Patent Application
20140240640
The present invention relates to a backlight, for example for use with an at least partially transmissive spatial light modulator. The present invention also relates to a display including such a backlight.
In particular, the invention relates to a thin and collimated backlight for use with monochrome displays or displays in which phosphors are used for color conversion.
U.S. Pat. No. 5,882,774 (James M. Jonza et. al, 3M, 10 Mar. 1995) discloses birefringent multilayer optical films in which the refractive indices in the thickness direction of adjacent layers are such that the Brewster angle is very large or nonexistent. This allows for multilayer film mirrors with high reflectivity for both planes of polarization for any incident direction. It also enables reflective polarizers with high reflectivity of the selected polarization direction for all incident directions. These properties can be maintained over a wide wavelength bandwidth.
WO 2010/059566 A1 (Michael F. Webber et. al., 3M, 19 Nov. 2008) discloses birefringent multilayer optical films which have reflectivity for normally incident light in an extended wavelength band of at least 75% for any polarization. The films have increased transmission for p-polarized light in the extended wavelength range in one plane of incidence at an angle θ1. P-polarized light incident on the film in a second plane of incidence orthogonal to the first one is subject to a reflectivity of at least 75% at any incident angle.
WO 2010/059568 A1 (Michael F. Webber et. al., 3M, 19 Nov. 2008) discloses a reflective film tailored to give a reflectivity for p-polarized light incident in one plane that decreases by at least 50% from its normal incidence value at an incident angle θ1. In a second plane, at the angle θ1, the reflectivity remains higher.
WO 2010/059579 A1 (Michael F. Webber et. al., 3M, 19 Nov. 2008) discloses a reflective film with angularly dependent polarizing properties. P-polarized light in one plane of incidence is substantially reflected at near-normal angles, but it substantially transmitted at an oblique angle.
According to an aspect of the invention an edge-lit lightguide based backlight is provided that emits collimated light substantially in a single polarization mode. These output characteristics are enabled by a specific form of reflective filter layer added to the backlight construction. The filter transmits only light with the desired characteristics, the remaining light being reflected and largely recycled within the backlight. The light re-cycling efficiency is improved by employing an efficient broad angle reflector beneath the lightguide and/or the inclusion of one or more diffuser sheets. All layers that are incorporated within the backlight construction show low absorption loss.
The enabling filter is based on stacked layers of two or more polymer materials. At least one of these materials is rendered optically anisotropic after a stretching procedure is applied. Preferentially, the filter is formed from bonding together two constituent multi-layer films. A uniaxial stretch is applied to each constituent film. Prior to bonding, the films are oriented such that the stretch axis direction of one constituent film is approximately orthogonal to the stretch axis direction of the other constituent film. The thicknesses of the layers within each constituent film after stretching are carefully chosen to give the required optical characteristics of the composite filter. The required thicknesses of each layer in the composite filter depend on the principle refractive index values of the layers after the stretching procedure.
The wavelength bandwidth over which such polymeric filters can provide collimated output as well as polarization selection is limited to less than around 100 nm if within the visible range. Thus, a single filter will not collimate a broadband white light source. The main embodiments of the invention pertain to narrow band collimated backlights. Such backlights are appropriate for use in phosphor luminescent displays (PLDs) and monochrome displays.
In a PLD, pixel color is produced by wavelength conversion in a patterned array of phosphor materials. Each phosphor element in the array is registered with a TFT sub-pixel aperture. A PLD in which the phosphor array is located above the liquid crystal panel, that is to say on the opposite side of the panel from the backlight, is of particular interest since its viewing properties are similar to those offered by OLED. Specifically, the weak luminance and color variation with angle enable an ultra-wide viewing angular range. For such displays, a collimated blue or UV backlight is needed to avoid incorrectly registered phosphors being excited (cross talk).
Both monochrome liquid crystal displays (LCDs) and PLDs can benefit from a collimated backlight since: 1) the light traversing the liquid crystal cell is close to being on-axis, thus improving contrast; 2) it enables light to be focused through thin film transistor (TFT) apertures so that device efficiency is improved and contrast is further enhanced due to reduced scatter from the electronics. For a monochromatic LCD, to ensure that the viewing angle range is sufficiently broad, it may be necessary to add a diffuser sheet above the liquid crystal cell and polarizers.
