GB2240213A

GB2240213A - Colour display device

Info

Publication number: GB2240213A
Application number: GB9001499A
Authority: GB
Inventors: Martin Weston
Original assignee: British Broadcasting Corp
Current assignee: British Broadcasting Corp
Priority date: 1990-01-23
Filing date: 1990-01-23
Publication date: 1991-07-24
Also published as: GB9001499D0

Abstract

A colour display device, in particular a shadow mask cathode ray tube, has a transparent screen 2 comprising an array of phosphor regions 3 emitting red, green and blue light, wherein in order to reduce both reflection of ambient light and "flare", respective colour filters 1 are located at each of the phosphor regions 3, as shown being interposed between the phosphors and the screen, and each respective filter is designed to pass light from the phosphor it covers, but to absorb light of other frequencies. The filters may comprise a chemical dye and may be deposited on the screen by a photo- sensitive process at the same time as, or before, the formation of the phosphor regions 3. Alternatively, the dye may be mixed with its respective phosphor and the two applied in a single operation. <IMAGE>

Description

COLOUR DISPLAY DEVICE This invention relates to colour display devices, such as for example cathode ray tubes (CRTs) and "beam indexed" displays, plasma panels and light emitting diodes (lets).

It is well known in displaying colour video images that a cathode ray tube known as a shadow-mask tube may be used. This contains three electron guns which produce three separate electron beams which move simultaneously in a scanning pattern over a fluorescent viewing screen so as to produce respectively a red, a green and a blue image. The fluorescent screen is composed of three separate sets of uniformly distributed phosphor dots on a glass screen. The dots of each set glow in a different colour.

Electrons discharged by the gun controlled by the red primary colour signal impinge only on the red-glowing phosphor dots and are prevented from impinging on the green-glowing and blue-glowing dots by a mask which contains many thousands of tiny holes, each of which is accurately aligned with the different coloured phosphor dots on the screen. Similarly, the electrons from the two other guns fall on the green-glowing and blue-glowing dots respectively. In this way three separate primary colour images are formed simultaneously.

The dots producing the three different colours are so small and so close together that the eye does not see them as separate points of light.

Such fluorescent viewing screens suffer from a disadvantage of limited effective contrast. This has two causes, reflection of ambient light from the fluorescent screen, and by a process known as flare, whereby light appears to originate at sane distance from its correct place on the fluorescent screen. This flare is a scattering of light caused by partial reflection of light at least once, both at the glass front surface then at the phosphor on the phosphorcarrying glass back surface.

These effects are usually minimised by making the screen out of dark glass. This attenuates the ambient light reflected off the phosphor which must pass through the glass twice, and also the flare whereby light must pass through the glass at least three times. As a result, both of these effects are attenuated more than the direct light output which only passes through the glass once. The contrast ratio is thus increased but at the expense of the brightness of the image.

According to the present invention there is provided a colour display device comprising a transparent screen viewable from one side, and an array of light emitting regions comprising lightemitting material and disposed on the other side of the screen, different regions having light-emitting material adapted to emit light of different colours, characterised in that the light emitting regions further osmprise colour filter material, the colour filter materials each having a pass-band corresponding to the colour of the light emitted by the light emitting material of the respective region, such that light passing through the screen to the lightemitting regions is reflected with reduced intensity outside the said pass-band of the filter material.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawing in which the sole figure is a sectional view of a fluorescent screen showing glass screen, phosphor dots and coloured filters.

The example illustrated is a face plate of a shadow-mask CRT.

The face place comprises a transparent glass screen 2 viewable from one side, namely the right-hand side as seen in the figure. On the surface of the glass 2 on the side within the CRT is an array of sets of phosphor regions 3. The phosphor regions respectively fluoresce with red, green and blue light in response to electrons from a respective red, green or blue green having passed through a shadow mask, in conventional manner. The different coloured light-emitting phosphor regions are designated R, G and B on the figure. The phosphor regions may be in various configurations such as patterns of dots or stripes, and will here be considered to be dots.

In accordance with this invention, colour filters 1 are disposed on the surface of the glass screen 2 between the phosphor regions and the screen 2. The colour filters 1 have similar shapes and are of similar area to the phosphor regions 3, the phosphor regions 3 being separated by a black matrix 4.

The processes of contrast enhancsnent will now be explained with regard to possible light ray paths (5-11, 15).

