GB2178226A

GB2178226A - Cathode ray tubes

Info

Publication number: GB2178226A
Application number: GB08614197A
Authority: GB
Inventors: Raymond Brogan; Martin William Redfern
Original assignee: GEC Avionics Ltd
Current assignee: Allard Way Holdings Ltd
Priority date: 1985-06-12
Filing date: 1986-06-11
Publication date: 1987-02-04
Also published as: GB8614197D0

Abstract

A shadow mask colour cathode ray tube has a fibre optic faceplate (11) with the customary distribution of phosphor dot groups (R, G, B) on the interior surface (13) of the faceplate, and on the exterior surface (15), a corresponding distribution of filter dot groups (RF, GF, BF), the spectral pass-band of each filter dot corresponding to the spectral characteristics of the light emitted by the phosphor dot to which it is optically coupled by way of the faceplate. The design of the faceplate enhances the contrast ratio between image luminance and background luminance, thus rendering the tube more usable under conditions of high ambient illumination. A photoresist method for forming the filter dots on the exterior of the faceplate is disclosed which uses light from the respective coloured phosphor dots. <IMAGE>

Description

SPECIFICATION Cathode ray tubes This invention relates to cathode ray tubes, in particular to shadow mask colour cathode ray tubes.

Cathode ray tube (CRT) displays are difficult to read under conditions of high ambient illumination. This is because the CRT emits light in selected areas of its phosphor screen to create its display image (rather than altering its reflectivity like a liquid crystal display) and this has to compete with the high luminance of the diffusely reflected ambient illumination.

At best the display image on small monochrome CRTs can be made of similar brightness to that caused by ambient illumination, but for high resolution raster displays it may perhaps be as little as one tenth as much. For a shadow mask colour CRT the display image is even dimmer because of power loss to, and thermal limitations of, the shadow mask within the CRT.

To overcome this difficulty it is necessary to increase the ratio of the luminance of the image to that of the reflected ambient luminance (contrast ratio) even if both luminances are reduced. If the ambient illumination cannot be reduced (as in some aircraft cockpits, for instance) then it is usual to use a filter in front of the CRT screen which will attenuate the ambient illumination twice (once before and once after reflection) but the display only once; this improves the contrast ratio but at the expense of luminance. Since the viewer's eyes may be adapted to viewing other bright images with the same ambient illumination there is a limit to how much the display may be usefully filtered. This may be overcome to some extent with monochrome displays by using a phosphor that emits within a very narrow spectral band.The filter can then be given a correspondingly narrow band-pass characteristic so that it attenuates the ambient illumination far more than, but does not attenuate the display luminance so much as, a neutral density filter giving the same contrast ratio.

In a shadow mask colour CRT the screen comprises a distribution of phosphor dot groups on the interior surface of the tube faceplate, the dots in each group emitting light of different colours. Normally there are three dots in each group which emit red, green and blue light respectively. The use of a conventional band-pass filter to improve the contrast ratio of the display of such a CRT has little benefit over a neutral density filter because the filter is spatially uniform and is required to hve three pass bands, consequently allowing more ambient light through.

It is an object of the present invention to provide a shadow mask colour CRT wherein this problem is overcome.

According to the present invention a shadow mask colour cathode ray tube has a fibre optic faceplate: on the interior surface of the faceplate, a distribution of phosphor dot groups, the dots in each group emitting light of different colours; and on the exterior surface of the faceplate a corresponding distribution of filter dot groups arranged so that each filter dot is optically coupled to the corresponding one of the phosphor dots by an individual group of fibres of the faceplate, the spectral pass band of each filter dot at least partly overlapping the spectral characteristic of the light emitted by the phosphor dot to which it is optically coupled by way of the faceplate.

One shadow mask colour cathode ray tube in accordance with the invention will now be described, by way of example, with reference to the accompanying single figure which is a diagrammatic sectional view of part of the shadow mask, faceplate and screen of the tube.

Referring to the drawing, the tube has a fibre optic faceplate 11, i.e. a faceplate consisting a compacted mass of individual optical fibres extending in a direction perpendicular to the main surfaces of the faceplate 11. On the interior main surface 13 of the faceplate 11, there are uniformly distributed a large number of groups of three phosphor dots R, G and B, the dots in each group consisting respectively of red, green and blue phosphors of narrow spectral characteristic. In the drawing, only two such groups are illustrated and the phosphor dots of these are shown arranged linearly, rather than in the usual triangular configuration, for the purposes of clarity. The exterior main surace 15 of the faceplate 11 has red, green and blue filter dots RF, GF and BF.

