EP1285288A2 - Afficheurs d'informations a couleurs compensees - Google Patents

Afficheurs d'informations a couleurs compensees

Info

Publication number
EP1285288A2
EP1285288A2 EP01933259A EP01933259A EP1285288A2 EP 1285288 A2 EP1285288 A2 EP 1285288A2 EP 01933259 A EP01933259 A EP 01933259A EP 01933259 A EP01933259 A EP 01933259A EP 1285288 A2 EP1285288 A2 EP 1285288A2
Authority
EP
European Patent Office
Prior art keywords
polarizer
recited
layer
absorbing polarizer
illuminant
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Ceased
Application number
EP01933259A
Other languages
German (de)
English (en)
Inventor
Robert S. Moshrefzadeh
Keith Kotchick
Gregory Gilligan
Ikuko Ebihara
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
3M Innovative Properties Co
Original Assignee
3M Innovative Properties Co
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by 3M Innovative Properties Co filed Critical 3M Innovative Properties Co
Publication of EP1285288A2 publication Critical patent/EP1285288A2/fr
Ceased legal-status Critical Current

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Classifications

    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
    • G02F1/1333—Constructional arrangements; Manufacturing methods
    • G02F1/1335—Structural association of cells with optical devices, e.g. polarisers or reflectors
    • G02F1/133528—Polarisers
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
    • G02F1/1333—Constructional arrangements; Manufacturing methods
    • G02F1/1335—Structural association of cells with optical devices, e.g. polarisers or reflectors
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F2203/00—Function characteristic
    • G02F2203/09—Function characteristic transflective

Definitions

  • the present invention is directed generally to systems for displaying information, and more particularly to information display systems having selected color compensation to enhance the color of the image viewed by the user.
  • TN Twisted Nematic
  • STN Super Twisted Nematic
  • LCD Liquid Crystal Displays
  • These displays offer ease of manufacture, passive multiplexed addressing schemes, and cost structures aligned with the end application.
  • the display usually operates in a reflective/transflective mode such that ambient light provides the primary illumination for the non-emissive display.
  • ambient illumination eliminates the need for a constantly activated system light source, which is a problem with devices that use transmissive displays, such as laptop computers.
  • transflective displays incorporate a supplemental backlight, which is commonly user activated. This allows the device to realize smaller size, lighter weight, smaller battery size, and longer battery life, all factors important in a portable, handheld device.
  • a TN or STN LCD typically requires a compensation film to provide color correction due to dispersion effects within the display. Without the added compensation film, the display creates spectrally peaked light and dark states instead of the desired white and black states.
  • STN displays may use a wide variety of design prescriptions, with a unique compensation film providing optimum performance for each design. Even with the proper compensation film however, the TN or STN display still does not provide the desired white/black performance. Instead, most common displays manifest a green/black appearance. This deficiency is attributable in large part to the spectral performance of elements within the display.
  • the present invention relates to a polarizer that has spectral characteristics particularly well suited to use in an LCD display.
  • One particular embodiment of the invention is directed to an optical device, comprising an absorbing polarizer having a double pass color shift of
  • Another embodiment of the invention is directed to an optical device, comprising an absorbing polarizer having a double pass color shift of ⁇ x ⁇ 0.005 and ⁇ y ⁇ 0.002 and a double pass contrast modulation of at least 90% under illumination by an A-illuminant.
  • the polarizer also has a double pass color shift of ⁇ x ⁇ 0.005 and ⁇ y ⁇ 0.005 under illumination by a C-illuminant.
  • Another embodiment of the invention is directed to a device for displaying information, comprising two or more layers stacked together, at least one of the layers being an absorbing polarizer having a double pass color shift of
  • Another embodiment of the invention is directed to a rear projection screen having a dispersing layer stacked together with first absorbing polarizer, the first absorbing polarizer having a single pass color shift with at least one of ⁇ x and ⁇ y being negative under illumination by a C-illuminant, and a polarization co- efficiency greater than 90%.
  • FIG. 1 schematically illustrates an embodiment of a display unit based on a liquid crystal display
  • FIG. 2A illustrates transmission spectra of light passing through an absorbing polarizer for both a single and a double pass, where the light is polarized parallel to the pass state of the polarizer
  • FIG. 2B illustrates transmission spectra of light passing through an absorbing polarizer for both a single and a double pass, where the light is polarized parallel to the block state of the polarizer
  • FIG. 3 illustrates reflection of an opaque and a transflective silver layer and transmission through the transflective silver layer
  • FIG. 4 illustrates an embodiment of a LCD display unit according to the present invention
  • FIGs. 5A and 5B respectively illustrate single pass transmission spectra for absorbing polarizer samples A-E for light polarized parallel to the pass and block states of the polarizer samples
  • FIGs. 6A and 6B respectively illustrate double pass transmission spectra for absorbing polarizer samples A-E for light polarized parallel to the pass and block states of the polarizer samples
  • FIG. 7 schematically illustrates a cross-section through a one particular embodiment of a transflector/polarizer layer
  • FIG. 8 is a graph showing the reflectivity spectrum of the transflector/ polarizer layer illustrated in FIG. 7, across the visible spectrum
  • FIG. 9 schematically illustrates a reflective display according to the present invention.
  • FIG. 10 schematically illustrates a rear projection system
  • FIG. 11 schematically illustrates reflection of ambient light within a rear projection screen
  • FIG. 12 schematically illustrates one embodiment of a rear projection screen according to the present invention
  • FIG. 13 schematically illustrates another embodiment of a rear projection screen according to the present invention. While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.
