Patent ID: 12189231

DETAILED DESCRIPTION OF THE INVENTION

The following is a detailed description of embodiments to illustrate the principles of the invention. The embodiments are provided to illustrate aspects of the invention, but the invention is not limited to any embodiment. The scope of the invention encompasses numerous alternatives, modifications, and equivalents. The scope of the invention is limited only by the claims.

While numerous specific details are set forth in the following description 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 are used throughout the drawings to refer to the same or like parts. References made to particular examples and implementations are for illustrative purposes and are not intended to limit the scope of the claims.

BACKGROUND AND PRIOR ART

FIG.1shows a typical prior art backlight100as used with transmissive or transflective display110. The backlight100is placed behind the display110from a viewer's perspective. Although, other backlight designs and principles exist, here the description shall be given using the example of an edge-lit lightguide-based backlight. The backlight100comprises a lightguide140, which is typically made from a transparent polymer such as PMMA, PC or glass.

Lightguide140is similar in size with respect to length and width dimensions to display110and serves the purpose of spreading the light uniformly across the display110. The lightguide140has features that may be in the bulk or on the surface. Such features in the surface may be dimples or prisms, in the bulk they may be randomly or regularly distributed alternate materials. The features are designed as to cause the light150to leave the lightguide140in a defined direction and with a uniform intensity distribution as shown inFIG.1.

The features may send the light150directly to display110or first to a reflector behind the lightguide140from observer's160perspective, from where the light150is reflected towards the display110. Lightguide140may incorporate other optical functions, such as diffusion or light shaping and directing, or these functions may be added in separate components (usually sheets of films) placed into the immediate vicinity of the lightguide140or adhered to it.

Illuminators130are placed along one or more edges of the lightguide140. In one embodiment, illuminators130can be side firing LEDs. It is important that the light150is effectively coupled into the lightguide140without excessive waste. This is achieved by the design of the interface between illuminator130and lightguide140as well as the design of the light sources such that the emitted light150exits the light source in a useful range of angles. Typically, a flexible printed circuit120provides the electrical current to operate the illuminators, which may be arranged electrically in series, parallel, or in parallel groups of smaller series.

In one embodiment, the LEDs comprising the illuminators130may be selected to emit white, red, green, blue, or infrared (IR) light or any combination thereof. One backlight may have LEDs with multiple, different emission wavelengths.

FIG.2shows a typical front light200as used in conjunction with reflective displays, such as reflective display210. The elements of front light200are similar to that of backlight100and include flexible printed circuit board220, illuminators230, and lightguide240. However, front light200is placed in front of the display210from an observer's260perspective, as compared to backlight100. Additional requirements for front light200are that the light150must be exclusively directed towards the display210and that the front light200must allow for a clear, sharp, and accurate color display image.

FIG.3shows a liquid crystal display300with backlight310according to U.S. Pat. No. 9,190,004. The backlight310is arranged behind the display300. The backlight310comprises LED light source320, a lightguide plate340, a reflector350behind the lightguide plate340, and optional light shaping films330, located between the lightguide plate340and the display300. The display300comprises a front polarizer360, an LCD cell370, and a rear polarizer380. The LCD cell370also comprises a liquid crystal layer390interspersed between a front substrate392and a rear substrate394.

U.S. Pat. No. 9,190,004 teaches that the LED light source320may emit visible and/or IR light and that the front polarizer360and rear polarizer380must be able to polarize both visible and infrared light in order for the display300to have contrast in the visible and infrared region of the electromagnetic spectrum. Typical liquid crystal display polarizers360and380work only in the visible range, where they absorb one polarization while transmitting the other polarization. Moreover, adding anisotropic IR absorbing chromophores to visible light polarizers would be required, and it is not disclosed how this can be done nor that such additional chromophores would reduce the visible light transmission and thus lead to a much dimmer display.

U.S. Pat. No. 9,191,004 teaches that certain reflective polarizers can be used such as wire grid polarizer sheets, birefringent polarizer sheets, or cholesteric liquid crystal polarizer sheets, which work both in the visible and IR region. The disadvantage of such polarizers is that they reflect, not absorb the unwanted polarization. While that can be dealt with in the rear polarizer380, it is undesirable in front polarizer360as it gives the display a mirror-like appearance. Bright image areas in such a display appear bright, while dark image areas appear like a metallic mirror. An observer will see both the display image and a reflected scene superimposed onto each other.

