EP0113218A1

EP0113218A1 - A visual display unit

Info

Publication number: EP0113218A1
Application number: EP83307617A
Authority: EP
Inventors: John Michael Payne
Original assignee: Individual
Current assignee: Individual
Priority date: 1982-12-17
Filing date: 1983-12-14
Publication date: 1984-07-11
Also published as: GB8333125D0

Abstract

A mask is positioned in front of the screen of a cathode ray tube visual display unit. The mask has electrically conductive properties, and is either earthed or is connected to a discharge electrode so that static electricity which builds up on the screen surface can be dissipated. If this static electricity charge is not dissipated, it can be harmful to the person viewing the screen. The conductive properties can be provided by a mesh of thin wires, or by a grid pattern printed in conductive ink, but other alternative constructions are described in the specification. The electrically conductive material can be combined in one mask with antiglare precautions.

Description

Field of invention

This invention concerns visual display units or VDU's as they are commonly referred to. Such devices normally incorporate a television tube for displaying information derived from a processor unit or computer unit associated therewith.
It has been discovered that operators sitting in front of visual display units can contract skin complaints particularly about the face and neck which seem to be attributable to exposure to the VDU when the latter is operating.
Early ideas concentrated on possible radiation hazards and the like but these have largely been discounted as a result of careful tests and measurements.
Applicant's own experiments have led him to an alternative potential source of inflammation which it is believed arises from dust particles accelerated by the electrostatic field emanating from the screen and it is believed that the inflammation of the skin is probably due to the dust particles impinging on the skin.
It is an object of the present invention to overcome this problem and to render visual display units less liable to produce skin inflammation.

