a rear substrate; and

a reflective rear polarizer for polarizing visible light and infrared light;

a lighting unit comprising a plurality of light sources, wherein at least one of the plurality of light sources emits infrared light;

a plurality of light sensors, wherein at least one of the plurality of light sensors is sensitive to infrared light; and

An electronic circuit capable of driving at least one of the plurality of light sources emitting infrared light.

15. The infrared light readable liquid crystal display system for an electronic license plate of claim 14, further comprising a microcontroller for receiving input from at least one of said plurality of light sensors that is sensitive to infrared light, and wherein based on said input, said microcontroller controls at least one of said plurality of light sources that emits infrared light.

16. The infrared light readable liquid crystal display system for an electronic license plate of claim 14, further comprising an independent circuit for receiving an input from at least one of the plurality of light sensors that is sensitive to infrared light, and wherein based on the input, the independent circuit controls at least one of the plurality of light sources that emits infrared light.

17. The infrared light-readable liquid crystal display system for an electronic license plate of claim 14, wherein the plurality of light sources comprises at least one infrared emitting light source and at least one visible light emitting light source.

18. The infrared light-readable liquid crystal display system for electronic license plates of claim 14, wherein the liquid crystal display maintains a stable visible image without requiring more than one refresh per second.

19. A method of operating an infrared light readable liquid crystal display system, comprising:

providing a liquid crystal display, the liquid crystal display comprising:

a liquid crystal display unit comprising:

a front substrate;

a rear substrate; and

a reflective rear polarizer for polarizing visible light and infrared light;

providing a lighting unit comprising a plurality of light sources, wherein at least one of the plurality of light sources emits infrared light; and

the plurality of light sources is controlled based on the external stimulus.

20. A method of operating an infrared light readable liquid crystal display system for an electronic license plate, comprising:

providing a liquid crystal display, the liquid crystal display comprising:

a liquid crystal display unit comprising:

a front substrate;

a rear substrate; and

A reflective rear polarizer for polarizing visible light and infrared light;

providing a lighting unit comprising a plurality of light sources, wherein at least one of the plurality of light sources emits infrared light;

providing a plurality of light sensors, wherein at least one of the plurality of light sensors is sensitive to infrared light;

providing an electronic circuit capable of driving at least one light source emitting infrared light; and

the plurality of light sources is controlled with the electronic circuit based on lighting conditions from the plurality of light sensors.