FIG.4shows prior art in accordance with U.S. Pat. No. 9,229,268, which is an improvement of U.S. Pat. No. 9,190,004 as it eliminates the undesirable mirrorlike appearance.

The backlight310again comprises LED light source320, a lightguide plate340, a reflector350behind the lightguide plate340, and optional light shaping films330, located between the lightguide plate340and the display400.

Backlight310is placed behind liquid crystal display400comprising the LCD cell370, a sequential stack of two front polarizers comprising visible light front polarizer460and IR light front polarizer465, and a sequential stack of two rear polarizers comprising visible light rear polarizer480and IR light rear polarizer485. The LCD cell370also comprises a liquid crystal layer390interspersed between a front substrate392and a rear substrate394.

In one embodiment, one each of the front polarizers460and465, and the rear polarizers480and485is a reflective type, such as a wire grid polarizer sheet, a birefringent polarizer sheet, or a cholesteric liquid crystal polarizer sheet, which work both in the visible and IR region. The other one of the front polarizers460and465and the rear polarizers480and485is a standard liquid crystal display polarizer sheet, which work only in the visible range by absorbing one polarization, while transmitting the other polarization. This works because the standard absorbing polarizer sheets are transparent for both polarization states in the IR region.

Hence, in the infrared region the display400still is a mirror-like display, while in the visible region the display looks as expected with black and bright image areas. A human observer will not see the reflected infrared light, while infrared equipment used to view the display has to be arranged and designed such that the reflected unwanted polarization is not detrimental to the image quality or function of the IR system, such as by using an optical pattern or character recognition system. Using the two front polarizers460and465and the two rear polarizers480and485, rather than one of each, adds two costly elements to the display400and since both types of polarizers are not 100% transmissive in the visible region of the electromagnetic spectrum, the brightness of the display is diminished by the second polarizer, which is unnecessary for visible light.

In addition, neither U.S. Pat. No. 9,190,004 nor U.S. Pat. No. 9,229,268 allow the use of a reflective display, which cannot be operated with a backlight. Neither patent teaches the use of a front light, but if the proposed structure were illuminated with a front light and backed with the necessary reflector, the display would be very dim as the light would travel through a sequence of eight polarizer layers, each absorbing a significant portion of the light.

Thus, this disclosure describes systems and methods of IR readable transmissive and reflective displays without a mirror appearance and without the unwanted dimming of the display due to sequential stacks of polarizers.

Infrared Transmissive LCD Display500A and B with a Reflective Rear Polarizer510and a Non-Polarized Infrared Backlight505A and B

FIG.5Ashows one example embodiment of this invention consisting of an infrared transmissive type LCD display500A with backlight505A. As an infrared transmissive type display, LCD display500A is generally transmissive to infrared light and reflective of ambient visible light.

Backlight505A comprises light sources such as LEDs515, emitting visible and infrared light, a lightguide520, an absorber525placed behind lightguide520from an observer's530vantage point, and in one embodiment light directing and diffusing films535in front of the lightguide520and behind LCD display500A. Optional light directing and diffusing films535may include optical films such as retardation films, compensation films, and other light management films that optimize the performance of the device.

LCD display500A further comprises an LCD cell540between a visible light front polarizer545and IR capable reflective rear polarizer510. LCD cell540comprises a liquid crystal layer550interspersed between a front substrate555and a rear substrate560.

The front polarizer545is of an absorptive type, which absorbs visible light of the unwanted polarization, while transmitting visible light of the desired polarization as well as all infrared light irrespective of polarization. The rear polarizer510is a reflective type such as a wire grid polarizer sheet, a birefringent polarizer sheet or a cholesteric liquid crystal polarizer sheet, which work both in the visible and IR regions.

An infrared sensitive image capture or recording device such as camera570is directed towards LCD display500A. In one embodiment, camera570comprises an infrared camera. Camera570comprises a lens575to focus the display image onto the sensor element (not shown) inside the camera570. It also comprises an IR analyzer580to avoid glare for reflective surfaces like the polarizers used in photographic equipment to reduce glare, only with its function optimized for IR wavelengths.

Also directed towards LCD display500A is observer530viewing LCD display500A via reflection of light from visible environmental light source590. Environmental light source590may be diffuse daylight, direct sunlight, room light, light from a dedicated illumination source or similar.

InFIGS.5-11, unpolarized light592is shown as arrows with a solid line, a dotted line illustrates a first polarization594, such as linear s-polarization or circular l-polarization, and a dashed line illustrates the second orthogonal to the first polarization596, such as linear p-polarization or circular r-polarization.