  • the present invention is applicable to absorbing polarizers, and is believed to be particularly suited to enhancing the spectral characteristics of information display systems that include one or more absorbing polarizers.
  • transflective display unit 100 based on a Super Twisted Nematic (STN) Liquid Crystal Display (LCD), is schematically illustrated in FIG.1.
  • the upper layer 102 is a first polarizer layer, typically an absorbing polarizer.
  • the unit 100 may include a compensation layer 104 that provides birefringence compensation within the unit 100 so that the light viewed by the viewer is of a particular combination of colors.
  • a reflective or transflective display based on a STN LCD presents black images to the viewer on a green background, as is commonly found with, for example a cell phone. Without the compensation layer, the unit would present a different combination of colors to the user.
  • Layers 106 and 110 are glass layers that sandwich the liquid crystal display (LCD) layer 108.
  • the LCD layer 108 includes the liquid crystal material, driver electronics, and other associated internal elements such as alignment layers, color filters, black matrices, and transparent electrodes.
  • the LCD layer 108 may include other types of liquid crystal different from super twisted nematic (STN) displays.
  • the LCD layer 108 typically includes many independently addressable picture elements (pixels) whose polarization rotating properties may be selectively adjusted. In one example, the pixels may each be adjustable between zero polarization rotation and polarization rotation through 90°.
  • a second absorbing polarizer layer 112 intercepts the light transmitted through the LCD layer 108.
  • the pass polarization direction of the second polarizer layer is approximately orthogonal to the pass polarization direction of the first polarizer layer 102.
  • a partial reflector 114 is disposed below the second polarizer layer to reflect a portion of the light passing through the second polarizer 112.
  • a light source 116 is disposed below the partial reflector 114 to provide light for backlighting the display 100 when ambient light is insufficiently bright for the viewer to see the information displayed in the LCD layer 108.
  • the partial reflector 114 includes any suitable type of optical element that partially reflects and partially transmits incident light.
  • the partial reflector may include a layer of metal, such as silver or aluminum, that is sufficiently thin to transmit a fraction of the incident light.
  • ambient light is typically unpolarized
  • ambient light may be considered to be formed from two independent components that are orthogonally polarized. These different components are considered separately to facilitate understanding of how the unit 100 operates.
  • Planar polarization states parallel to the plane of the figure are illustrated in FIG. 1 with a line
  • polarization states perpendicular to the plane of the figure are illustrated with a circle.
  • a second component 122 of the ambient light 122 has a polarization parallel to the pass direction of the first polarizer layer 102, and is transmitted through the first polarizer 102 and through the compensation layer 104 to the LCD layer 108.
  • the polarization of the light 122 is not rotated by the LCD layer 108.
  • the second polarizer layer 112 is oriented so that its pass polarization direction is orthogonal to the pass polarization direction of the first polarizer layer 102. Therefore, the 122 is absorbed in the second polarizer layer 112.
  • Light 124 has a polarization that is transmitted through the first polarizer layer 102.
  • Light 124 passes through a portion of the LCD layer 108 that rotates the polarization of the light, and so the light 124 emerges from the LCD layer 108 in a polarization state perpendicular to the polarization state when it entered the LCD layer 108. Light 124, therefore, passes through the second polarizer layer
  • a first portion of the light 124 is transmitted through the partial reflector 114 as beam 126.
  • a second portion of the light 124 is reflected as beam 128.
  • the polarization direction of light beam 128 is parallel to the pass direction of the second polarizer layer 112, and is transmitted through to the LCD layer 108, which rotates the polarization of the light beam 128.
  • Light beam 128 then passes up through the compensation layer 104 and is passed through the first polarizer layer 102, and emerges from the display unit 100 to be seen by the viewer.
  • the ambient light incident on the display unit 100 may be selectively absorbed or reflected to the viewer.
  • Control of the polarization rotation state of the different pixels of the LCD layer 108 results in control of the display image viewed by the viewer.
  • the back light 116 may be activated.
  • Light emitted by the backlight 116 passes up through the display.
  • light beam 130 having a polarization direction that is parallel to the pass direction of the second polarizerlayer 112 passes up through the second polarizer layer and through the LCD layer 108.
  • Light 130 passes through a pixel of the LCD layer 108 that rotates the polarization of the light, and so light beam 130 emerges from the LCD layer in a rotated polarization state that is passed by the first polarizer layer 102, as light beam 132.
  • a fraction of light beam 130 is reflected at the partial reflector 114 as reflected beam 131.
  • Some of the light emitted by the backlight 116 may be in a polarization state that is absorbed by the second polarizer layer 112.
  • Some of the light emitted by the backlight 116 may be in a polarization state that is passed through the second polarizer layer 112, but also passed through a pixel of the LCD layer 108 that does not rotate the polarization of light, and so is absorbed in the first polarizer layer 108.
  • the different layers 102-114 need not be separated as illustrated, but any or all of the layers 102-114 may be bonded together using, for example, an optically transparent adhesive. Optical adhesive layers are omitted from the figure for clarity.
  • the display unit 100 may include other layers, such as a touch panel or a cover lens, which may be located above layer 102, or located within the layer stack.
  • the display unit 100 may operate in manner different from the illustrated embodiment.
  • the display unit 100 is operated such that ambient light reflected to the viewer is polarization rotated by the LCD layer 108.
  • the pass directions of the first and second polarizer layers 102 and 112 are parallel, the ambient light reflected to the viewer may not be polarization rotated by the LCD layer 108.
  • the pass directions of the polarizers 102 and 112 need not be either parallel, in other words set at 0°, or perpendicular, set at 90°, but may be oriented at some value between 0° and 90°.
