GB2155679A

GB2155679A - Liquid crystal display illumination

Info

Publication number: GB2155679A
Application number: GB08406074A
Authority: GB
Inventors: Edmund Gray
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1984-03-08
Filing date: 1984-03-08
Publication date: 1985-09-25
Also published as: GB8406074D0

Abstract

Normal or near normal back lighting of a matrix addressed scattering type liquid crystal cell 4 is provided using a lens array 7 in conjunction with a shadow mask 3. The mask has an array of opaque masking spots 9 set in a transparent field, and these spots are positioned and dimensioned to intercept all the unscattered light. When the pixels of the cell 4 are in straight lines, light source 1 may be a line source, lens elements 7 being cylindrical and mask 3 being formed with marking lines. <IMAGE>

Description

SPECIFICATION Liquid crystal display illumination This invention relates to liquid crystal display illumination and in particular to a back-lighting arrangement for scattering type matrix addressed liquid crystal display cells.

If a scattering type liquid crystal cell is not to be lit from the front, for instance by ambient lighting, it is conventional practice to light it obliquely from the rear or from the side edge. In either instance the illumination is necessarily at a relatively large oblique angle so that undeviated light can be prevented from reaching the eye of the observer.

This in turn means that any light that the observer does see is light that has had to be deviated by scattering through a sizeable angle. In a typical scattering type liquid crystal cell the intensity of scattered light falls off fairly sharply with increasing angle of deviation, and hence efficiency of illumination tends to be quite small. The present invention is concerned with an alternative arrangement of illumination affording improved efficiency of illumination.

According to the present invention there is provided a liquid crystal display device including a point light source positioned behind a matrix addressed scattering type liquid crystal display cell to provide illumination of the pixels of the liquid crystal cell from the rear; located between the source and the pixels a convergent lens array whose individual elements register with the individual pixels; and located in the image plane of the light source, in front of the pixels, a shadow mask having, in a transparent field, an array of non-transparent masking spots also registering with the pixels, each spot being small compared with the area of an individual pixel whilst being large enough to intercept the whole of the image of the point light source formed by the element in the lens array associated with that spot.

There follows a description of display devices embodying the invention in preferred forms. The description refers to the accompanying drawings, in which: Figure 1 depicts a schematic cross-section through one of the devices, and Figure 2 is a diagram of a layout employed for a polychromatic version of the device of Fig. 1.

The device of the accompanying Fig. 1 is a monochromatic display device and has a single white-light point source 1 contained within a matt black enclosure 2. A sufficiently close approximation to a point source may be provided for instance by the filament of a tungsten halogen lamp. Located in one wall of the enclosure 2 is a shadow mask plate 3 whose construction and function will be described later. Behind this plate 3 is located a matrix addressed scattering type liquid crystal display cell 4. This cell is typically a smectic cell, and some of its individual pixels are indicated at 5.

Between the liquid crystal cell 4 and the point source 1 is a lens array whose individual lens elements 7 register with the individual pixels 5. Each lens element 7 is required to form an image of the point source 1 in the plane of the shadow mask 3. For this purpose it is convenient to introduce a collimating lens 8 between the lens array and the point source 1 so that all the lens elements 7 may be identical. This collimating lens 8 may be a Fresnel type lens.

The shadow mask is constituted by a transparent plate provided with a set of nontransparent, preferably matt black, masking spots 9. Each masking spot registers with an individual one of the pixels, and a corresponding one of the lens elements. The size and position of each spot is such that all the light passing through the lens element from the source, and from thence undeviated through the pixel, is totally intercepted by the masking spot. Since the masking spots partially obstruct a view of the pixels, the optical system is designed so that the area of an individual masking spot is small compared with that of an individual pixel. Typically the area of a masking spot is made not more than one-fifth that of a pixel.

The device of Fig. 1 has been represented as a monochrome display device, but it will be evident that the system is readily adaptable for polychromatic displays where the various colours of the display are built up by mixing in appropriate proportions contributions from a palette of 'primary' colours. Typically this palette consists of the triad red, green and blue, in which case, instead of having a single point source of white light, there are now three point sources, one red, one green and one blue. Fig. 2 shows an arrangement where these three sources lie at the vertices of an equilateral triangle.

Fig. 2 shows a number of masking spots 901 to 915 corresponding to the masking spots 9 of Fig. 1, a number of lens elements 701 to 706 corresponding to the lens elements 7 of Fig. 1, and a number of pixels 501 to 518 corresponding to the pixels 5 of Fig. 1. Collimated light from each source is incident at a small oblique angle upon the lens elements as represented by the arrows R, G and B so that the image of the red light source formed by lens element 701 is intercepted by masking spot 901, while the image of the green light source formed by lens elements 702 is intercepted by masking spot 906, and the image of the blue light source formed by lens element 706 is intercepted by masking spot 915.With the equilateral arrangement depicted in Fig. 2 it has been possible to reduce the required number of masking spots by having the same masking spot serving to intercept one image of each of the three colours formed by different lens elements. Thus, for instance, masking spot 908 intercepts red, green and blue images formed respectively by lens elements 706, 703 and 702. The arrangement of pixels associated with each lens element is a set of three pixels that lie between that lens element and the images of the three light sources formed by that element. Thus associated with lens element 701 are pixels 501, 503, and 506, which are respectively red, green, and blue pixels, and are positioned to lie in the cones of converging light coming to a focus on masking spots 901, 905 and 907.

The foregoing description has related exclusively to the display devices using point sources in conjunction with shadow masks provided with masking spots. It will however, be evident that with the pixels arranged in straight lines it is possible to use one or more line sources instead of point sources, these lines being aligned with the pixel lines, and the co-operating masking areas of the shadow mark also being in the form of masking lines rather than masking spots. In this case the individual lens elements of the lens array are preferably cylindrical lenses rather than spherical.

Claims

1. A liquid crystal display device including a point light source positioned behind a matrix addressed scattering type liquid crystal display cell to provide illumination of the pixels of the liquid crystal cell from the rear; located between the source and the pixels a convergent lens array whose individual elements register with the individual pixels; and located in the image plane of the light source, in front of the pixels, a shadow mask having, in a transparent field, an array of non-transparent masking spots also registering with the pixels, each spot being small compared with the area of an individual pixel whilst being large enough to intercept the whole of the image of the point light source formed by the element in the lens array associated with that spot.

2. A liquid crystal display device as claimed in claim 1, wherein a collimating lens is located between the light source and the lens array.

3. A liquid crystal display device as claimed in claim 1 or 2, which device includes one or more additional point light sources of contrasting colour which are also located such that each image of these additional point light sources formed by the elements of the lens array is totally intercepted by one of the masking spots.

4. A liquid crystal display device as claimed in claim 3, wherein the arrangement of the point light sources in relation to the elements of the lens array and the shadow mask is such that different light sources are imaged by different line elements to produce coincident images on individual masking spots.

5. A liquid crystal display device substantially as hereinbefore described with reference to Fig. 1 or Figs. 1 and 2 of the accompanying drawings.

6. A liquid crystal display device including a line light source positioned behind a matrix addressed scattering type liquid crystal display cell to provide illumination of the pixels of the liquid crystal cell from the rear; located between the source and the pixels a convergent lens array whose individual elements are aligned with the line of the line source and are registered with and aligned with the lines of the individual pixels; and located in the image plane of the light source, in front of the pixels, a shadow mask having, in a transparent field, an array of non-transparent masking lines also registering with the pixel lines, each masking line width being small compared with the width of an individual pixel line whilst being large enough to intercept the whole of the image of the line source formed by the element in the lens array associated with that masking line.