According to one aspect of the invention, a backlight includes: a lightguide having a light receiving face for receiving light emitted by a light source, a first major face and a second major face; extraction features arranged relative to the lightguide, the extraction features configured to extract light from the second major face; and a filter including a first multilayer birefringent polymeric film arranged on a side of the lightguide corresponding to the second major face, wherein over at least a portion of a bandwidth of the light source the first multilayer birefringent polymeric film reflects light in one polarization state at substantially all angles of incidence, reflects light in another polarization state only at angles of incidence greater than a predetermined threshold, and transmits a majority of light that is not reflected by the first multilayer birefringent polymeric film as collimated light.
According to one aspect of the invention, a reflector arranged on a side of the lightguide corresponding to the first major face.
According to one aspect of the invention, the backlight includes a plurality of narrow band light sources arranged relative to the light receiving face so as provide light to the lightguide.
According to one aspect of the invention, the plurality of narrowband light sources comprise light emitting diodes (LEDs).
According to one aspect of the invention, the backlight includes at least one light controlling layer.
According to one aspect of the invention, at least one light controlling layer comprises a brightness enhancing film (BEF) or a diffuser sheet.
According to one aspect of the invention, the backlight includes at least one brightness enhancement film (BEF) arranged between the multilayer birefringent polymeric film and the second major face.
According to one aspect of the invention, the backlight includes a second multilayer birefringent polymeric film arranged on a side of the lightguide corresponding to the second major face, wherein the first and second multilayer birefringent polymeric films are configured to operate over different wavebands.
According to one aspect of the invention, the multilayer birefringent polymeric film comprises a first part and a second part adjacent to the first part, wherein the first part is configured to reflect a single polarization of light over at least part of a bandwidth of the light source, and the second part is configured to reflect an orthogonal polarization of light only at angles greater than a predetermined threshold relative to a normal of the face of the multilayer birefringent polymeric film that receives the light.
According to one aspect of the invention, the predetermined threshold is less than 40 degrees.
According to one aspect of the invention, the first part and the second part of the multilayer birefringent polymeric film each comprise a plurality of polymers.
According to one aspect of the invention, a first polymer of the plurality of polymers is birefringent, and a second polymer of the plurality of polymers is isotropic.
According to one aspect of the invention, the multilayer birefringent polymeric film comprises a first part and a second part adjacent to the first part, wherein each of the first part and the second part is configured to provide reflection of a single polarization state over a target angle and wavelength range.
According to one aspect of the invention, each of the first part and the second part is rendered anisotropic by an applied stretch, and the first and second parts are arranged such that a stretch direction of the first part is orthogonal to a stretch direction of the second part.
According to one aspect of the invention, the first multilayer birefringent polymeric film comprises more than two different types of polymers.
According to one aspect of the invention, a display device includes a liquid crystal panel, and a backlight as described herein.
According to one aspect of the invention, the display device is a monochrome display device or a phosphor luminescent display. To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.
In the annexed drawings, like references indicate like parts or features:
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.
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 θc relative to the normal to the backlight plane. The angle θc thus 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°.
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 TiO2 and SiO2 due 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 in
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 lightguide 30 having a light receiving face 30a for 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 lightguide 30 further includes a first major face 30b and a second major face 30c, and extraction features 32 arranged relative to the lightguide and configured to extract light from the second major face 30c. 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.
The nature of the bottom reflector 33 that reflects the majority of light reflected at the filter 41 influences the recycling efficiency. Preferentially, the filter 33 has a total reflectivity above 95% over the backlight bandwidth. The reflector 33, which may be arranged on a side of the lightguide corresponding to the first major face, may have a reflectivity above 98%. The reflector 33 may be a diffuse reflector. A diffuse reflecting characteristic can act to improve the recycling in propagation direction compared to a specular reflector.
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 in
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.
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 in
These values are typical for polymeric layers used in birefringent filters, as disclosed for example in U.S. Pat. No. 5,882,774 (James M. Jonza et. al, 3M, 10 Mar. 1995).
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.
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 polarizer 3 of 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 TiO2 and SiO2 layers, 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.
Although the invention has been shown and described with respect to a certain embodiment or embodiments, equivalent alterations and modifications may occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including a reference to a “means”) used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein exemplary embodiment or embodiments of the invention. In addition, while a particular feature of the invention may have been described above with respect to only one or more of several embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application.
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.