  • Ambient light generated for example by the sun, overhead incandescent or fluorescent lamps, or any other type of light source, is usually normally perceived by human eyes as being white.
  • Many artificial light sources emit red, green, and blue components, either broadband or at discrete wavelengths, that are integrated by the human eye so that the resultant observed color is white. If the display unit 100 maintains this input spectral power distribution, the light emitted from the display also appears to the viewer to be white. A number of different components, however, affect the color quality of the light passing through the display unit 100 so that the image under ambient lighting conditions has a color that is different from the ambient light incident on the display.
  • the first and second polarizing layers 102 and 112 do not maintain equality in their spectral performance. Under ambient light operation, the light interacts with each of the first and second polarizing layers 102 and 112 twice, once on the input path and once on the return path. This double pass operation further separates actual performance from the desired spectrally neutral performance.
  • FIGs. 2A and 2B Graphs showing single pass (1X) and double pass (2X) transmission profiles of a commercially available polarizer (Sanritz LLC2-5518), typically used in polarizing layers 102 and 112, are illustrated in FIGs. 2A and 2B.
  • Transmission of light in the pass state is illustrated in FIG. 2A: curve 202 illustrates single pass transmission and curve 204 illustrated double pass transmission.
  • the transmission of light polarized in the blocking state is illustrated in FIG. 2B: curve 212 illustrates single pass transmission and curve 214 illustrates double pass transmission.
  • the curves in FIGs. 2A and 2B show that the performance in the blue region of the spectrum, for example in the range of approximately 400 nm - 480 nm, is different from the region of the spectrum above about 500 nm, particularly for light having a wavelength less than about 430 nm. This phenomenon is referred to as blue rolloff.
  • the absorption of a portion of the blue component of the light polarized in the polarization transmission direction results in a shift in the color of the ambient light ultimately reflected to the viewer.
  • Efficiency is a measure of the display brightness expressed simply as the average photopic double pass polarized transmission of the top polarizer calculated against a standard illuminant. For a perfect polarizer, the efficiency equals 100%.
  • a standard illuminant such as an A-illuminant, B-illuminant, C-illuminant, D-illuminant or E- illuminant, which approximates the emission from a standard type of optical source.
  • An A-illuminant for example, replicates the emission spectrum from a tungsten filament having a color temperature of 1800 K.
  • a C-illuminant is described as average daylight without the ultraviolet portion, and is commonly used for uniform color calculations.
  • Standard illuminants are often presented in a look-up table as a function of spectral intensity as a function of wavelength.
  • Color shift expresses the shift in color after double passing polarizer when using a specific illuminant. It is common to express color shift under illumination by a standard illuminant, such as an A-illuminant, B-illuminant, C-illuminant, D- illuminant or E-illuminant. Color shift is calculated using the 1931 CIE chromaticity coordinates (x, y) and is expressed as the change ( ⁇ x, ⁇ y) between the color co-ordinates of the illuminating light and the light after double passing the optical element under test. The transmission is measured for light that is passed normally through the polarizer.
  • One common method of determining color shift is to measure the transmission spectrum of the polarizer for light in both the pass and block polarization states and use the measured transmission spectra to calculate the spectrum of light, emitted by a standard illuminant, after double passing through the polarizer.
  • Contrast modulation is the ratio of the difference of the double pass average photopic transmitted pass and block values over sum of the double pass average photopic transmitted pass and block values, when using a specific illuminant. Contrast modulation is a bounded metric ranging from 0 (no contrast) to 1 (perfect contrast). In other words, the contrast modulation, CM, is given by the expression:
  • l p and l b are the light intensities transmitted through a pair of the polarizers whose transmission axes are parallel and crossed respectively, averaged over the visible spectrum.
  • the polarizers have high contrast modulation and good efficiency, but all demonstrate a significant color shift towards a yellow color, particularly when compared with a C-illuminant.
  • the polarizer that shows the smallest color shift is the Nitto polarizer, where the color shift is less than or equal to 0.005 for both ⁇ x and ⁇ y when compared with an A-illuminant.
  • the color shift of the Nitto polarizer increases due to blue drop- off.
  • the polarizer whose characteristics are illustrated in FIGs. 2A and 2B were to be used on a display in ambient positive mode (dark text with a light background), the ambient appearance would be very black text on a yellow background. It has been found that users do not prefer yellow displays, which are perceived as being of lower quality or "dingy". Consequently, display manufacturers use the first polarizer 102 to give very dark black state appearance, but adjust the other system components, for example by adding color filters in the LCD layer 108 to further change the color shift and substitute a green background for the otherwise yellow background. The choice of green for the bright state coincides with the eye photopic peak in the green portion of the visible spectrum and is more pleasing to the user's eye than yellow, but does not result in the desired neutral or white appearance.
  • the performance of the display unit is further degraded due to losses introduced by the elements below the LCD layer 108, namely the second polarizer layer 112 and the partial reflector 114.
  • the second polarizer layer 112 may exhibit the same double pass performance as the first polarizer layer 102, thus causing compounded efficiency losses and color shifts.
  • the partial reflector 114 since the partial reflector 114 performs the two conflicting functions of reflecting ambient light and transmitting backlight, its performance in performing each function is compromised. In order to transmit some of the backlight, the partial reflector 114 cannot reflect all of the incident ambient light. Likewise, in order to reflect some of the ambient light, the partial reflector 114 cannot transmit all of the backlight light. This results in a reduced system efficiency.
  • the reflectivity of the partial reflector 114 is typically selected to be in the range of 50% to 90%.