In visible light observation of LCD display500A, unpolarized light592from the environmental light source590enters LCD display500A. The portion of the unpolarized light592with the undesired polarization is absorbed in front polarizer545. Light of the desired polarization is transmitted through front polarizer545into LCD display500A, where in the liquid crystal layer550the light either retains its polarization596or has its polarization morphed or changed into the orthogonal polarization594, depending on the state of the liquid crystal layer550.

Unchanged light is transmitted through reflective rear polarizer510and backlight505A until it gets absorbed in absorber525. The corresponding display area looks black or dark to the observer530. Light with a changed polarization state is reflected by the rear polarizer510and changed back to its original polarization state in the liquid crystal layer550, and therefore has the correct polarization state to pass the front polarizer545and then travels to the observer530. The corresponding area of LCD display500A appears bright to the observer530. For visible observation of light from environmental light source590, backlight505A is not required, however, absorber525must be provided.

For infrared observation, the IR LEDs515are activated. Unpolarized IR light from IR LEDs515spreads through lightguide520and illuminates LCD display500A uniformly from behind. After passing the light shaping and diffusing films535, the light is polarized in reflective rear polarizer510as only one polarization state is transmitted, while the other polarization state is reflected. The reflected portion of the light is returned into lightguide520and may get absorbed in absorber525.

The polarized light travels through liquid crystal layer550and depending on the alignment of the liquid crystals in liquid crystal layer550, the light retains its polarization or changes to another polarization state. ‘Bright’ and ‘Dark’ areas of the image emit the same amount of light but with different polarization states. One of these states can pass IR analyzer580of camera570while the other polarization is rejected.

Therefore, bright and dark areas are projected by lens575onto the image sensor inside camera570, corresponding to the polarization state emitting from the respective areas of the display. If desired, the contrast of the image can electronically be inverted before image analysis or before displaying the image on LCD display500A. Because only one polarizer is used on either side of LCD display500A there is no additional cost, thickness, and undue dimming of the brightness of LCD display500A. Because the front polarizer545is an absorbing type, there is no mirror-like image display surface.

In an alternative embodiment, backlight505A can be used in conjunction with visible light from LEDs515. In this embodiment, the visible light image using backlight505A has inverted contrast. Such a display arrangement may use a visible light sensor (not shown) and activate backlight505A while simultaneously electronically reversing the contrast of the image, so that the observer sees the proper contrast (twice inverted).

It should be clear to those skilled in the art that LCD display500A can be operated with the transmission axes of front polarizer545and rear polarizer510either orthogonal (crossed) or parallel and with the liquid crystal layer550being arranged to retain the polarization either when powered or not or either in one or another stable state. This leads to several possible alternative embodiments often referred to as normally white and normally black with direct or inverse contrast.

FIG.5Bshows an alternative embodiment of LCD display500A shown inFIG.5A. InFIG.5B, backlight505B has reflector527as the element furthest from observer530, instead of absorber525in backlight505A. In this embodiment, LCD display500B has visibly opaque (black) layer526added to LCD display500B near backlight505B. Visibly opaque (black) layer526is transparent for infrared light but absorbs visible light. Visibly opaque (black) layer526can be printed with special dyes such as Epolight dyes, or it can be formed by a visible light absorptive polarizer with its transmission axis orthogonal to (crossed) that of the reflective rear polarizer510. In other embodiments, visibly opaque (black) layer526can comprise other colors such as a blue opaque layer that transmits IR which leads to an embodiment of LCD display500B with a blue and white contrast rather than a black and white contrast.

LCD display500B has the advantage of reduced parallax shadow in visible light operation as absorber525is closer to liquid crystal layer550. In infrared operation, the reflected polarization of the infrared light at the rear polarizer510is recycled in the backlight505B. This increases the efficiency of backlight505B.

Infrared Transmissive LCD Display600with Absorptive Visible Light Rear Polarizer610and Polarized Infrared Backlight605

FIG.6shows another embodiment of the invention. Infrared transmissive type LCD display600is similar to LCD display500A except reflective rear polarizer510has been replaced by absorptive visible light rear polarizer610. Backlight605is similar to backlight505B except the IR polarizer612is added between the IR LEDs515and lightguide620. In this embodiment, lightguide620is polarization conserving.

