GB2373620A

GB2373620A - Light source arrangements for displays

Info

Publication number: GB2373620A
Application number: GB0107122A
Authority: GB
Inventors: Rafie Mavaddat
Original assignee: Cambridge University Technical Services Ltd CUTS
Current assignee: Cambridge University Technical Services Ltd CUTS
Priority date: 2001-03-21
Filing date: 2001-03-21
Publication date: 2002-09-25
Anticipated expiration: 2021-03-21
Also published as: GB2373620B; GB0107122D0

Abstract

A display comprises a light source 18 for producing activation light at a predetermined narrow range of wavelengths; a light modulator 30 such as an LC cell formed from an array of pixels and adapted to modulate the excitation light; and a screen, preferably a photoluminescent screen 16, arranged to emit a visible output when struck by the narrow-band excitation light passing through the cell. The light source contains a set of light-emitting point sources such as LEDs 24, and an optical arrangement 26 is included between the sources and the LC cell to form the excitation light into a slightly divergent beam. In this way a seamless composite image can be produced on the screen.

Description

LIGHT SOURCE ARRANGEMENT FOR DISPLAYS

The present invention relates to backlit photoluminescent liquid-crystal displays (PL-LCD) in which the illuminating light is modulaced by a liquid crystal cell and then hits a phosphor screen, exciting the phosphor and producing a display. Displays of this kind are known for instance from WO 95/27920 (Crossland et al. ) in which collimated monochromatic UV light is incident on an LC cell, and a full-colour display can be produced by virtue of suitable phosphors excited by the light modulated by the cell. Such displays, however, need to be used with well collimated light to avoid"crosstalk"between the respective pixels; light that has passed through a particular pixel on the liquid-crystal panel needs to hit the corresponding phosphor in order to give an accurate image. Moreover a narrow range of transmission angles is essential if one is to achieve a sufficient LCD contrast ratio.

The backlight illumination needs to have a wavelength which will excite the phosphor screen, which is why UV or near-UV visible light is generally used in such displays. However, collimation is not readily achievable with known sources since they tend to be extended sources, for example UV fluorescent lamps (FL). A recent development is the availability of near-visible UV light-emitting diodes (LEDs) ; their large potential market, and hence the possibility of their dramatic price reduction, makes them a viable alternative to FL backlight design. An array of LEDs can be assembled to form a diffuse extended light source, which is similar to FL back illumination. A paper published by R Yamaguchi et al. of Akita University ("Fluorescent liquid crystal display using a UV light emitting diode", IDW 98, pp. 25-28) shows the

use of such a display illuminated by an extended source which is made up of many UV or near-UV LEDs. However, this document does not address the problem of collimation in such a display.

Accordingly, embodiments of the present invention aim to eliminate the need for collimating a diffuse source in a display of the liquid-crystal/phosphor type.

According to the present invention there is provided a PL-LCD display comprising: a light source for producing activation light at a predetermined narrow range of wavelengths; a light modulator such as an LC cell formed from an array of pixels and adapted to modulate the excitation light; and a photoluminescent screen arranged to emit a visible output when struck by the narrow-band excitation light passing through the cell; wherein the light source contains one or more light-emitting point sources, and an optical arrangement is included between the sources and the LC cell to form the excitation light into a slightly divergent beam.

The point sources are preferably LEDs and can be arranged in a regular fashion, with a corresponding lens arrangement to form a suitably shaped beam, each illuminating a portion of the LC cell. To avoid crosstalk the light from each LED can be stopped down so that each pixel receives light from only one LED.

In doing this the present invention takes advantage of the inherent near-point-source property of the light emitting diodes, which the prior art does not do.

Preferably each LED has a beam-shaping device to ensure that it emits a uniform cone of light in the direction of the modulator.

In the invention, instead of being collimated to a parallel beam, the light from the point source is formed to a slightly divergent beam before striking the

LC cell. This enables the image on the screen to be made a little larger than that on the LC panel. In this case the region of each LC panel illuminated by a single source must be spaced slightly from the adjacent region illuminated by the next point source The gap between the LC panel regions can be a physical gap where no modulating pixels exist, or, preferably for most purposes, an"addressing gap"where pixels exist but are not used.

