HIGH EFFICIENCY POLARIZED AND COLLIMATED BACKLIGHT

A thin collimated backlight is provided for use in a monochrome liquid crystal display or a color liquid crystal display with color converting elements. The color converting elements are located on the opposite side of the liquid crystal panel to the backlight. The backlight is illuminated by narrow-band light sources such as single color LEDs. The backlight is formed by a lightguide, one or more conventional light controlling sheets and a polymeric filter sheet. The polymeric filter acts to: 1) reflect one polarization direction over the bandwidth of source at all angles of incidence and 2) reflect the orthogonal polarization direction over the bandwidth of the source only at high incident angles. Light in this orthogonal polarization passes through the filter when incident close to the normal to the filter sheet. Light reflected by the filter is efficiently recycled within the backlight.

DESCRIPTION OF REFERENCE NUMERALS

1refers to a collimated backlight with a narrow emission bandwidth.2refers to a lens array for focusing light from the backlight.3refers to the lower polarizer of a liquid crystal display panel.3′ refers to the upper polarizer of a liquid crystal display panel.4refers to apertures in a TFT layer.5refers to a liquid crystal cell.6refers to apertures in a black mask.7refers to a diffuser layer above a liquid crystal panel21R refers to a chamber registered with a red sub-pixel TFT aperture.21G refers to a chamber registered with a green sub-pixel TFT aperture.21B refers to a chamber registered with a blue sub-pixel TFT aperture.22R refers to a black mask aperture registered with a red sub-pixel.22G refers to a black mask aperture registered with a green sub-pixel.22B refers to a black mask aperture registered with a blue sub-pixel.30refers to a lightguide.30arefers to a light receiving face of the lightguide.30brefers to a first major face of the lightguide.30crefers to a second major face of the lightguide.31refers to a narrow band light source.32refers to lightguide extractions features.33refers to a reflector.34refers to a diffuser sheet.35refers to a first brightness enhancement film35′ refers to a second brightness enhancement film36refers to a reflective polarizer film37refers to a band pass filter.41refers to a polymeric reflective filter giving angle and polarization selection51refers to a stacked arrangement of two or more polymeric reflective filters giving angle and polarization selection.

DETAILED DESCRIPTION OF INVENTION

The present invention will now be described in detail with reference to the drawings, in which like reference numerals are used to refer to like elements throughout.

FIG. 1shows two types of liquid crystal display (LCD) that involve relatively narrow-band light passing through the liquid crystal (LC) cell5. The form shown inFIG. 1(A)is a monochrome display. Light from a monochrome backlight1is subjected to spatial modulation by transmission through a conventional liquid crystal panel11containing: a lower polarizer3, an actively addressed TFT layer with apertures4, an LC cell5, a black-mask array with apertures6and an upper polarizer3′. The efficiency of the display is impacted by absorption and scatter in the black-mask and TFT electronics. If light from the backlight1is collimated, a focusing lens sheet2can be used to increase the efficiency by focusing the light through the TFT apertures4and the black mask apertures6. The associated reduction in scattering within the panel also leads to an increased contrast ratio (CR). The CR is further improved using a collimated backlight since the angular spread of light passing through the LC cell5and polarizers3and3′ is reduced. A diffusing layer7can be placed above the LC panel11in order to increase the angular range over which the display may be viewed.

FIG. 1(B)shows a configuration that makes use of a blue backlight1′. A liquid crystal panel11is again used to spatially modulate the light. Each pixel is now divided into three color sub-pixels. The light passing through the LC panel enters an array of chambers21R,21G, and21B. Each chamber21R is registered with a red sub-pixel aperture within the TFT. Similarly, each chamber21G is registered with a green sub-pixel aperture and each chamber21B with a blue sub-pixel aperture. If the backlight is sufficiently collimated, a lens sheet2may be used to focus the light through each aperture in the TFT and black-mask into the correctly addressed chamber21R,21G or21B. Without collimation, some backlight light will pass through a TFT aperture and enter an incorrectly registered chamber. Such cross-talk processes degrade the displayed image. A red-emitting phosphor is housed in each chamber21R, a green phosphor in each chamber21G and diffusive material in each chamber21B. The phosphors are chosen to give adequate absorption over the spectrum of the backlight. The red, green and blue light radiance distributions escaping from the front of the display panel, having emanated from all of the sub-pixel chambers21R,21G and21B and passed through a black-mask with apertures22R,22G and22B, constitute the viewable image. Color filters can be included at the apertures of the black mask in order to sharpen the displayed image. The blue backlight1′ can be replaced by a UV backlight, in which case a blue emitting phosphor is housed in the chambers21B instead of wavelength preserving scattering material.