For visible light observation, unpolarized light592from environmental light source590enters LCD display600. The portion of the unpolarized light592with the undesired polarization is absorbed in front polarizer645. Light of the desired polarization is transmitted into LCD display600, where, in the liquid crystal layer550, the light either retains its polarization596or has its polarization morphed or changed into orthogonal polarization594, depending on the alignment of the liquid crystals in liquid crystal layer550. Unchanged light is absorbed in rear polarizer610. The corresponding area of LCD display600looks black to observer530.

Light with a changed polarization state is transmitted through rear polarizer610and backlight605and is reflected by reflector527. The reflected light passes through rear polarizer610and is changed back to its original polarization state in the liquid crystal layer550and therefore has the correct polarization state to pass through front polarizer545and then travel to the observer530. The corresponding area of LCD display600appears bright to observer530. For visible observation with light from environment light source590, backlight605is not required, but reflector527must be provided.

In an alternative embodiment, backlight605can be used with optional visible light sources (not shown) if there is insufficient environmental light. In this case, unpolarized light592exiting lightguide620is polarized by rear polarizer610and remains unchanged or has its polarization state changed in the liquid crystal layer550, depending on the orientation of the liquid crystals. Light with a changed polarization state passes through front polarizer545and reaches the observer530. The corresponding image area of LCD display600looks bright to observer530. Light with an unchanged polarization state gets absorbed in the front polarizer545. The corresponding image area of LCD display600looks dark to observer530. This arrangement does not cause a contrast inversion.

For infrared observation, IR LEDs515are activated. Unpolarized IR light from IR LEDs515is polarized with IR polarizer612before entering lightguide620. Polarized IR light spreads through lightguide620and illuminates LCD display600uniformly from behind.

After passing the optional light shaping and diffusing films535, the light passes rear polarizer610unchanged as this polarizer type appears transparent to IR light. The polarized light travels through liquid crystal layer550where, depending on the alignment of the liquid crystals in liquid crystal layer550, the light retains its polarization or changes to another polarization state. ‘Bright’ and ‘Dark’ areas of the image emit the same amount of light but with different polarization states. One of these states can pass IR analyzer580of camera570, while the other polarization is rejected.

Therefore, bright and dark areas are projected by lens575onto the image sensor inside camera570, corresponding to the polarization state emitting from the respective areas of LCD display600. If desired, the contrast of the image can electronically be inverted before image analysis or before displaying the image on LCD display600. Since only one polarizer is used on either side of LCD display600there is no additional cost, thickness, and undue dimming of the display brightness. Because front polarizer545is an absorbing type, there is no mirror-like image display surface for LCD display600.

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

Reflective LCD Display700with Reflective Rear Polarizer510and with Non-Polarized IR Front Light705

FIG.7illustrates another embodiment of the invention, based on a reflective type LCD display700. A front light705, comprising LED illuminators515and a light guide720, is placed between the front polarizer545and LCD cell740. The front polarizer545is absorptive, which works for visible light, but appears transparent to infrared light. Rear polarizer510is reflective and works for both visible and infrared light. Located behind the rear polarizer510is an absorber525.

For visible light observation, unpolarized light592from environmental light source590enters LCD display700. The portion of the light with the undesired polarization is absorbed in front polarizer545. Light of the desired polarization596is transmitted through front polarizer545and front lightguide720and into LCD display700, where, in the liquid crystal layer750, it either retains its polarization or has its polarization morphed or changed into the orthogonal polarization594, depending on the alignment of the liquid crystals in liquid crystal layer750.

Unchanged light596is transmitted through reflective rear polarizer510and gets absorbed in absorber525. The corresponding area of LCD display700looks dark to observer530. The observer530‘sees’ the black absorber525. Light with a changed polarization state594is reflected by rear polarizer510, changed back to its original polarization state596in liquid crystal layer750, and therefore has the correct polarization state to pass through front polarizer545and then travel to observer530. The corresponding area of LCD display700appears bright to observer530. For visible observation with light from environment light source590, front light705is not required.

In an alternative embodiment, front light705can be used with optional visible light sources (not shown) and a visible light polarizer (not shown) between the visible light sources and lightguide720if there is insufficient light from environmental light source590. In this case, polarized light exiting the lightguide720takes the same path as environmental light after passing through front polarizer745.

In another alternative embodiment, front light705can be positioned in front of front polarizer745. In this case, the visible light polarizer (not shown) between visible light source (not shown) and lightguide720is not necessary.

