GB2157049A

GB2157049A - Back lit display device

Info

Publication number: GB2157049A
Application number: GB08408995A
Authority: GB
Inventors: Edmund Gray
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1984-04-06
Filing date: 1984-04-06
Publication date: 1985-10-16
Also published as: GB8408995D0

Abstract

In a back lit scattering type matrix addressed liquid crystal display a coloured filter mosaic 4 is provided whose tessera register with the individual pixels of the display to provide a coloured display. This filter mosaic is imaged upon the liquid crystal layer 6 so as to avoid the chemical compatibility problems of a filter in direct contact with the liquid crystal material and the parallax problems of a filter located on an external surface of the cell. <IMAGE>

Description

SPECIFICATION Back-lit display device This invention relates to display devices incorporating matrix addressed display cells, for instance of the liquid crystal type in which an individual pixel of the display cell can be electronically converted between a first state in which the pixel is substantially transparent and non-scattering to light and a second state in which that light passing through the pixel is scattered. Cells of this kind may or may not be able to exist in intermediate states between these two states and thereby be directly capable of portraying grey-scale. One class of such cell is constituted by cells of the liquid crystal type. In particular this invention relates to such display devices incorporating pixels of different colour.

One way of providing a back-lit liquid crystal light-value type matrix addressed display cell with pixels of contrasting colour is to place a colour filter mosaic in contact with one face of the cell, and to arrange for the individual tessera of that mosaic to lie in registry with the individual pixels.

If the filter mosaic is located on the outside surface of the cell, then it is spaced from the liquid crystal layer itself by the thickness of one of the confining walls of the cell. The result is that the viewing angle becomes limited by the effects of parallax. Parallax is minimised by reducing the separation between the liquid crystal layer and the filter, but a lower limit to this separation is usually set by the constraints imposed by the need to adopt a cell construction that will enable a close control of the liquid crystal layer thickness.

If the filter mosaic is located inside the liquid crystal cell other problems are presented. Compatibility between the material of the liquid crystal layer and the materials used in the filter mosaic is a primary difficulty. Each material is liable to degrade the other, and extensive life tests would be necessary to find suitable materials. Furthermore, the scattering of light in the filter could seriously degrade contrast, and the need to avoid this sort of problem rules out the use of some convenient methods of making the filters.

An object of the present invention is to circumvent these problems of parallax and compatibility.

According to the present invention there is provided a display device including a matrix addressed scattering type display cell back-lit via a coloured filter mosaic imaged upon the cell with the individual tessera of the filter mosaic registering with the individual pixels of the display cell.

There follows a description of a liquid crystal display device embodying the invention in a preferred form. The description refers to the accompanying drawing which schematically depicts the device.

Light from a lamp 1 is directed through a heat filter 2 to a condenser lens system 3, and from there to a colour filter mosaic 4. On the opposite side of the filter mosaic a lens system 5 forms an image of the mosaic in the thickness of the liquid crystal layer of a matrix addressed valve-type liquid crystal display cell 6, so that the individual tessera of the filter mosaic register with the individual pixels of the display cell. In this way parallax between the filter and the liquid crystal layer is eliminated.

The viewing angle for the display cell for observation of the scattered light can typically be made in the region of + 80 except for a region near the axis where the illumination source is visible. If the lens system 5 is of symmetrical construction and the optical system between the filter and the display cell is symmetrical, then geometrical distortion is not significant, and chromatic aberration is also absent, both by the principle of reversibility.

For a display cell with a display area measuring 70 x 70mm, a lens system of 1 35mm focal length and f 5.6 aperture is suitable. If a 250 watt tungsten halogen incandescent lamp is used with a suitable condenser to focus the lamp filament on to the lens system 5, then the display has good visibility in normal room lighting. The condenser lens system for this purpose is advantageously about f 1.0. A reflector 7 placed behind the lamp improves efficiency.

In making the display cell, errors may occur to the non-squareness of the intersections of electrode row and column electrodes on the plates that define the pixels. However, these distortions can be corrected if the lens system 5 incorporates a variable anamorphic element.

Problems of matching the shape of the filter mosaic tessera with that of their corresponding pixels can be avoided by using the display cell itself to create the filter pattern in a photographic transparency film. For this purpose an unexposed photographic colourtransparency film is located in the position eventually to be occupied by the developed filter mosaic. The film is then exposed in turn to light of the various primary colours of the display through the lens system 5 whilst using the display cell as a selectively programmable mask. For each colour in turn the display is arranged so that only pixels of that colour are set into their transparent state, while the remainder are held in their light blocking state.It will be evident that, once a 'master' photographic filter mosaic has been made in this way, copies of that master can be made by normal photographic copying methods without recourse to the special masking techniques employed to generate the master.