It will be clear that both display forms described above greatly benefit from use of a collimated backlight with a relatively narrow emission wavelength range. In most display applications, the collimating backlight needs to be thin, efficient, offer good spatial uniformity and also be relatively cheap to produce. Conventional light-guide based backlights do not satisfy the collimation requirements. Direct-view backlights, for example based on an array of single-reflection LEDs (SRLEDs), can provide adequate collimation but are not sufficiently thin. In order to improve the collimation properties of a light-guide based backlight, a reflective filter can be added that reflects high angle light yet allows collimated light to pass through. High angle light is here defined to propagate at angles higher than a value θcrelative to the normal to the backlight plane. The angle θcthus sets the required collimation level, with a typical value being θc=20°. The light reflected by the filter is recycled in the backlight. The efficiency of the recycling is set by losses in the various backlight layers as well as in the filter. A thin, low-loss reflective filter that allows only collimated light to transmit is not currently available for broad band light such as white light. For a narrower bandwidth, an interference band pass filter (BPF) can fulfill this function.

Preferentially, the collimation angle θc′ is in the range 10° to 30°.

FIG. 2shows a typical edge lit light-guide backlight geometry with an added BPF37. The light sources31emit narrow band light. Preferentially, the bandwidth of the sources is below 100 nm. Light is ejected from the lightguide30, which includes a light receiving face30a, a first major face30band a second major face30c, by means of extraction features32. Any light that propagates downwards below the lightguide30is reflected in the reflector33. A diffusive layer34above the lightguide30improves the spatial homogenization of the light and smoothens the luminance distribution. Brightness enhancement films (BEFs)35and35′ provide some reflective angular filtering but some high angle light survives and is transmitted upwards from these layers (the diffusive layer and/or the BEFs may be considered light controlling layers). A reflective polarizing sheet (DBEF)36can be added to selectively transmit the polarization direction aligned with the pass direction of the lower polarizer of the TFT panel (not shown). The orthogonal polarization is largely reflected for recycling within the backlight. The BPF37is placed above the DBEF36in the example configuration shown inFIG. 2.

The BPF can be fabricated using known techniques. Various forms are possible, all involving multiple layers of at least two types of material. The layers may, for example, be deposited by sputtering. Typical constituent materials used in this process are TiO2and SiO2due to the relatively low loss and high refractive index contrast of these materials. The layers may alternatively be polymeric. A co-extrusion process may be used to deposit alternating layers of constituent polymers that give an adequate refractive index difference. The multilayer stack thus formed may be stretched to produce a filter with layer thicknesses and refractive index values that give rise to the targeted BPF characteristic.

An experimental investigation into light recycling processes within a conventional backlight with a BPF was undertaken. The studied geometry adheres to the form shown inFIG. 2with blue GaN LEDs used as light sources. The BPF was formed from TiO2and SiO2layers and gives a long wavelength cut-off to transmission at around 455 nm. All light above this wavelength will not pass through the filter and is ultimately lost. Hence, only the recycling efficiency of light components below this wavelength was considered. The study showed that loss in the backlight layers severely restricts the recycling efficiency. Absorption in the DBEF36and BPF37was found to account for the majority of this loss. In order to improve light recycling and hence the backlight efficiency, the combined loss in these filters needs to be reduced.

A conventional DBEF is optimized to reflect one polarization over the entire visible spectrum. It does not provide significant angular filtering. The disclosed invention relies upon a polymeric filter that combines the roles of a reflective polarization filter and a reflective angular filter. The filter is designed to be effective over the relatively narrow bandwidth of a source such as a blue LED. Preferentially, the narrow bandwidth of the LED is less than 100 nm. The filter can give rise to less absorption loss per pass than a conventional DBEF despite its added angular filtering capability.

A backlight in accordance with the present invention emits collimated light in substantially a single polarization mode. The backlight can include a lightguide30having a light receiving face30afor receiving light emitted by a light source, such as one or more narrow band light sources (e.g., one or more LEDs configured to emit narrow band light). The lightguide30further includes a first major face30band a second major face30c, and extraction features32arranged relative to the lightguide and configured to extract light from the second major face30c. A filter including multilayer birefringent polymeric film is arranged on a side of the lightguide corresponding to the second major face. The filter is configured such that, over at least a portion of a bandwidth of the light source, light is reflected in one polarization state with a reflection coefficient greater than 50% at all angles of incidence, yet reflects light in another polarization state with a reflection coefficient greater than 50% only at angles of incidence greater than a predetermined threshold θc′. The majority of light that is not reflected by the birefringent polymeric filter is transmitted as substantially collimated light.