For infrared observation, IR LEDs515are activated. Unpolarized IR light from the light source590spreads through lightguide720and illuminates LCD display700uniformly from the front. Because front polarizer545is transparent to IR light, in a modification of this embodiment, front light705can also be in front of front polarizer545from the vantage point of observer530. The light exiting lightguide720towards LCD display700travels unchanged through liquid crystal layer750to rear polarizer510, where light of one polarization is reflected into LCD display700while light of the other polarization is transmitted through reflective rear polarizer510and is absorbed in absorber525.

The polarized light reflected into LCD display700travels through the liquid crystal layer750where, depending on the alignment of the liquid crystal in layer750, the light retains its polarization or changes to another polarization state. The front light705and front polarizer545are transparent to IR light. ‘Bright’ and ‘Dark’ areas of the image emit the same amount of light but with different polarization states. One of these states can pass the IR analyzer580of camera570while the other polarization is rejected.

Therefore, bright and dark areas are projected by lens575onto the image sensor inside camera570, corresponding to the polarization state emitting from the respective areas of the display700. If desired, the contrast of the image can electronically be inverted before image analysis or before displaying the image on LCD display700. Since only one polarizer is used on either side of LCD display600there is no additional cost, thickness, and undue dimming of the display brightness. Because the front polarizer545is an absorbing type, there is no mirror-like image display surface for LCD display600.

Reflective LCD Display800with Reflective Rear Polarizer510and Polarized Infrared Front Light805

FIG.8illustrates another embodiment of the invention. Reflective type LCD display800is similar to LCD display700except IR polarizer812is placed between the IR LEDs515and front lightguide820. Front lightguide820must be polarization maintaining.

The light path and functional principle for visible light observation for LCD display800is the same as for LCD display700. For infrared observation, IR LEDs515are activated. Unpolarized IR light from IR LEDs515is polarized with IR polarizers812at the light source. Polarized IR light spreads through the polarization maintaining lightguide820and illuminates the LCD display800uniformly from the front. In another modification of this embodiment, because the front polarizer545is transparent to IR light, the front light805can also be in front of the front polarizer545.

The polarized light exiting lightguide820into LCD display800travels unchanged to liquid crystal layer750where, depending on the orientation of the liquid crystal in liquid crystal layer750, the light retains its polarization or changes to another polarization state. Unchanged light596is transmitted through reflective rear polarizer510and is absorbed in absorber525. The corresponding area of LCD display800is imaged via lens575as a black area onto the sensor inside camera570since no light is traveling to the camera.

Light with a changed polarization state594is reflected by rear polarizer510and is changed back to its original polarization state596upon passing liquid crystal layer750a second time. Front polarizer545is transparent to IR light, so the light continues to camera570. The corresponding area of LCD display800is imaged via lens575as a bright area onto the sensor inside camera570. In this configuration, IR analyzer580is not required. If the camera system has an antiglare IR polarizer580, the polarization directions of LCD display800must be configured such that the light596traveling to camera570is substantially vertically polarized. This ensures the light596can pass the antiglare IR polarizer580, which blocks horizontally polarized light.

Digital License Plate with Reflective LCD, Front Light and Combined Infrared and Visible Light Illumination

FIG.9illustrates display system900, which in one embodiment is the display system of a digital license plate that requires visible readability and IR optical pattern or character recognition. In this example, for display system900, LCD display800is placed into housing905. However, in other embodiments, LCD display500, LCD display600, or LCD display700could also be used in place of LCD display800in display system900. The housing905comprises a front lens950, an IR light sensor955sensitive only to IR light, and a daylight sensor960sensitive only to visible light wavelengths. In addition, in one embodiment, additional optional visible light LEDs915may be added.

Digital license plates (not shown) must be readable by automated license plate recognition (ALPR) camera system975. ALRP camera system975comprises one or more ALRP cameras970, working with visible and IR light and infrared light illuminators980, next to the ALRP camera970. ALRP camera system975is optimized to read retroreflective license plates or license plates with diffuse, Lambertian reflectance and is necessarily placed above or to the side of the roadway or on another vehicle.

The illumination from IR illuminator980is coaxial with ALPR camera970. Such light, however, is reflected by a digital license plate substantially away from ALPR camera970, rather than back towards ALPR camera970. This necessitates the use of internal IR illumination of the license plate.

Display system900has suitable electronic circuits (as shown inFIG.10) that activate IR LEDs515when IR light sensor955senses a rapid change in IR intensity, which occurs, for example, if LCD display800is being flashed with ALPR camera970or if a vehicle drives into an IR flood illumination zone. Such electronic circuits may be based on a microcontroller unit and firmware determining when to turn on IR LEDs515.