1. A display device including a matrix addressed scattering type display cell back-lit via a coloured filter mosaic imaged upon the cell with the individual tessera of the filter mosaic registering with the individual pixels of the display cell.

2. A display device as claimed in claim 1, wherein the display cell is a liquid crystal display cell.

3. A display device as claimed in claim 1 or 2, wherein the filter mosaic is constituted by a photographic transparency generated by using the display cell as a programmable mask to form the latent image from which the filter pattern has been photographically developed.

4. A display device as claimed in claim 1 or 2,

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Back-lit display device This invention relates to display devices incorporating matrix addressed display cells, for instance of the liquid crystal type in which an individual pixel of the display cell can be electronically converted between a first state in which the pixel is substantially transparent and non-scattering to light and a second state in which that light passing through the pixel is scattered. Cells of this kind may or may not be able to exist in intermediate states between these two states and thereby be directly capable of portraying grey-scale. One class of such cell is constituted by cells of the liquid crystal type. In particular this invention relates to such display devices incorporating pixels of different colour. One way of providing a back-lit liquid crystal light-value type matrix addressed display cell with pixels of contrasting colour is to place a colour filter mosaic in contact with one face of the cell, and to arrange for the individual tessera of that mosaic to lie in registry with the individual pixels. If the filter mosaic is located on the outside surface of the cell, then it is spaced from the liquid crystal layer itself by the thickness of one of the confining walls of the cell. The result is that the viewing angle becomes limited by the effects of parallax. Parallax is minimised by reducing the separation between the liquid crystal layer and the filter, but a lower limit to this separation is usually set by the constraints imposed by the need to adopt a cell construction that will enable a close control of the liquid crystal layer thickness. If the filter mosaic is located inside the liquid crystal cell other problems are presented. Compatibility between the material of the liquid crystal layer and the materials used in the filter mosaic is a primary difficulty. Each material is liable to degrade the other, and extensive life tests would be necessary to find suitable materials. Furthermore, the scattering of light in the filter could seriously degrade contrast, and the need to avoid this sort of problem rules out the use of some convenient methods of making the filters. An object of the present invention is to circumvent these problems of parallax and compatibility. According to the present invention there is provided a display device including a matrix addressed scattering type display cell back-lit via a coloured filter mosaic imaged upon the cell with the individual tessera of the filter mosaic registering with the individual pixels of the display cell. There follows a description of a liquid crystal display device embodying the invention in a preferred form. The description refers to the accompanying drawing which schematically depicts the device. Light from a lamp 1 is directed through a heat filter 2 to a condenser lens system 3, and from there to a colour filter mosaic 4. On the opposite side of the filter mosaic a lens system 5 forms an image of the mosaic in the thickness of the liquid crystal layer of a matrix addressed valve-type liquid crystal display cell 6, so that the individual tessera of the filter mosaic register with the individual pixels of the display cell. In this way parallax between the filter and the liquid crystal layer is eliminated. The viewing angle for the display cell for observation of the scattered light can typically be made in the region of + 80 except for a region near the axis where the illumination source is visible. If the lens system 5 is of symmetrical construction and the optical system between the filter and the display cell is symmetrical, then geometrical distortion is not significant, and chromatic aberration is also absent, both by the principle of reversibility. For a display cell with a display area measuring 70 x 70mm, a lens system of 1 35mm focal length and f 5.6 aperture is suitable. If a 250 watt tungsten halogen incandescent lamp is used with a suitable condenser to focus the lamp filament on to the lens system 5, then the display has good visibility in normal room lighting. The condenser lens system for this purpose is advantageously about f 1.0. A reflector 7 placed behind the lamp improves efficiency. In making the display cell, errors may occur to the non-squareness of the intersections of electrode row and column electrodes on the plates that define the pixels. However, these distortions can be corrected if the lens system 5 incorporates a variable anamorphic element. Problems of matching the shape of the filter mosaic tessera with that of their corresponding pixels can be avoided by using the display cell itself to create the filter pattern in a photographic transparency film. For this purpose an unexposed photographic colourtransparency film is located in the position eventually to be occupied by the developed filter mosaic. The film is then exposed in turn to light of the various primary colours of the display through the lens system 5 whilst using the display cell as a selectively programmable mask. For each colour in turn the display is arranged so that only pixels of that colour are set into their transparent state, while the remainder are held in their light blocking state.It will be evident that, once a 'master' photographic filter mosaic has been made in this way, copies of that master can be made by normal photographic copying methods without recourse to the special masking techniques employed to generate the master. CLAIMS

4. A display device as claimed in claim 1 or 2, wherein the filter mosaic is constituted by a photographic transparency photographically produced from a master photographic transparency generated by using the same or a similar display cell as a programmable mask to form the latent image from which the filter pattern of the master photographic transparency has been photographically developed.

5. A display device substantially as hereinbefore described with reference to the accompanying drawing.