GB2161969A - Edge lit liquid crystal display device - Google Patents

Abstract

An edge-lit scattering mode type matrix addressed liquid crystal polychromatic display device has a set of filter elements in an layer (16) that register with the individual pixels of the liquid crystal layer (15). Light is piped into device to the rear of the liquid crystal layer via a leaky waveguide formed by the dielectric material lying between the rear face of the display device and an apertured reflector (19). in this way light to illuminate the central region of the display does not have to make multiple passes through the filter elements. The leaky waveguide may include a partial reflector or a total reflector provided with apertures. The light illumination may be made uniform by providing a gradation in the sizes of the apertures across the reflector. <IMAGE>

Description

SPECIFICATION Edge lit liquid crystal display device This invention relates to edge illuminated scattering mode type matrix addressed liquid crys tal display devices.

A matrix addressed scattering mode type liquid crystal display device that incorporates neither dye nor any colour filter elements can be relatively efficiently illuminated from the side by making use of total internal reflection.

Such edge illumination is described in the specification of our Patent Application No.

8312277, to which attention is directed. One of the reasons why such illumination works relatively well is that, even in the scattering state, the aborption of the liquid crystal is relatively low, and such light that is scattered at angles at which it is still trapped by total internal reflection is not wasted because much of it serves to illuminate other regions of the display area.

As soon as an attempt is made to introduce colour filter elements into the display device in order to be able to display polychromatic images the propagation of light by total internal reflection becomes heavily attenuated as the light has to pass through a filter between each successive pair of total internal reflections. In a simple display it is not possible to avoid this requirement because the separation between the liquid crystal layer and the colour filter elements is much less than the pixel spacing. For a typical device the separation needs to be in the range 0.3 to 0.03 mm. In the case of a conventional three-colour dye filter array a single pass through the filter will absorb at least two thirds of the light energy and possibly as much as 90%.Clearly, under these circumstances, after no more than a very few total internal reflections there is effectively no illumination to reflect.

According to the invention there is provided an edge illuminated scattering mode type matrix addressed liquid crystal display device incorporating colour filter elements of different colour in optical registry with the pixels formed in the liquid crystal layer of the device by the intersection of the electrodes, which device includes a leaky optical waveguide extending ac-oss the whole extent of the pixel display area by means of which the display is adapted to be obliquely lit within an angular range such that substantially all of the light is prevented by total internal reflection from emerging from the front surface of the display device unless first scattered by a region of the liquid crystal layer.

The solution provided by the invention to this problem of illumination by total internal reflection in the presence of colour filter elements is seen to lie in the use of a leaky feeder to introduce the illumination to the display area gradually over the full width of the display rather than have the light introduced all at once at the edge. This leaky feeder may for instance be provided by incorporating a partial reflector between the liquid crystal layer and the rear face of the display cell so that this reflector and the rear face cooperate to form the leaky optical waveguide.

Alternatively, a total reflector may be used which is provided with apertures to form a slotted optical waveguide, these apertures being in optical registry with the individual pixels. In either instance the effective reflectivity may be graded across the width of the display area to compensate for the progressive reduction in intensity of the propagating light on account of the leakage. Thus in the case of the partial reflector its reflectivity may be reduced with distance from the illumination sources, whereas in the case of the slotted waveguide the effective reflectivity may be reduced by increasing the size of the apertures.

There follows a description of display devices embodying the invention in preferred forms. This description refers to the accompanying drawing depicting a schematic crosssection through part of the device.

Referring to the drawing, a liquid crystal cell is formed by sealing together front and back transparent sheets 1 0,11 with a perimeter seal 1 2. The inward facing surfaces of the two sheets are respectively provided with sets of transparent row and column electrodes 13 and 14. The two sheets 10, 11, and the perimeter seal 12, combine to form an envelope of well-defined thickness which is filled with a liquid crystal material, typically a smectic or a nematic, to form a liquid crystal layer 1 5. Uniformity of thickness of the layer is normally assisted, particularly in the case of large area cells, by the trapping of a scattering of short lengths of glass fibre (not shown) of uniform diameter between the two electroded sheets.These spacers may be present not only in the display area enclosed by the perimeter seal but also in the seal itself where they are employed to determine the seal thickness.

The front sheet 10 is a composite structure incorporating a set of colour filter elements 1 6 that individually register with the individual pixels of the display that are defined by the areas of intersection of the various row electrodes with the various column electrodes.

These filter elements need to be as close as possible to the liquid crystal layer, and hence the front sheet comprises a glass substrate 1 7 supporting the filter elements 1 6 which are covered by a thin membrane 1 8 carrying the transparent electrodes 1 3.