  • the partial reflector 114 may impart some color shift upon reflection.
  • Silver is becoming a common choice for the partial reflector 114 mirror plane because of its high reflectivity: it is used to increase efficiency at a given reflection/transmission ratio.
  • the reflectivity of silver falls off in the blue region of the visible spectrum, further exaggerating the yellow shift resulting from the first and second polarizing layers 102 and 112.
  • the spectral reflectivity of an opaque layer of silver is illustrated as curve 302 in FIG. 3, showing a significant reduction in reflection at blue wavelengths, for example in the range 400 nm - 480 nm, compared to yellow (580 nm) or red (630 - 700 nm) wavelengths.
  • Curve 304 illustrates the reflection of a transflective silver layer having an average reflectivity of 82%, compared with the average reflectivity of 94% for the opaque example of curve 302.
  • Curve 306 illustrates the transmission through the transflective layer, with an average reflectivity of 13%.
  • the difference between the reflectivities of the opaque and transflective silver layers are amplified, however, in the blue region of the spectrum. In the red portion of the spectrum, for example at 630 nm, the difference in transmission between the opaque and the transflective silver layers is about 10%.
  • the difference in transmission between the opaque and transflective layers is about 20%. Furthermore, the transmission of the transflective layer at about 600 nm is 10%, whereas the transmission at 400 nm is over 25%. Accordingly, the silver transflective layer also introduces a shift in the spectrum of both reflected and transmitted light.
  • the ambient spectral performance of the display unit 100 is determined by the convolution of the spectral performance of each of the elements 102-114. Discrete spectral losses, particularly in the blue portion of the spectrum, cause significant color shifts that prevent white ambient operation. In addition, amplitude losses reduce overall efficiency resulting in a lower brightness display.
  • An embodiment of another LCD-based display unit 400 is schematically illustrated in FIG. 4.
  • the first polarizer 402 is optically tuned to increase efficiency while reducing color shift. This is achieved by balancing contrast modulation against absorption for both pass and block polarization states.
  • the first polarizing layer 402 may use an oriented polyvinyl alcohol (PVA) matrix with an iodine stain to provide absorption. Adjusting stain concentration, stain duration, and PVA thickness creates various leak amplitudes with corresponding pass state profiles.
  • PVA polyvinyl alcohol
  • Other constructions, for example dye based PVA, K-type, and lyotropic polarizers are also suitable for use in the first polarizing layer 402, for example as described in U.S. Patent Application Serial No. 09/426,288, incorporated herein by reference.
  • An optional retardation compensating layer 404 may be placed below the first polarizer 402.
  • An optional touch panel 416 may be provided with the display unit 400 to permit the user to enter information to the device using the display unit 400.
  • the touch panel 416 may be coupled to the controller 409. Although the touch panel 416 is illustrated in a position above the first polarizer 402, the touch panel 416 may be placed in any suitable position in the stack of layers forming the display unit 400.
  • Layers 406 and 410 are glass layers that sandwich the liquid crystal display (LCD) layer 408.
  • a controller 409 is typically coupled to the LCD layer 408 to control the polarization rotation state of the different pixels of the LCD layer 408, so as to control the information seen by the viewer.
  • a polarizer/transflector layer 412 is disposed below the LCD glass layer
  • the polarizer/transflector layer 412 may be a reflective polarizer, in other words a polarizer that reflects light at one polarization and transmits light in the orthogonal polarization.
  • the polarizer/transflector layer 412 may also include one or more diffusive layers to provide efficient, broadband reflectivity and system viewing angle, rather than a metallic, partial mirror.
  • a backlight 414 is disposed below the polarizer/transflective layer 412 that provides light to the viewer under conditions where there is insufficient ambient light to view the display 400.
  • Ambient light ray 420 has a polarization orthogonal to the pass polarization state of the first polarizer 402, and is absorbed in the first polarizer 402. In the illustrated embodiment, the pass polarization direction of the first polarizer 402 is in the plane of the figure.
  • Ambient light ray 422 has a polarization that is transmitted by the first polarizing layer 402, and is transmitted through the LCD 408 without having its polarization rotated. The ray 422 is transmitted through the polarizer/transflective layer 412, and may be subsequently absorbed or diffusely attenuated.
  • Another ambient ray 424 is transmitted through the first polarizing layer 402 and the LCD 408.
  • Ray 424 passes through a region where the LCD layer 408 rotates the polarization of ray 424.
  • the polarization-rotated ray 424 is, therefore, reflected by the polarizer/transflective layer 412 as ray 426, which is polarization rotated on its passage back through the LCD 408, and is transmitted back through the first polarizer 402 for viewing by the user.
  • backlight ray 430 When operating under backlighting, backlight ray 430 is transmitted through the polarizer/transflector 412.
  • the polarization of ray 430 is not rotated by the LCD 408, and so ray 408 passes through the first polarizing layer 402 to be viewed by the viewer.
  • Backlight ray 432 is transmitted through the polarizer/transflector 412, and through a portion of the LCD 408 that rotates polarization of incident light. Therefore, the polarization of ray 432 is in a state that is blocked by the first polarization layer 402 and is not transmitted to the viewer.
  • Backlight ray 434 has a polarization that is not transmitted through the polarizer/transflector 412.
  • the display may be monochromatic, or may be a color display, with different pixels including different color filters, so as to produce different colors. It will also be appreciated that some embodiments of displays that use polarizer/transflective layers below the LCD are configured to avoid inverting the image upon use of the backlight.
  • One approach to providing a non- inverting display is to have the transmission polarization axis of the transflector 412 set at an angle between 0° and 90°, for example as described in U.S. 6,124,971 , incorporated herein by reference.