The filter dots RF, GF and BF are positioned on the faceplate 11 in a corresponding manner to the phosphor dots R, G and B, and each filter dot RF, GB or BF is optically coupled to the corresponding red, green or blue phosphor dot R, G or B by way of an individual group of optical fibres of the faceplate 11. Each filter dot has a spectral pass-band substantially matched to the spectral characteristic of the light emitted by the phosphor dot to which it is optically coupled. Areas of the exterior surface 1 5 of the faceplate 11 not occupied by a filter dot RF, GF or BF are coated with a black filler material 17.

Within the tube adjacent the interior surface 13 of the faceplate 11 there is positioned a shadow mask 19 of conventional form. The mask 19 comprises a metal plate in which are formed a large number of apertures 21, one for each group of phosphor dots R, G and B.

The apertures 21 are positioned so that electrons 23, 25 and 27 from red, green and blue electron guns (not shown) of the CRT respectively can impinge via the apertures 21 only on the red, green and blue phosphor dots R, G and B respectively.

In operation when electrons from an electron gun of the cathode ray tube are incident on a phosphor dot R, G or B, light emitted by that dot is directed by the associated small group of optical fibres to the corresponding filter dot RF, GF or BF which, having a spectral pass-band corresponding substantially to the spectral characteristic of the associated phosphor dot, transmits light from the phosphor dot to a viewer. The fibre optic faceplate 11 thus prevents lateral spreading of the light from the phosphor dot R, G or B within the thickness of the faceplate 11. Of the ambient light incident on the faceplate 11 only the red, green or blue component, as the case may be, reaches the phosphor dot via the associated filter dot.Hence, any ambient light so transmitted to and subsequently reflected by a phosphor dot to the viewer is subjected to two-fold attenuation by the filter dot, as opposed to the single attenuation which light emitted by the phosphor dot undergoes, thus enhancing the contrast ratio. The fibre optic faceplate 11 further enhances the contrast ratio since it limits the acceptance angle of ambient illumination. In the limit, with a faceplate of sufficient thickness, this angle can be made small enough for the viewer's head to block all direct ambient illumination. This is practicable, of course, only where a limited viewing angle is acceptable, e.g. for a display in a single seat aircraft.

The filter dots may be formed by various methods. For example, a multi-stage photoresist process similar to that employed in forming the phosphor dot groups in a conventional shadow mask tube may be used.

In this method the filter dots of each required particular colour are formed, as follows.

Firstly, the exterior surface of the faceplate is coated with a material which will act as a filter of the required colour and the coating covered with photo-sensitive material. The photo-sensitive material is then exposed via the fibre optic facepiate by exciting the phosphor dots corresponding to the filter dots being formed using the appropriate gun of the CRT. After exposure, the unexposed photosensitive material and underlying filter material is removed leaving dots of the material in the required positions. After the filter dots of each required colour have been so formed, the interstices between the fliter dots are filled with black filler material, e.g. using a squeegee.

In an alternative method of forming the filter dots a multi-layer photo-sensitive coating of the kind used in colour photo transparencies is utilised on the exterior surface 1 5 of the faceplate 11. The coating is exposed using all electron guns, and the exposed coating is developed. The coating, whiist similar in construction to a colour transparency, may differ from the latter in in the selection of dyes to produce filter dots of the desired spectral pass bands.

Claims

1. A shadow mask colour cathode ray tube having a fibre optic faceplate; on the interior suface of the faceplate a screen comprising a distribution of phosphor dots groups, the dots in each group emitting light of different colours; and on the exterior surface of the faceplate a corresponding distribution of filter dot groups arranged so that each filter dot is optically coupled to the corresponding one of the phosphor dots by an individual group of fibres of the faceplate, the spectral pass-band of each filter dot at least partly overlapping the spectral characteristic of the light emitted by the phosphor dot to which it is optically coupled by way of the faceplate.

2. A cathode ray tube according to Claim 1 wherein each filter dot has a spectral passband substantially matched to the spectral characteristic of the light emitted by the corresponding phosphor dot.

3. A cathode ray tube according to Claim 1 or Claim 2 wherein each group of phosphor dots comprises three dots which emit red, green and blue light respectively.

4. A cathode ray tube according to any one of the preceding claims wherein said faceplate is sufficiently thick for a viewer's head at a predetermined viewing position to block substantially all direct illumination of the phosphor dots by ambient light passing through the faceplate.

5. A cathod ray tube according to any one of the preceding claims wherein said filter dots comprise dots of material formed on the faceplate using a multi-stage photo-resist process.

6. A cathode ray tube according to any one of Claims 1 to 4 wherein said filter dots comprise portions of a multi-layer photo-sensitive coating exposed to light produced by the phosphor dots via the faceplate and developed.

7. A shadow mask colour cathode ray tube substantially as hereinbefore described with reference to the accompanying drawing.