The invention

According to the present invention, there is provided a visual display unit having an electroconductive material located in front of the visual display unit cathode ray tube screen, the electroconductive material being connected to charge dissipating means and being arranged relative to the screen so that electrical charge appearing on the screen is conducted to the dissipating means.
It is found that by dissipating the charge which builds up on the CRT screen, so the tendency for skin inflammation on the part of an operator is reduced.
The charge can be dissipated through an electrical connection made between the conductive material and an actual or effective earth.
Alternatively, a corona discharge electrode pointing away from where the operator would normally be situated may be provided and the electroconductive material connected thereto so that the charge is dissipated using the known principle of electrical discharge using a point discharge electrode.
Where the point discharge device is located close to the VDU, a separate conductive screen may be required between it and the operator to prevent any air flow between the discharging point and the operator.
In one embodiment the conductive material may be applied directly to the surface of the cathode ray tube of the V_DU. The material may be a film, and a suitable film is one manufactured by National Adhesives Limited and marketed by them under trade names such as TL121, TL125 and _TL126. It is to be understood that reference to these specific products is not intended to limit the invention to the use of these particular materials which are specified by way of example only.
It is believed that the materials concerned are water- based polymers which when cured (dried) do not depend on the atmospheric humidity for their conductive properties. To this end such materials are intrinsically electroconductive and it is advantageous if the film is of such material or possesses such properties so that the conductive nature of the film is not affected by the prevailing humidity of the environment in which the VDU is located.
Electrical connection to such a film is established either by creating a conductive region in electrical contact with the remainder of the film to which an electrical lead is attached for dissipating any charge built up on the surface of the VDU; or the coated region is surrounded by a conductive frame which is connected through a suitable conductor for charge dissipation purposes.
In another and preferred embodiment of the invention, the conductive material forms part of a separate self- supporting mask which is adapted to be placed in front of the cathode ray tube either in toto or around its boundary region or may be simply stretched across in front of the cathode ray tube in a suitable bounding frame or supporting structure or may be stuck to a frame which normally surrounds the glass CRT screen. In each case, where an electrical connection is required to the mask, a suitable conductive terminal is provided such as a metal eyelet or solder tag to which an electrical connection can be made.
The conductive material may be in the form of a fine grid of lines which may be all parallel one to the other or may intersect for example at right angles to form a rectilinear array. Typically a grid of lines at a pitch of 20 to the inch and each line having a width of approximately 0.010 inches may be used. If not transparent, there will be a reduction of light from the surface of the CRT in use by by appropriate choice of pitch and line width, the light loss can be kept to a minimum, where it is not significant.
This last mentioned embodiment has an advantage over a plain conductive surface in that each line will additionally act as a discharge corona wire and less of the conductive material would in practice be necessary. The charge may of course be dissipated as described earlier by electrical connection to an appropriate earth or discharging electrode.
The grid may be formed as a mesh which is then mounted on a transparent carrier sheet member, desirably a flexible sheet.
Metal mesh of typical size 100 micron aperture and .05mm dia. wire is necessary to achieve the optical properties required. The kinds of metals which can be used commercially to make mesh of this fineness is limited by the ability to draw the metal to the small diameter required; the metals commonly used are brass and stainless steel. For the VDU shield of this invention, some form of blacking process is required so that the mesh is rendered as non-reflective as possible in order to enhance the optical properties. A possible method is to electroplate a brass mesh, with, for example, black chrome. The problem with this is that the deposit of chrome (or other plating material) in effect tends to weld or bond together the wire mesh at the points where one wire crosses another. This means that the mesh is no longer highly flexible and will not readily follow the three-dimensional contours of the VDU screen. However, it is important that the mesh is in contact with the screen in order to avoid moire patterns caused by the shadow of the mesh interacting with the mesh itself. A solution to this problem is to form the mesh, after electroplating, using a male and female press tool, thereby to give the mesh a permanent dished shape which substantially coincides with the contours of the VDU screen.
A method of blacking metal mesh, without causing it to become inflexible, comprises an oxidising process which can be applied to stainless steel mesh (which is also more robust than brass mesh). This process, for example available under licence from the company Inco, is a non- electrolytic oxidising process, whereby the stainless steel mesh is immersed in a bath of acids and additives. To further enhance the non-reflective properties of the mesh, it is necessary to roughen the inherent polished surface of the wire; this may be done by chemical etching, or by electrolytic action, or by ion bombardment of the kind which is normally used to etch a metal surface prior to bonding with paint, rubber, adhesive or the like. Importantly, it has been found that blasting with a ceramic, aluminium oxide, or ferrous grit imparts a uniform matt surface to the wire. A grit size smaller than the aperture size of the mesh should be used, typically 50-75 microns. These roughening processes are 3esirably carried out before the blacking process.
Masks may be constructed from a conductive fabric mesh such as dark or black nylon. This material has an inherent sheen on the surface of each fibre and overall there is a certain amount of light reflectivity. In a preferred embodiment, the fine mesh is formed from a metallic material which is chemically treated or electroplated to give a matt black surface to all of the metal content of the mesh so as to reduce overall light reflectivity of the mask.