FIG. 3schematically illustrates the operation of the filter in a lightguide-based backlight. The backlight layers are the same as described previously, except that the DBEF36and BPF37are replaced by the disclosed filter41. Light in one polarization state is reflected for substantially all incident directions, with the reflection coefficient at all angles preferentially being larger than 50% over the wavelength bandwidth of the light source31. Light in the orthogonal polarization direction is reflected, with a reflection greater than 50%, only for incident angles larger than a value θc′. The angle θc′ thus sets the chosen collimation level, a typical value for θc′ being 20°. A majority of light in this orthogonal polarization state incident at an angle less than θc′ to the normal to the plane of the filter passes through. The light reflected by filter is recycled in the backlight.

The nature of the bottom reflector33that reflects the majority of light reflected at the filter41influences the recycling efficiency. Preferentially, the filter33has a total reflectivity above 95% over the backlight bandwidth. The reflector33, which may be arranged on a side of the lightguide corresponding to the first major face, may have a reflectivity above 98%. The reflector33may be a diffuse reflector. A diffuse reflecting characteristic can act to improve the recycling in propagation direction compared to a specular reflector.

FIG. 4(A)shows a second embodiment of the invention. This configuration corresponds to removing the one or more BEF layers35and35′ of the preferred embodiment shown inFIG. 3. The polarization/angle filter is fully relied upon to improve the collimation from the backlight. The absence of the BEF layers leads to a thinner and cheaper collimated backlight solution. The small sharp features protruding from the surface of BEF layers can become worn down, particularly if a touch panel exists above the liquid crystal display. Their removal can therefore lead to a more robust display.

FIG. 4(B)shows a third embodiment of the invention, where a second multilayer birefringent polymeric film is arranged on a side of the lightguide corresponding to the second major face, and the first and second multilayer birefringent polymeric films are configured to operate over different wavebands. More specifically, two or more polymeric films51, that behave as combined angular and polarization filters, are placed above the backlight layers. Each one of the filters targets operation over distinct but overlapping wavebands. In this way, angle and polarization selection can be enhanced.

FIG. 5(A)schematically shows the construction of a preferred embodiment of the enabling filter. It is composed of two polymeric constituents (a first part and a second part). One of the constituents (a first part) reflects a single polarization over at least part of the bandwidth of the source illumination and all incident directions with a reflection coefficient greater than 50%. The second constituent (a second part), which may be arranged adjacent to the first part, reflects the orthogonal polarization with more than 50% efficiency only at incident angles greater than an angle θc′ that defines the collimation. Preferentially, the second constituent acts to reflect at least 80% of backlight light power in this polarization that is incident at angles larger than 40° relative to the normal to the surface of the film that receives the light as measured in air. The two constituents may be optically bonded together, using known techniques, to form a single composite filter. The resulting composite filter is shown schematically inFIG. 5(B).

Both of the constituent films may comprise a plurality of polymer layers. Each constituent may be formed using a co-extrusion process. In a preferred embodiment of the filter, shown inFIG. 5, each constituent film contains two types of polymer. Both constituents are separately subjected to a uniaxial stretch. After the stretch, a “type 1” polymer is rendered birefringent, whereas a “type 2” polymer remains largely isotropic. The polymers are chosen so that the principle refractive index values of the two layers are rendered similar after the stretching process except for along the stretch direction. Preferentially, the difference in refractive indices of the two layers is less than 0.02 except along the stretch direction. The two constituent films are oriented such that their stretch directions are orthogonal, as indicated inFIG. 5(A).

The thicknesses of the layers in each constituent film are carefully chosen to give the desired optical characteristics after the stretching processes have been applied. The number of layers required in each constituent depends on the source bandwidth, the principle refractive index values of the layers after stretching and the required rejection characteristics. A person having ordinary skill in the art would know how to choose the thickness to give a desired optical characteristic and how to select the number of layers based on the above-referenced characteristics.

FIG. 6shows an embodiment of a polymeric film that is composed of more than two separate constituent films (e.g., a first part, a second part adjacent to the first part, and a third part adjacent to the second part). Each constituent film provides reflection of a single polarization state over a target angle and wavelength range. Each constituent film is rendered anisotropic by an applied stretch. The constituents are ordered such that the stretch direction of each constituent is orthogonal to the stretch direction of neighboring constituents.

