GB2272277A

GB2272277A - An optical display backlight

Info

Publication number: GB2272277A
Application number: GB9221915A
Authority: GB
Inventors: Colin Teck Hooi Yeoh; Andrew Russell Keith Webb; Carolyn Devereux
Original assignee: GEC Marconi Ltd; Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1992-10-19
Filing date: 1992-10-19
Publication date: 1994-05-11
Anticipated expiration: 2012-10-19
Also published as: GB2272277B; GB9221915D0

Abstract

An optical display backlight includes a source of unpolarised light and a group of transparent surfaces 3, 5, 7, 9, 11, 13 and 15 each inclined at its Brewster angle to direction of the light is reflected by the surfaces to provide a polarised light source for an optical display 39. <IMAGE>

Description

Display Devices This invention relates to display devices, and particularly to liquid crystal display panels.

Most transmissive liquid crystal displays (LCDs) employ two dichroic polarisers to present the difference between dark and bright states. The brightness of these displays is limited by the absorption of the polarisers. The loss of transmitted light at the input polariser is 60% and a further transmission loss of 20% occurs at the output polariser (analyser). Dichroic polarisers are therefore very inefficient.

It is an object of the present invention to provide an improved polarised light source for a liquid crystal display.

According to the invention there is provided an optical display backlight, comprising a source of unpolarised light; and a group of transparent surfaces disposed at the Brewster angle to the direction of said light impinging on the surfaces, so that s-polarised light is reflected by the surfaces to provide a polarised light source for the optical display.

Preferably the surfaces are disposed such that light from the source passes through the surfaces in sequence, and output light after passage through the surfaces is primarily p-polarised light.

Preferably means is provided to rotate the polarisation of the output light through 90" and to return it through the group of surfaces so that further s-polarised light is emitted by each surface for illuminating the optical display.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which Figures 1(a) and 1(b) and 1(c) are schematic cross-sectional views illustrating successive passes of light through a first embodiment of the invention.

Figures 2(a) and 2(b) are graphs showing s-polarised light output from each Brewster plate in successive passes in Figure 1, Figure 3 shows the resultant s-polarised light output obtained across the width of the device of Figure 1, Figure 4 is similar to Figure 3, but for a device having thirteen Brewster plates, Figure 5 illustrates different refractive indices of Brewster plates of a further embodiment, Figure 6 is similar to Figure 3, but in relation to the device of Figure 5, Figures 7,8,9 and 10 are schematic cross-sectional views of respective further embodiments of the invention, Figure 11 shows the reflectance of s-polarised light and p-polarised light at differnet light wavelengths in the device of Figure 10, and Figures 12 and 13 are schematic cross-sectional views illustrating respective applications of devices in accordance with the invention.

Referring to Figure 1 of the drawings, a backlight 1 for illuminating an optical display, such as a liquid crystal display, comprises a row of seven equally spaced-apart transparent plates 3,5,7,9,11,13 and 15, each inclined at its Brewster angle to the direction 17 of unpolarised light from a source 19 such as a fluorescent or incandescent lamp. The Brewster angle e measured from the normal to the plate is given by tan e = n where n is the refractive index of the plate.

The light is split into two components by the plates, namely s-polarised light, i.e. light which is polarised normal to the plane of incidence on the plate, which is reflected by the plates along paths indicated by arrows 21, and p-polarised light 23 which is emitted from the end of the row of plates.

Referring to Figure 1(b),the output p-polarised light 23 is passed through a i-wavelength phase plate 25 and is reflected back through the phase plate 25 by a mirror 27, so that the polarisation of the light is rotated by 90" and the light becomes s-polarised. This light, indicated by an arrow 29, is passed back through the row of plates 15-3. Some of the light is reflected at each of the plates along paths indicated by arrows 31. This light is reflected by a mirror 33 along paths 35 and most of it passes through the plates 3-15 to augment the s-polarised light already passing along the paths 21. The remainder of the light is reflected by the plates, along paths 37, towards the phase plate 25.

The resulting light emitted along the paths 21 and 35, which is therefore entirely s-polarised, is used to illuminate a transmissive optical display 39.

Assuming that the plates 3-15 are made of glass with a refractive index n of 1.5, the reflectivity of s-polarisation at successive plates in the first pass is as shown in Figure 2(a). The reflectivity (i.e. the percentage of the input light at each plate which is reflected as s-polarised light) follows a decaying function. In the second pass back through the plates, the reflectivity similarly follows a decaying function (Figure 2(b)).

Combination of the functions for the three passes gives an approximately uniform distribution of light for illuminating the display 39. This is illustrated in Figure 3 (full line) in comparison with a theoretical polariser having 100% polarisation efficiency (dashed line). The polarisation efficiency PE is given by the ratio of total output polarised light intensity to input unpolarised light intensity. The value of PE for Figure 3 is 79%.

Figure 4 is similar to Figure 3, but illustrates the light distribution obtained from a backlight comprising thirteen Brewster plates, for which n = 1.3 and PE = 79%.

In an alternative seven-plate arrangement, the Brewster plates are made of different materials, such that their refractive indices vary from one plate to another as shown in Figure 5. This has the effect of raising the centre of the light distribution curve, as shown in Figure 6. A polarisation efficiency of, say, 83% can be achieved. The plates having high refractive index can be produced from thin evaporated metallic films, or doped glass or suitable polymeric materials.

An alternative configuration in accordance with invention is shown in Figure 7. This uses two groups 41,43 each of six Brewster plates, coupled end-to-end, with a ±wavelength phase plate 45 therebetween. As in the Figure 1 embodiment, a mirror 47 is located at the back (i.e. non-output) side of the plates. Light is fed into the ends of the groups 41 and 43 from light sources 49,51, respectively. This configuration provides a large area backlight.