Individual sources can be mounted together in a single mounting to form a backlight unit. The optical arrangement can most simply be a set of lenses, one for each source. If only a slight magnification is required then the LEDs can lie on the axes of their lenses. However, normally the modulating gap at the edge of an LC panel requires more divergence then this; here therefore the LEDs should be arranged with an offset from the optic axis that increases towards the edge of the LC panel.

Many modules of displays as outlined above can then be put together to make a larger display; in this case, although there are gaps between the LC panels, the images or projections on the phosphor screen are tiled seamlessly together to give a uniform display.

For a better understanding of the invention embodiments of it will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 shows a pixel of the photoluminescent (PL) screen consisting of a 3x3 array of red, green and blue sub-pixels ; Fig. 2 shows a PL panel corresponding to a complete LC module, divided into 3x4 regions; Fig. 3 is a schematic diagram showing a display module in accordance with the invention, with an LED backlight assembly with collimating lenses, an LC cell

and a PL screen ; Fig. 4 is a schematic. diagram showing the pixel arrangement on the LC panel ; and Fig. 5 shows a side view in section of the system.

Figure 1 shows how a pixel 10 of a Pu screen is made up of nine coloured sub-pixels 12. A screen such as this allows a visible coloured display to be formed from a monochromatic, non-visible light source.

Figure 2 shows a module of the PL panel 14, or rather an area of the PL panel corresponding to one LC cell or module. Each module is made up of 3x4 square regions 16, which in turn are made up of 64x64 pixels 10. The complete panel is made up of any desired number of such modules, as described below.

Figure 3 shows a schematic representation of a single LED backlight unit or BLU 18. Each of these units illuminates a corresponding region 30 of an LC modulator panel and, if transmitted, the PL panel region 16. Each LED-BLU plug-in unit is designed as a self-contained unit that is accessible and easily replaceable in case of any LED failure. It consists of a 4x4 array of near-blue UV LEDs 24, together with an array of 4x4 aspheric lenses 26. The light from each LED is substantially collimated by the aspheric lens 26 but the beam is left with a small divergence.

The LC panel region 30 is placed immediately behind the lens array 26, so that the modulated activation light spreads out before hitting the PL panel 14. The phosphor on the PL panel is then excited and visible light is emitted. The pixels on the LC panel are formed in blocks or portions 32, each corresponding to one LED 24, and this slight magnification, as described in more detail below, eliminates the necessary but undesirable spacing on the LC panel required for the seal and LCD drivers at the

edge of the panel, which is typically at least 5 mm.

Each of the 3x4 regions 16 shown in Fig. 2 is illuminated by a respective LED backlighc assembly 18, and respective modules consisting of an LC panel and its twelve BLUs can be joined together to form a seamless image.

In a specific example we assume that a large display panel consists of any required number of tiled modules each with the dimensions 30.72 cm x 23.04 cm.

For example, tiling three by four modules would produce a display screen of approximately 92 cm x 92 cm and tiling 4x4 modules would produce a display screen of approximately 123 cm by 92 cm. As in the example in Figure 2 we further assume that the PL panel of each module 14 consists of 4x3 regions 16, each region having 64x64 pixels so that there are 256x192 pixels in total on each module. Each of these pixels is made up of red (R), green (G) and blue (B) sub-pixels with dimensions 0.4 mm by 0.4 mm, and arranged as shown in Figure 1.

In the example shown the LC panel is divided into twelve regions about 8x8 cm square, and there are twelve BLUs, one for each region. If the BLUs are, for simplicity, to be identical, then each LC region has to have a 4 mm border, even though this additional space could be said to be wasted. An alternative would be to have a BLU that covered the whole LC panel, with, say, 12x16 = 192 LEDs, with the progressive offset towards the edge of the panel, though this is a fairly complex arrangement. A further alternative would be to have 8x8 cm LCD panels, each with one BLU. Clearly many permutations of this sort are possible.

Figure 4 shows the LC panel region 30 to be imaged on the corresponding region 16 of the PL panel 14. In this example it is assumed that the sub-pixels of the LC panel have dimensions of 0.333 mm by 0.333 mm, and

form pixels 31 with dimensions of 1 mm by 1 mm. The pixels 31 are divided into an array of 4x4 blocks 32, each containing 16x16 pixels 31, with inter-block spacing 34 of 1.6 mm and a border 36 of 4 mm. The 4-mm border between respective portions of the LC panel 30 provides enough spacing for the wiring of the LC panel.