  • more than one reflecting polarizer may be used as the transflector 412, for example as described in U.S. Patent Application Serial No. 09/551 ,111, incorporated herein by reference.
  • polarizers were fabricated by passing polyvinyl alcohol (PVA) film into a series of aqueous baths that allow the PVA film to accept iodine molecules and, with the use of borates in the third bath, cross-link the PVA film.
  • PVA film may be pre-stretched or may be stretched during the process. Films that are stretched during the process may be stretched during either or both the staining or cross-linking stages.
  • a typical sequence includes: washing by immersing the PVA film in a first bath to remove plasticizers; staining by immersing the film in an iodine bath containing free iodine and potassium iodide (Kl); cross-linking by immersing the film in a boration bath; and rinsing in a final rinse bath.
  • the cured film is then transported through a chemical rinse to adjust the iodine content and to remove surface deposits from the surface of the film.
  • the first bath may be omitted where the PVA film has been pre-stretched.
  • the concentration of free iodine in the second bath controls the amount of iodine present in the final polarizer film.
  • the boration bath includes borax, boric acid and/or Kl, and may also contain zinc chloride.
  • Dwell times in the staining bath typically range from 5 to 60 seconds depending on iodine concentration.
  • Dwell time in the boration bath typically ranges from 20 to more than 180 seconds depending on the temperature of the bath.
  • the temperature of the boration bath typically ranges from 50 °C to 80°C, depending on the properties of the PVA film.
  • DIW de-ionized water
  • the single pass transmission spectra for light in the pass polarization state and in the block polarization state were measured for each of the samples A-E, and are presented in FIGs. 5A and 5B respectively.
  • the different curves presented in FIGs. 5A and 5B are labeled according to the labels presented in Table III.
  • curve 502 in FIG. 5A represents the single-pass, pass state transmission for Sample A
  • curve 530 represents the single pass, block state transmission for Sample E.
  • the double pass transmission through polarizer samples A-E is illustrated in FIGs. 6A and 6B for light polarized parallel to the pass and block states respectively.
  • the double pass transmission curves were calculated by convolving the curves of FIGs. 5A and 5B.
  • Table III also lists the relationship between the curve numbers presented in FIGS. 6A and 6B and the representative samples. Table III. Summary of Curves Related to Sample Type
  • FIG. 5A Sample FIG. 5A FIG. 5B FIG. 6A FIG. 6B
  • Controlling the magnitude of the extinction leak in polarizer samples A-E allows for a balanced pass state amplitude giving a neutral color. Color shifts for the samples are below the minimum threshold for color discrimination while efficiency and contrast modulation tradeoffs may be balanced. In all of the samples, the efficiency was higher than 60%, and in all of the samples except one, the efficiency was higher than 65% and higher than 68%, for a C-illuminant. Furthermore, all but one of the samples demonstrated a contrast modulation in excess of 0.90, which is adequate performance. However, enhanced performance is seen with a contrast modulation in excess of 0.95, a value exceeded by samples B and E. Samples A and E both produced contrast in excess of 0.97.
  • Samples B and C present a small color shift where the absolute values (
  • the sample polarizers fabricated according to the method described above, in which the losses at the blue portion of the spectrum are less than with conventional polarizers, and which therefore produce small color shifts, may be referred to as color-neutral polarizers.
  • the benefits of neutral color performance of the polarizer samples A-E may be realized in a system by using a transflector/polarizer layer 412 having a flat reflective spectral response.
  • the transflector/polarizer layer 412 is a reflective polarizer.
  • the spectral properties of the reflective polarizer may be tuned to give a relatively flat spectrum that can maintain the input spectral power distribution.
  • the reflective polarizer may be, for example, a multilayer reflective polarizer, a cholesteric reflective polarizer, a dispersed phase reflective polarizer, or a wire grid reflective polarizer.
  • transflector/polarizer layer 412 is a TDF film, produced by 3M Company, St. Paul, Minnesota, and shown schematically in FIG. 7 as element 700.
  • Layer 702 is a diffusing adhesive used to bond the element 700 to the liquid crystal display glass layer 410.
  • Layer 702 is preferably polarization preserving with a diffusion profile optimized for display viewing and reflected brightness.
  • Layer 704 a multilayer reflective polarizer and optional layer 706 is a partial absorber layer with an average transmission preferably between 30% and 70%.
  • the multilayer reflective polarizer 704 typically includes a first set of isotropic layers, with a second set of layers interleaved between the layers of the first set, the layers of the second set including uniaxially oriented film.
  • the refractive index of the uniaxial layers, for light in one polarization state is typically very close to the refractive index of the isotropic layers.
  • the refractive index of the uniaxial layers, for light in the orthogonal polarization state is different from that of the isotropic layers. Where the layer thickness is selected to be around one quarter wavelength thick, the stack of layers reflects light in the orthogonal polarization state.
  • a reflective polarizer 704 produces a bright pixel in the transflective display 400 by reflecting one polarization state and produces a dark pixel in the transflective display 400 by transmitting the orthogonal polarization state. The light thus transmitted is passed to the partial absorber layer 706.
  • the reflective polarizer transmits only one polarization state, producing an inverted image, as indicated above. Consequently, an inverted image is created when switching from ambient to backlit operation.
  • one of the main advantages of using a reflective polarizer 704 is that the reflection and transmission axes are independent, and so the reflective polarizer operates at near optimum efficiency for both reflection and transmission. This reduces the deleterious effects of trading off reflection and transmission that arise when using metallic based transflectors.