FIG.10shows display system900in the form of block diagram1000consisting of microcontroller unit (MCU)1040connected to battery1010, day light sensor960, IR sensor955, optional peripherals1050, LCD display800, and illuminator unit1070consisting of visible light LEDs915, second light source1082, IR LEDs515, and fourth light source1086. Illuminator unit1070is used to illuminate display800. In alternative embodiments, additional light sources may also be used.

Alternatively, for lower power consumption and faster response, such circuits may connect the IR light sensor955to an operational amplifier, which drives a current source for IR LEDs515, causing IR LEDs515to flash back in sync with being flashed by IR light without the digital license plate and its microcontroller system having to wake up from a low power state.

FIG.11shows such an alternative layout of display system900in the form of block diagram1100consisting of MCU1040connected only to battery1010, optional peripherals1050, and LCD display800. Light sensor circuit1110is connected directly to the battery and controls illuminator unit1070, visible light LEDs915, and IR LEDs515. Illuminator unit1070is used to illuminate display800.

FIG.12shows an example of light sensor circuit1110that can be used to detect light and drive the IR LEDs515without involvement of MCU1040, consisting of IR sensing board1210and LED driving board1220.

Turning back toFIG.9, the internal IR illumination of display system900represents an active response to interrogation, directed towards ALPR camera970resulting in a brighter image, lower signal-to-noise ratio and hence a better accuracy of the optical pattern or character recognition. The IR wavelength of the interrogating system and the IR wavelength of the active response are independent. The IR wavelength of internal IR LEDs515illuminating LCD display800can be chosen to achieve the best possible contrast and accuracy in the IR image capture and recording system.

The reflective LCD display800may be a bi-stable or multi-stable LCD due to the low power requirements of such displays compared to displays requiring constant updating. In one embodiment, the liquid crystal display maintains a stable visible image without being refreshed more than once per second. One such bistable LCD type may be a memory-in-pixel LCD, another may be a bistable nematic LCD known as Binem, or a bistable nematic display known as ZBD.

The LCD display800can work with a reflective rear polarizer510, such as a multilayer polymer stack available from 3M™ known as DBEF, a wire grid polarizer such as WGF from Nagase, or similar. Such reflective polarizers have usable contrast from about 380 nm to greater than 850 nm.

Front lightguide820may be located on top or below front polarizer845. It is illuminated from the edge with optional white light LEDs915for night visibility, controlled by daylight sensor960, and with a plurality of IR LEDs515selected for a desired wavelength or multiple desired wavelengths depending on the requirements of the location where such a license plate is issued. Automated license plate recognition systems operate at specific infrared wavelengths, such as 740 nm, 850 nm, 940 nm and others. For example, IR LED's may comprise several 740 nm and several 850 nm LEDs if that matches the requirement. One of skill in the art would understand that other combinations are possible as well.

Polarizer812may be a dye type polarizer with dye selected for infrared operation and it is not required to have good transmission or polarization efficiency in the visible spectrum as no visible light is required to pass through it. Another suitable type of polarizer may be a wire grid type polarizer or multi-layer stack polarizer as such polarizers are simpler and easier to produce at a lower cost than wire grip polarizers for the visible range.

Polarization preserving lightguide820may be made from transparent polymers, glass, or a combination of different transparent materials and may be coated with materials of a different refractive index.

Also shown inFIG.9is the light path990for observer530in night mode. If the daylight sensor960detects a dark environment, it may activate the visible light LEDs915. Unpolarized light from visible light LEDs915travels through the lightguide820from where it is directed uniformly towards LCD display800. Because the light is unpolarized it passes through the display unchanged.

Part of the light is reflected at rear polarizer510, while light of the undesirable polarization passes through rear polarizer510and is absorbed by absorber525. The reflected polarized light will either remain unchanged or its polarization will be changed by the liquid crystal layer850, depending on the orientation of the liquid crystals in liquid crystal layer750.

Light with an unchanged polarization state will be absorbed by front polarizer545. The corresponding image areas of LCD display800appear dark to observer530. Light with a change in polarization state passes front polarizer545and reaches observer530. Corresponding display areas of LCD display800appear bright. In an alternative embodiment, front light805can also be placed in front of front polarizer845. In this case visible light exiting lightguide820will first be polarized by the absorptive front polarizer845before passing to the display in an analogous fashion. In yet another alternative embodiment, a visible light polarizer can be placed between visible light LEDs915and lightguide820.

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