FIG. 7shows a filter constituent that comprises a first multilayer birefringent film with more than two different types of polymer. After a stretch is applied, at least one of the constituent layers is rendered birefringent.

Simulations have been performed in order to assess the backlight performance that can be expected with the addition of the combined polarization and angular filter. First, a filter design was found that gives the desired optical performance. The optical characteristics of the filter are calculated using a 4×4 transfer matrix formulation that will be familiar to those skilled in the art. Second, the filter is included in a backlight simulation based on a ray-tracing method.

The filter design is based on two constituent films as shown inFIG. 5. Each constituent is based on quarter wave (QW) stacks. At the available index contrasts, the reflection band associated with a single QW stack is not broad enough to cover the target spectral and angular regions. A number of QW stacks are therefore concatenated together to cover the required range. The step in layer thicknesses between neighboring QW stacks is such that some overlap in their reflection bands occurs. This allows for a finite tolerance to the layer thickness and refractive indices in the fabricated filter. The principle reflective index values used for the example filter are given in the table below:

The lower constituent of the example filter contains a total of 252 material layers. The thicknesses of the layers in the QWs were chosen such that high reflection is maintained over the wavelength range of a typical blue GaN LED in the polarization direction with maximal projection along the x-direction (x-polarization). This reflection occurs for all angles of incidence from air. The orthogonal polarization (y-polarization) suffers little reflection from the film until close to grazing incidence is reached.

The second constituent film contains a total of 168 material layers. The layer thicknesses were chosen to give reflection of high angle y-polarized light over the bandwidth of a typical blue LED, yet allow most y-polarized light from this source to pass through when directed close to the normal to the filter plane. The x-polarized light largely passes through the second constituent film unless close to grazing incidence.

FIG. 8(A)shows the calculated transmission of the y-polarized state through the example polarization/angle filter.FIG. 8(B)shows corresponding data for the orthogonal polarization state. The spectrum from a typical blue LED is also shown in these figures. To make the regions of high and low transmission more clear,FIGS. 8(C) and 8(D)show in white regions where the transmission is above 50%, and in black regions where the transmission is below 50%.FIG. 8(C)gives this information for y-polarized light andFIG. 8(D)gives this information for x-polarized light. The typical blue LED spectrum is again shown for reference.

FIG. 9presents data from a simulation of a backlight with the example combined polarization and angle filter included. The backlight is of the form shown inFIG. 3. A ray-tracing package was used for the simulation.FIG. 9(A)shows the normalized intensity distribution emitted by the backlight into air as a function of angle8relative to the backlight normal. The intensity distribution has been averaged over azimuthal angles. Also shown is the intensity distribution from the backlight without the filter present. It is confirmed that the intensity of high angle components have been heavily suppressed by the action of the filter.FIG. 9(B)shows the fraction of backlight power emitted into a cone of half-angle8centered at the normal to backlight. It is seen that, with the filter present, less than 5% of light power is emitted at angles above θ=40°. Without the filter present, this power fraction is around 25%.

The backlight efficiency was also found by simulation. The efficiency is defined as the fraction of the LED light power that passes through the lower polarizer3of the display. Absorption loss in the various layers of the backlight arrangement, as well as the filter, was included. With the example filter present, the efficiency was found to be 29.3%.

A model of a conventional reflective polarizer was also constructed. The polarizer reflects one polarization state over the visible waveband and all incident angles. The material properties of the layers used in its construction are the same as was used in the angle and polarization filter described above. The filter contains a total of 630 layers. A model of a conventional BPF, formed from TiO2and SiO2layers, was also built. This filter gives comparable angle selection to that of the example polymer filter. The example polymer filter was replaced by the polarization filter and BPF in the backlight model. The efficiency was found to have decreased to 20%. This confirms the advantage of using a combined polymeric polarization and angle filter that has been optimized for use over a selected wavelength range.

INDUSTRIAL APPLICABILITY

The invention pertains to a backlight that can be used in liquid crystal displays. In essence the invention relates to backlights that emit well collimated light substantially within a single polarization mode. The disclosed backlights are enabled by a particular form of birefringent polymeric interference filter. Other than for the addition of such a filter layer, the disclosed backlights are largely of a standard lightguide-based composition, enabling cheap construction. The disclosed backlights can be used in monochrome liquid crystal displays with improved contrast ratio. The disclosed backlights can be used to enable thin and efficient phosphor luminescent displays with high contrast ratio and low cross-talk.