The rear sheet 11 is similarly a composite structure, incorporating an apertured reflector 1 9 whose individual apertures register with the individual pixels. The portion of the rear sheet to the rear of the reflector 1 9 needs to be relatively thick compared with the width of an aperture so that the great majority of the light launched into this region from strip lamps 20 mounted along the sides of the display is able to propagate well beyond the first line of apertures, suffering multiple reflections alternately at the reflector surface 1 9 and, by total internal reflection, at the rear surface 21 of sheet 11.

The apertured reflector 19, and the part of the rear sheet 11 to the rear of that reflector, thus combine to function as a leaky waveguide that serves to distribute the illumination in a controlled manner to the front part of the display device that contains the liquid crystal layer and the colour filter elements. One feature of using this apertured reflector is that, because the apertures are in registry with the pixels, it can easily be arranged that the peripheral regions of the pixels, where scattering may be difficult to control, are not illuminated. The light distribution across the area of the display may be made more nearly uniform by having different sizes of apertures in the reflector 19.Those nearest the lamps 20 are smallest because here the light intensity is strongest in the leaky waveguide, whereas in regions more remote from the lamps the apertures are graded to a larger size to compensate for the reduced light intensity consequent upon the leakage of the waveguide.

If the holes in the reflector 1 9 are uniformly about 10% of the pixel area, then the reflectivity will be about 90%. After ten reflections the light will be reduced to about 35% of its original intensity upon entering the leaky waveguide in a distance of about 20 t where 't' is the thickness of the leaky waveguide.

For a leaky waveguide thickness of 1 Omm this distance is 200mm. By adjusting the size distribution of the apertures substantially uniform pixel illumination may be obtained, provided that the leaky waveguide is of adequate thickness. The 1 Omm thickness quoted above is a typically convenient thickness to implement.

It is preferable for the liquid crystal layer 1 5 to be between the reflector 1 9 and the filter elements 16, rather than for the filter elements to lie between the reflector and the liquid crystal layer. This is because the light that the viewer will see from a scattering pixel will be mainly that that is derived from illumination from the rear that has been scattered through a relatively small angle, but the viewer will also see a contribution that is derived from illumination from the front (after reflection in the front surface) that has been backscattered through a large angle.If a scattering pixel lies in front of its filter element this illumination from the front can be of a different colour to that that it is receiving from the rear, and hence the backscattered illumination from the front will tend to desaturate the wanted colour derived from small angle scattered illumination from the rear. If the filter lies in front of the pixel, the illumination from the front is filtered so that the only illumination to reach the pixel from the front is light of the appropriate colour.

In applications for which illumination efficiency is of particular importance it may be preferred to use interference type filter elements rather than light absorbing dye type filter elements. That is so that the light not transmitted by the filter elements shall be reflected for use elsewhere in the display. The design of interference filter elements will need to take account of the fact that the illumination is at an oblique angle, and should be designed to minimise the wavelength dependence of the filter pass band upon angle of incidence.

Claims (7)

1. An edge illuminated scattering mode type matrix addressed liquid crystal display device incorporating colour filter elements of different colour in optical registry with the pixels formed in the liquid crystal layer of the device by the intersection of the electrodes, which device includes a leaky optical waveguide extending across the whole extent of the pixel display area by means of which the display is adapted to be obliquely lit within an angular range such that substantially all of the light is prevented by total internal reflection from emerging from the front surface of the display device unless first scattered by a region of the liquid crystal layer.

2. A display device as claimed in claim 1, wherein the leaky optical waveguide includes a partial reflector.

3. A display device as claimed in claim 1, wherein the leaky optical waveguide includes an apertured reflector whose apertures are in optical registry with the pixels.

4. An edge illuminated scattering mode type matrix addressed liquid crystal display device, which device includes a liquid crystal layer sandwiched between front and rear transparent electroded sheets, wherein contained within the front sheet are a plurality of filter elements of different colours which elements are in optical registry with the display pixels of the matrix array, whereby the device is operable to display polychromatic images, wherein contained within the rear sheet is a reflective layer having apertures formed therein in optical registry with the display pixels, and wherein the display device incorporates display illumination means for directing light into the rear sheet behind its reflective layer within an angular range such that, at least over the display area, substantially all of the light is prevented from emerging from the externally facing major surface of either sheet by total internal reflection at these two externally facing surfaces unless first scattered by a region of the liquid crystal layer.

5. A display device as claimed in claim 3 or 4, wherein the size of the apertures is graded across the display area.

6. A display device as claimed in any preceding claim, wherein the colour filter elements are of interference filter type.

7. A display device as claimed in any preceding claims, wherein the colour filter elements are located in front of the liquid crystal layer.