Figure 8 shows a further alternative configuration of backlight in accordance with the invention. This comprises an arrangement similar to Figure 1 but with the planar mirror of Figure 1 replaced by a mirror 53 of sawtooth cross-section. The light reflected by the inclined faces 55 of the mirror is normal to the inclined faces, and the angle of the plates 3-15 is chosen accordingly. This configuration reduces the overall thickness of the backlight. A plurality of such faces 55 may be provided for each plate.

In another embodiment (Figure 9) the plates are replaced by prisms 57,59,61 having faces 63 and 65, 67 and 69, and 71 and 73, respectively. Mirrors 27 and 33 and a i-wavelength phase plate 25 are provided as in Figure 1. Each face 63-73 acts as a Brewster plate.

Figure 10 shows a modification of the Figure 9 embodiment, in which the plates of Figure 1 are replaced by refracting surfaces 75,77,79,81,83 and 85 embedded in a transparent substrate 87. Each refracting surface comprises a stack of dielectric films of i-wavelength thickness, the films being formed of high and low refractive index materials alternately. For example, the high refractive index value (nH) may be 1.7 and the low refractive index value (nL) may be 1.5, the refractive index of the substrate material being, for example, 1.59. As in the Figure 1 embodiment, a i-wavelength phase plate 25 and a mirror 27 are provided. The bottom surface 89 (as seen in Figure 10) of the substrate is made reflective or a separate mirror is located in contact with that surface.

The reflectance/wavelength characteristic of such a stack having the refractive indices mentioned above and a 45" angle of incidence of the light on each refracting surface is shown in Figure 11, where R5 and Rp are the refl ectances to s-polarised and p-polarised light, respectively.

Figure 12 illustrates schematically an application of a backlight in accordance with the invention, wherein collimated light 91 received from the backlight is passed through a microlens 93 which is aligned with a liquid crystal picture element 95 between two masking areas 97,99. The microlens 93 focuses the light on to the liquid crystal pixel, so that the light is concentrated at the pixel and a smaller proportion of the light is lost. A brighter pixel is thereby achieved. The brightness can be increased by a factor of 2. Furthermore, higher pixel resolution can be achieved, since the pixel can be made smaller without changing the inter-pixel gap. It is not essential to separate the pixels by using the conventional black matrix masking areas 97,99 in the inter-pixel gaps, because the light is focused only at the pixel areas.The omission of the black matrix simplifies the manufacture of the display. The microlens 93 is part of a microlens array 101.

Figure 13 illustrates an extension of the Figure 12 embodiment in which the collimated light 91 passes through a colour filter array 13 before reaching the microlens array 101. Since the light is focused by each microlens, there is no problem with parallax from the colour filters. This use of external colour filters also increases the yield and decreases the cost of liquid crystal display (LCD) panels, because the processing of the external colour filter array using photographic film is less critical than the processing used for filters within the display panel.

Conventional LCD backlights are only about 25% efficient, i.e. of the light leaving the source only 25% subsequently arrives at the rear of the LCD. The usual input dichroic polariser used in an LCD introduces a further 60% loss and this gives an effective overall efficiency of 10%. Therefore brighter sources are used, with the result that most of the power consumed in LCD panels is due to the backlights. The present invention offers a highly efficient means of providing uniform polarised light for LCD panels because the evenly-spaced Brewster plates provide an efficient means of polarising light and distributing the light uniformly. Furthermore, the combination of polarisation invertion and beam combination allows more efficient use of the available light.

Claims

1. An optical display backlight, comprising a source of unpolarised light; and a group of transparent surfaces disposed at the Brewster angle to the direction of said light impinging on the surfaces, so that s-polarised light is reflected by the surfaces to provide a polarised light source for the optical display.

2. A backlight as claimed in Claim 1, wherein the surfaces are disposed such that light from the source passes through the surfaces in sequence, output light after passage through the surfaces being primarily p-polarised light.

3. A backlight as claimed in Claim 2, comprising means to rotate the polarisation of said output light through 90* and to return it through the group of surfaces so that further s-polarised light is provided by each surface for iluminating the optical display.

4. A backlight as claimed in Claim 3, wherein the means to rotate the polarisation comprises a i-wavelength phase plate through which the output light passes, and mirror means for reflecting that light back through the phase plate and thence back through the group of surfaces.

5. A backlight as claimed in Claim 4, inclucing further mirror means for reflecting back throught the surfaces light which has been reflected by the surfaces in a direction opposite to that required for illuminating the optical display.

6. A backlight as claimed in Claim 5, wherein said further mirror means has a number of inclined reflecting surfaces, each associated with one or more respective said transparent surfaces.

7. A backlight as claimed in any one of Claims 1-6, wherein each said transparent surface is provided by a transparent plate.

8. A backlight as claimed in any one of Claims 1-5, wherein each said transparent surface is a surface of a prism.

9 A backlight as claimed in any one of Claims 1-5, wherein each said transparent surface is provided by a stack of dielectric layers of i-wavelength thickness and of alternately high and low refractive index.

10. A backlight as claimed in Claim 9, wherein the stacks are embedded in a transparent substrate.

11. A backlight as claimed in Claim 1, comprising two of said groups of transparent surfaces disposed in end-to-end configuration with means therebetween to rotate the polarisation of light received from each group before passing it to the other of said groups; and sources of unpolarised light located respectively adjacent opposite ends of the configuration.

12. A backlight as claimed in any preceding claim, comprising a microlens arrangement located to receive said s-polarised light and to focus it on a pixel of the optical display.

13. A backlight as claimed in Claim 18, further comprising a colour filter arrangement disposed between the s-polarised light source and the microlens arrangement.

14. An optical display backlight substantially as hereinbefore described with reference to the aocompanying drawings.