  • the reflection spectrum of the reflective polarizer may be adjusted by controlling the optical properties of the reflective polarizer in order to maintain the input spectral power distribution.
  • an appropriate distribution of layer thicknesses may be used to provide reflection over a desired wavelength band.
  • the chiral pitch is provided with the appropriate gradient to cover the desired wavelength band.
  • Analogous criteria may be met for the disperse phase reflective polarizer and the wire grid polarizer to achieve the desired chromatic performance.
  • the measured reflective performance of one example of a multilayer reflective polarizer 704 is shown as curve 802 in FIG. 8.
  • the need for white displays also extends to purely reflective display systems, for example as may be used in a pocket calculator.
  • An embodiment of a reflective display 900 is illustrated in FIG. 9.
  • the reflective display includes a first polarizing layer 902 formed from an absorbing polarizer.
  • the first polarizing layer 902 may be optically tuned to maximize while minimizing color shift, and may be of the type discussed above with respect to polarizer layer 402.
  • An optional compensating retarder layer 904 may be provided below the first polarizing layer 902.
  • An optional touch panel 916 may be also be provided with the reflective display 900.
  • Layers 906 and 910 are glass layers that sandwich the LCD layer 908.
  • a polarization sensitive reflector 912 is disposed below the LCD glass layer 910.
  • the polarization sensitive reflector 912 reflects light of only one polarization and may be, for example, a multiple layer reflective polarizer, a cholesteric reflective polarizer, a disperse phase reflective polarizer or a wire grid polarizer.
  • the polarization sensitive polarizer may also be an absorbing polarizer having a highly reflective sublayer to reflect light transmitted through the absorbing polarizer.
  • the polarization sensitive reflector 912 may also include one or more diffusive layers to provide efficient, broadband reflectivity and system viewing angle.
  • the display 900 operates in a manner similar to that for transflective display 400.
  • Ambient light ray 920 is absorbed in the first polarizer layer 902.
  • Ambient light ray 922 is transmitted through the first polarizer layer and through the LCD layer 908 without its polarization being rotated, and so is either transmitted through or absorbed in the polarization sensitive reflector 912.
  • Ambient light ray 924 is transmitted through the first polarizer layer 902 and is polarization rotated on passing through the LCD layer 908 and is, therefore, reflected from the polarization sensitive reflector 912 as ray 926.
  • Ray 926 is polarization rotated on passing back through the LCD layer 908 and emerges from the first polarizer layer to be viewed by the user.
  • a frontlight 914 may be used above the first polarizer layer 902 for supplemental viewing if ambient lighting is insufficient for viewing the displayed . image.
  • the absorbing polarizer may be of the type of absorbing polarizer discussed above with respect to polarizer 402.
  • a spectrally flat reflector such the Enhanced Specular Reflector (ESR), available from 3M Company, St. Paul, Minnesota, may be coupled with the absorbing polarizer using a suitable adhesive to form the reflective polarizer.
  • ESR is a stack of a first set of layers interleaved with a second set of layers.
  • the refractive index of the first set of layers is different from the refractive index of the second set of layers.
  • the layer thickness is selected to be around one quarter wavelength, the light is reflected.
  • the ESR may be highly reflective over a range of wavelengths.
  • the polarization sensitive reflector 912 may be a TDF layer, as described with respect to FIG. 7, with an opaque back layer to provide for ambient black state performance. These combinations maintain the input spectral power distribution for a paper white reflective display.
  • the polarization sensitive reflector 912 may be an absorbing polarizer followed by two reflecting polarizers having their optical transmission axes crossed relative to each other.
  • the polarization sensitive reflector 912 may be an absorbing polarizer followed by a reflecting polarizer having its transmission axis crossed relative to the transmission axis of the absorbing polarizer.
  • Other types of polarization sensitive reflector 912 may also be used.
  • the first sample unit referred to as Sample 1
  • Sample 1 was a commercially available unit having a structure like that illustrated in FIG. 1.
  • Sample 1 used conventional absorbing polarizers for the first and second polarizing layers 102 and 112, having spectral characteristics similar to those of the Sanritz 5518 polarizer.
  • the transflector layer 114 used a thin layer of aluminum.
  • the second sample referred to as Sample 2
  • Sample 2 was fabricated by taking a display unit like that of Sample 1 , and replacing the first polarizer layer with a color neutral first polarizer 402 and replacing the second polarizing layer 112 and transflector layer 114 with a TDF layer 700.
  • the resultant structure was like that illustrated in FIG. 4, and included a compensator 404.
  • the first polarizer layer 402 was formed from polarizer Sample B listed in Table II, and the TDF layer 700 operated as the polarizer/ transflector layer 412.
  • the TDF layer 700 was placed onto the lower LCD glass layer 410 with its reflection axis oriented parallel to the transmission axis of the original second absorbing polarizer.
  • the first polarizer layer 402 was aligned onto the cell by eye judging against the best color performance. Both systems incorporated a compensation film between the first polarizer
  • the color shift of Sample Display Unit 2 is less than the color shift of
  • Sample Display Unit 1 and so Sample Display Unit 2 is more chromatically neutral than Sample Display Unit 1.
  • Sample Display Unit 2 also demonstrates an efficiency of approximately twice that of Sample Display Unit 1. Although the contrast modulation for Sample Display Unit 2 was less than for Sample Display Unit 1 , it was still easy for a viewer to read the information on Sample Display Unit 2. Moreover, the increased efficiency of Sample Display Unit 2 produced almost a doubling of the reflected brightness, resulting in a perceived contrast improvement. The image on Sample Display Unit 2 looked distinctly more white/black than the image on Sample Display Unit 1. Overall contrast modulation of Sample Display Unit 2 may be improved with redesign of the compensation film.