This gap does not show on the output produced by the PL screen because of the above-mentioned magnification of the image. The border spacing and the inter-block spacing helps with the seamless imaging of LC pixels 31 onto the pixels 10 of the PL panel 14.

Figure 5 shows a section through one column of the region, with its LED and lens arrangement, and a stop 40 ensuring that the light from any one LED reaches only its corresponding block 32. In this example the source 24 is 29.6 mm away from the lenses 26, and the LC cell (lens and the LC being close together) is 48 mm away from the PL screen 14. Each source 24 is located slightly closer towards the central horizontal plane of the central axis of the group, than the middle of its corresponding block 32, i. e. than its optic axis, so that the light from each block is"tilted"slightly outwards in order to make up the 4 mm border. Moreover the amount of offset increases towards the border 36.

The height of the section 16 of the PL screen over which the four sources are being imaged in this example is 76. 8 mm, and the border spacing 36 is 4 mm, as explained previously. This means that for a lens with an F-number of 2 the maximum angle of off-normal divergence required for the beam is:

which is acceptable for the contrast requirement of the LC panel 20. The lens has to be carefully designed to minimise any aberrations, and must be placed as close

as possible to the panel to minimise the length cf the optical system. The point source is placed close to the focus of the lens 2f, with the distance adjusted to give the required magnification. It is also important to place refractive or reflective beam-forming optics close to the LED source 24 to produce a uniform illumination across each lens. In this example the focal length of the lens is 32 mm and the LEDs have been placed a distance of 29.6 mm from the lens. This gives a small magnification of 76.8/68. 8 = 1.12.

To examine the possibility of image degradation by diffraction of propagating light between LC and PL panels, a short'Glad'programme was used. Assuming a coherent source with a wavelength of 0.376 m illuminating a single sub-pixel, the intensity variation at the pixel is shown below in Figure 6 and its image, 4. 8 cm away, is shown in Figure 7. As can be seen, the diffraction of light to adjacent sub pixels is not appreciable.

Clearly other arrangements of blocks than 4x4, and other module sizes, can be used, but the present example suits currently available LC cells and other components. Also the arrays need not be square-a hexagonal array of sources, for instance, would be advantageous. Furthermore, the invention is not restricted to PL-LCD-type displays: with white LEDs a projection display of the more conventional type can be made, using a diffusing instead of a photoluminescent screen.

Claims

CLAIMS 1. A display comprising : a light source (18) for producing activation light at a predetermined narrow range of wavelengths ; a light modulator (30) such as an LC cell formed from an array cf pixels and adapted to modulate the excitation light; and a screen, preferably a photoluminescent screen (16), arranged to emit a visible output when struck by the narrow-band excitation light passing through the cell; wherein the light source contains a set of lightemitting point sources (24), and an optical arrangement (26) is included between the sources and the LC cell to form the excitation light into a slightly divergent beam.
2. A display according to claim 1, wherein the radiation emitted by the point sources (24) is in the UV or near-UV visible part of the spectrum.
3. A display according to claim 1 or 2, in which the point sources are LEDs and are arranged in a regular array, the optical arrangement being a corresponding array of lenses each forming a suitably shaped beam illuminating a portion of the LC cell.
4. A display according to any preceding claim, wherein the light source (24) is placed between the lens and the focal point of the lens so as to form the said slightly divergent beam.
5. A display according to any preceding claim, wherein the pixels (31) on the LC cell (30) are divided into blocks (32) with gaps between them, each block

corresponding to one point source (24).
6. A display according to claim 5, wherein the divergence of the radiation passing through the pixels (31) on the LC cell before it hits the PL screen (14) is such as to eliminate the gaps-in the image caused by the said gaps (34) between the blocks of pixels (31) of the LC cell (30).
7. A display according to any preceding claim, in which each LED has a beam-shaping device to ensure that it emits a uniform cone of light in the direction of the modulator.
8. A display according to any preceding claim, in which the point sources are arranged with a transverse offset from the optic axis of their respective lenses that increases towards the edge of the LC panel, so that an additional border (36) can be present round the outer edge of the modulator (30).
9. A display module comprising one or more displays according to claim 8, in which the light modulator is an LC panel and the additional border (36) coincides with the opaque edge of the panel.
10. A display assembly comprising an array of modules according to claim 9 arranged adjacently so as to form a continuous image on the screen.