Each filter 1 is designed to pass light fram the phosphor region 3 it covers, but to absorb light at other frequencies. For example, the filter 1 covering red-phosphor 16 should pass red light 15 but absorb green and blue.

Ambient light 5 will only be reflected 6, 7, 8 if it landed on a phosphor region 6', 7', 8' of the correct colour. Because phosphor regions emit light over a fairly narrow range of frequencies, the filters 1 can be designed to absorb most of the incident ambient light 5, thus showing an improvement of greater than three to one. Thus the image contrast is enhanced due to the reduction in the reflection of ambient light.

Flare is also reduced because the light 9, 10, 11 reflected back into the screen is absorbed by tw9 thirds of the phosphor regions 12, 13, 14. It is only reflected out again if it lands on a phosphor region 12, 13, 14 of the correct colour. An example is that the red component of the ambient light will be absorbed by some phosphor regions (12,13) but reflected if it lands on a phosphor region of the correct colour (14). Thus, the degree of reduction will be of the order of two thirds.

By these means, the filters 1 significantly increase the image contrast, without reducing the brightness of the display.

In one embodiment of the invention, the colour filters are made of chemical dyes for example as used on the manufacture of photographic film. These are applied to the glass screen in a similar process to the application of phosphor regions, whereby the dye is mixed with a photo-sensitive binding material and spread over the inside surface of the glass screen. Light is then shone through the holes or slots of the shadow mask in order to fix the dye in the desired places. The unexposed dye is then washed off, and the process repeated for the other two dyes. The various colour-glowing phosphor regions are then applied on top of the colour filter material in a similar way.

Alternatively the number of operations can be reduced by applying the phosphor layer on top of the unexposed dye before fixing, or mixing the dye with the phosphor into a single mixture for application. In the latter case a single composite phosphor/filter layer will result.

Dichroic filters are unsuitable for use in the invention because they reflect, rather than absorb, unwanted frequencies.

In another embodiment of the invention, again by way of example, there is provided a "Beam Indexed" display. This is a cathode ray tube with a transparent screen covered with narraw stripes of phosphor on its internal surface. Colour filters are disposed between the phosphor stripes and the screen which reduce the reflection of ambient light and flare. There is no shadowmask and only a single election gun. This gun is modulated as it scans across the phosphor stripes. The phosphor stripes emit a variety of visible light or non-visible electromagnetic radiation.

For example, four types of phosphor stripes, red-light emitting, green-light emitting, blue-light emitting and ultra-violet emitting might be present. Then to produce a red image, for example, the gun is turned on while the election beam it produces lands on the red-light emitting phosphor regions. The exact position of the election beam is checked by detecting pulses of light (usually ultra-violet) produced by the fourth set of "index" stripes.

The system described can also be adapted for use with other types of colour display device, including for example plasma panels and displays based on light emitting diodes.

Claims

1. A colour display device comprising a transparent screen viewable fran one side, and an array of light emitting regions comprising light-emitting material and disposed on the other side of the screen, different regions having light-emitting material adapted to emit light of different colours, characterised in that the light emitting regions further ccarrprise colour filter material, the colour filter materials each having a pass-band corresponding to the colour of the light emitted by the light emitting material of the respective region, such that light passing through the screen to the light-emitting regions is reflected with reduced intensity outside the said pass-band of the filter material.

2. A colour display device according to claim 1, in which the colour filter materials are interdisposed between the screen and the regions of light-emitting material.

3. A colour display device according to claim 1, in which the light emitting regions each comprise a mixture of light emitting material and colour filter material.

4. A colour display device according to any preceding claim, in which the light emitting material is a fluorescent material.

5. A colour display device according to any preceding claim, in which the array of light emitting regions comprises sets of redemitting, green-emitting and blue-emitting regions.

6. A colour cathode ray tube, comprising a transparent face plate, and an array of light-emitting regions comprising a fluorescent phosphor material and disposed on the interior surface of the face plate, different regions having phosphors adapted to emit light of different colours, characterised in that the light-emitting regions further comprise colour filter material, the colour filter materials each having a pass-band corresponding to the colour of the light emitted by the respective phosphor, such that light passing through the screen to the light-emitting regions is reflected with reduced intensity outside the pass-band of the filter material.

7. A colour display device, substantially as herein described with reference to and as shown in the accompanying drawing.