  • Sample Display Unit 2 Since the compensation film was designed to operate with polarizers that manifest a blue drop-off, the polarizers used in Sample Display Unit 2 add additional dynamic color range requiring a different degree of compensation.
  • the comparison between Sample Display Units 1 and 2 shows that the use of an optimized polarizer system, both first polarizer and polarizer/ transflector, in which efficiency and color shift are optimized, allows new performance advantages to be realized.
  • transflective displays operate primarily in ambient mode, most systems have monochromatic or quasichromatic backlights where spectrally flat polarizers have limited differentiation.
  • the use of spectrally flat polarizers provide greater advantages in backlit situations when using broadband or tri-emission (red, green, blue) light sources.
  • transflective display having an inverted image may also be used in transflective displays that have a non- inverted image, for example as described in WO 97/01788 and U.S. patent application serial no. 09/551 ,111 , both incorporated herein by reference.
  • the reflective element in a reflective or transflective display reflects substantially all of the blue ambient light that passes through the LCD.
  • Another advantage of the invention is that the transmission spectra of the polarizers and/or the reflective spectrum of the reflector/transflector are balanced so that the ambient light reflected by the display to the viewer is substantially perceived as being white.
  • FIG. 10 one other approach to information display, illustrated in FIG. 10, is to use a rear projection display 1000, where the information to be shown to one or more viewers is projected by a light image projector 1002 to a rear projection screen 1004.
  • the light image projector may be coupled to a controller 1006 that controls the image projected from the light image projector 1002.
  • the light image projector 1002 may be a LCD-based color image projector and the controller 1006 may be a computer.
  • the screen 1004 includes a disperser layer to disperse the light so that the viewer can see the image from all points of the screen.
  • the disperser layer may include any suitable type of dispersing layer, for example a bulk diffusing layer (scattering particles disposed randomly within a bulk medium), a lenticular lens array, a micro-structured surface, or a beaded layer.
  • the disperser layer may also include a combination of more than one dispersing layers, of the same or of differing types.
  • Important characteristics of a projection system include the screen gain, a representation of the screen's brightness; the viewing angle, the angle relative to the axis at which the gain of the screen drops to half of the peak gain, or to half of the one-axis gain; and the contrast.
  • Contrast is generally the ratio of luminance of a projected white image to that of a projected black image.
  • some of the ambient light may be reflected from the surface of the screen or from within the screen or the projector system.
  • the reflected light typically includes both specular and diffuse components. The ambient reflection tends to decrease the contrast of the screen.
  • the contrast ratio is also dependent upon the ability of the screen to avoid reflecting ambient light back to the viewer.
  • a screen Another important characteristic of a screen is its overall spectral performance, in other words its ability to maintain the spectrum of the light incident on its input surface. Where the screen is formed from one or more polymer layers, the screen often tends to display a decreased ability to transmit blue light, since the blue portion of the visible spectrum is preferentially absorbed in the polymer. Thus, images often suffer a color shift when being displayed on a projection screen.
  • resolution Another important characteristic of rear projection screens is resolution, which is becoming increasingly more important where there are higher resolution requirements, for example in high definition television.
  • the resolution of a screen is generally defined as a measure of the finest detail that can be distinguished in an image projected on the screen. One method of measuring resolution is accomplished by projecting an image on the screen and measuring the modulation depth, as is further described in U.S. Patent No. 6,163,402, incorporated herein by reference. However, since the resolution is related to the screen contrast, a reduction in contrast resulting from ambient light also results in a reduction in the resolution.
  • Ambient light 1010 from the viewing side 1008 of the screen 1004 may be diffusely reflected or specularly reflected. Specular reflection is commonly reduced by using an anti-reflection (AR) coating on the viewing surface of the screen, or by a matte finish, or by a combination of the two. Specular reflection at the interfaces between different layers of a screen is commonly low because of index-matching between layers. Index-matching is, however, typically not perfect, and some specular reflection does occur.
  • AR anti-reflection
  • FIG. 11 shows a screen 1100 in a back-illumination configuration, with an image projector 1102 on an input side 1101.
  • Ambient light 1104 is incident on the output side 1106 of the screen 1100.
  • the screen 1400 includes a number of different layers, 1110, 1112, and 1114.
  • a fraction, R 0 of the incident ambient light is reflected by the front surface 1406 of the screen 1400.
  • Further fractions Ri and R 2 are reflected at the interface between the first and second layers 1110 and 1112, and at the interface between the second and third layers 1112 and 1114.
  • a fraction, R 3 , of the remaining ambient light 1104 is reflected off the input surface 1116 of the fourth layer 1114. Furthermore, a fraction, R D , of the ambient light 1104 is diffusely reflected light at one of the layers, typically a scattering layer. A significant fraction of all the reflected light is transmitted back out of the screen 1100 in the direction towards a viewer on the output side of the screen 1100. This ambient light exiting from the screen reduces the contrast of the desired image, and therefore negatively affects the resolution. It is, therefore, important to reduce the amount of ambient light that is reflected within the screen 1100 towards the viewer. It will be appreciated that the screen may be formed from a different number of layers from that illustrated.
  • One approach to enhancing the color of the viewed image, as well as the contrast, is to use a color neutral polarizer 1202 along with the dispersing layer 1204, as shown in FIG. 12 for the screen 1200.
  • the color neutral polarizer 1202 absorbs half of the ambient light entering the screen 1200 and also absorbs ambient light reflected within the screen whose polarization has been rotated relative to the polarization state of the light entering the screen 1200.
  • the screen 1300 includes a disperser layer 1302 and a color neutral absorbing polarizer 1304.
  • a retarder layer 1308 is disposed between the polarizer 1304 and the disperser layer 1302. In this embodiment, ambient light that passes through the absorbing polarizer 1304 passes through the retarder layer 1308 and has its polarization state changed.
  • the retarder layer 1308 is a quarter-wave retarder layer, and so ambient light entering the disperser layer 1302 is approximately circularly polarized. Any ambient light that is reflected by the disperser layer 1302 passes through the retarder layer 1308 once more, and the polarization of the light is further changed. If the light reflected from the disperser layer 1302 is circularly polarized, and the retarder layer 1308 is a quarter wave retarder, then the light is linearly polarized upon leaving the retarder layer 1308 into the polarizer 1304. Furthermore, the direction of polarization is rotated from that of the ambient light that originally passed through the polarizer 1304, and so the ambient light reflected from the disperser layer 1302 is absorbed in the polarizer 1306 and 1308.
  • the polarizer 1304 absorbs one fraction of the ambient light as it enters the screen 1300 and absorbs the other fraction that is reflected within the screen 1300 towards the viewer. Therefore, the polarizer 1304 reduces the contrast-diminishing effects of the ambient light.
  • image light passing through the screen 1300 should be polarized in a specific manner in order to avoid losses in the polarizer 1304.
  • the reflected ambient light propagates through the disperser layer 1302 towards the viewer with one handedness of circular polarization.
  • the image light passing through the disperser layer 1302 is circularly polarized with the opposite handedness from the reflected ambient light. Therefore, when the image light passes through the retarder layer 1308, the polarization of the image light is changed by the retarder layer 1308 to a linear polarization that is parallel to the polarization transmission direction of the polarizer 1304, and so the image light is transmitted to the viewer.
  • the image light may be passed through another retarder layer 1310 on the input side 1312 of the disperser layer 1302. Since it is pleasing to the viewer's eye that the screen appear a little blue, the polarizer may permit some degree of leakage of blue light in the block polarization state. Therefore, the polarizer need not be of as high contrast as is generally required in an LCD display.
  • the polarization co-efficiency is defined as the square root of the contrast modulation.
  • the polarization co-efficiency of Samples B and C is greater than 0.90, and in both cases is greater than 0.97.
  • Application Serial No. 09/274,585 may use a color neutral absorbing polarizer.
  • the present invention is applicable to display devices, and is believed to be particularly useful for maintaining reduced color shift in display systems that include a polarizer.
  • the present invention should not be considered limited to the particular examples described above, but rather should be understood to cover all aspects of the invention as fairly set out in the attached claims.
  • Various modifications, equivalent processes, as well as numerous structures to which the present invention may be applicable will be readily apparent to those of skill in the art to which the present invention is directed upon review of the present specification. The claims are intended to cover such modifications and devices.

Abstract

La présente invention concerne un filtre polariseur à faible décalage de couleur particulièrement bien adapté pour une utilisation dans un afficheur LCD. Ce filtre polariseur possède un décalage de couleur en double passe de ?Δx0? ≤ 0.005 et ?Δx0? ≤ 0.005 pour une illumination par un illuminant C, et une modulation de contraste en double passe d'au moins 90 %. Cet afficheur comprend au moins deux couches empilées ensemble, au moins une de ces couches étant un filtre polariseur absorbant possédant un décalage de couleur en double passe de ?Δx0? ≤ 0.005 et ?Δx0? ≤ 0.005 pour une illumination par un illuminant C. Un filtre polariseur à décalage de couleur sélectionné est également utilisé avec un écran de projection par transparence possédant une couche dispersante empilée avec le premier filtre polariseur absorbant. Ce filtre polariseur possède un décalage de couleur en simple passe, un des Δx and Δy au moins étant négatif sous illumination par un illuminant C, et une polarisation de co-efficience supérieure à 90 %.
EP01933259A 2000-05-11 2001-05-10 Afficheurs d'informations a couleurs compensees Ceased EP1285288A2 (fr)

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US6624936B2 (en) * 2000-05-11 2003-09-23 3M Innovative Properties Company Color-compensated information displays
AU2003270750A1 (en) * 2002-09-20 2004-04-08 Honeywell International, Inc. High efficiency viewing screen
JP2005208568A (ja) * 2003-10-17 2005-08-04 Seiko Instruments Inc 液晶表示装置

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US5659408A (en) * 1995-05-24 1997-08-19 Polaroid Corporation Reflective image-providing display viewed with holographically diffused ambient light

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US4166871A (en) * 1977-06-29 1979-09-04 Polaroid Corporation Iodine stained light polarizer
US4416946A (en) * 1982-04-05 1983-11-22 American Hoechst Corporation High stability polarizer
US4591512A (en) * 1985-01-25 1986-05-27 Polaroid Corporation Method of making light polarizer
EP0871923A1 (fr) * 1995-06-26 1998-10-21 Minnesota Mining And Manufacturing Company Dispositifs transflectifs a transflecteur de polarisation reflechissant
JP3591699B2 (ja) * 1997-10-09 2004-11-24 日東電工株式会社 偏光素子、光学素子、照明装置及び液晶表示装置
US6163402A (en) * 1998-06-11 2000-12-19 3M Innovative Properties Company Rear projection screen

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US5659408A (en) * 1995-05-24 1997-08-19 Polaroid Corporation Reflective image-providing display viewed with holographically diffused ambient light

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CN1440511A (zh) 2003-09-03
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