In order to reduce electrostatic levels as efficiently as possible, it is preferably to mount the conductive mesh in contact with the cathode ray tube face.
The mask may be secured to the VDU screen using transparent adhesive material located at least in the corner regions of the mask and adapted to adhere to the mask material and to glass. The transparent adhesive material may be transparent adhesive double-sided tape.
Alternatively, as is preferred, the transparent sheet member may be formed from a plastics material of the type which adheres to glass without the use of an adhesive.
To attach the mask to a screen, one corner, which may contain an earth wire, is fixed first.
The diagonally opposite corner is then fixed, the slack being taken out. The other two diagonally opposite corners are then stretched and fixed simultaneously to eliminate wrinkles.
It has been found that if adhesives strips are used along the whole of the edges of the mask it is exceedingly difficult to align and fix. The corner pieces of adhesive material, if used, allow some misalignment and post smoothing of the mask.
In an alternative embodiment, such a grid of lines or conductive film may be applied directly to the glass of the cathode ray tube in the visual display unit with appropriate electrical connection if required for dissipating the charge.
The transparent sheet member which is used may be formed from a material haivng filtering characteristics to reduce any ultra violet or other radiation which may escape from the cathode ray tube under normal use of the tube. In this way a visual display unit can be rendered even more acceptable to operators having particularly sensitive skins.
According to a further preferred feature of the invention, the improved mask is formed oversize and is adapted to be cut down to just fit the CRT screen to which it is to be applied.
According to a further aspect of the present invention, a kit for fitting a mask to a cathode ray tube screen comprises a mask as set forth above, of overall dimensions as large as the largest screen for which the kit is intended, and a template having printing and indicia thereon and capable of being offered up to the cathode ray tube to which the mask is to be fitted so that two adjoining edges are aligned with the edges of the aperture containing the cathode ray tube screen (for example, one bottom edge and one side edge) and the template material is such that after it has been smoothed over the CRT screen, the two other bounding edges of the screen can be felt through the template and marked therein by means of a sharp edge or point to provide cutting lines to allow the template to be cut down exactly to the size of the cathode ray tube screen. The template can then be used to trace around the mask or to allow the mask to be cut directly to the correct size.
The mask is convenient to pack and transport, in that it can be rolled up with the template and packed in the centre of a cardboard tube along with the earth wire and adhesive pieces, if used.
Although the finished VDU shield may be supplied as a universal fitting; ie a large rectangle which the customer cuts to fit a particular VDU screen, it could alternatively be supplied as a pre-cut device, the customer giving the size or the model of his VDU so that the mesh can be cut to fit this size, typically by die cutting, by the VDU shield manufacturer.
In use, the VDU shield is to be connected by a conductive lead to a convenient earthpoint, thereby to dissipate the static from the VDU screen and to alternate the radiation therefrom. A wire may be used for this purpose, but as a retrofit device and fitting externally to an existing VDU, may be considered untidy.
A further feature of this invention concerns the use of a self-adhesive metal strip. Typically this would be 50 microns thick aluminium approx. 10mm wide, having an adhesive backing, eg acrylic, which would be protected before use with a silicon paper backing strip. Such a material is available from companies such as Sellotape, 3_M and Barrier. This material is attached at one end to a convenient edge or corner of the VDU screen, before fitting the shield, and thence led and stuck to the contours of the screen frame, around the cabinet, to the back of the VDU, where it is stuck to a convenient earthpoint, eg chassis, co-axial, or power earth connection. This results in a neat appearance for the the earth connection, more acceptable than a wire.
For home use, in particular where the television set is used in conjunction with a micro computer, a readily detachable plastics shield may be preferred. In this case, the television may be used for normal viewing without the shield, or with the shield when using the computer. This means that the shield has to be fitted onto the screen and then removed many times, so that the shield material has to be robust and have an easy fitting method. A suitable material is transparent PVC of a grade known as "self-cling" or "window-cling". This is typically about .010 inch thick, and is commonly used for stickers on windows as notices and signs, as for example as a wallet to retain and fix the motor vehicle licence to the windscreen of the car. The material has several manufacturers, including Hoescht.
The equivalent of the aforementioned metal mesh is introduced to the surface of the transparent PVC in the form of a printed black, conductive gird. A typical grid size would be an aperture size of 100 microns with a line width of 50 microns, a matt black conductive ink of the type manufactured by Coates Bros (Industrial Finishes) Limited, or Acheson Industries, may be used. This contains a graphite or nickel powder to give the conductivity required.
The printing methods available to give the fineness of grid required is limited, but one such suitable printing method is known a d.p. or direct printing, whereby a flat metal plate has a grid of grooves etched into it corresponding to the grid pattern required. The conductive ink is put into the grooves, the excess being wiped from the surface of the plate. The plate is then brought into contact, under pressure, with the surface of the PVC sheet, sometimes with the plate heated also. The ink is transferred from the grooves to the PVC sheet which is subsequently dried. Additionally, the printed PVC may be coated, on top of the printed grid, with a transparent varnish which can act to further retain the ink, and also may be in a matt or semi-matt form in order to make the surface of the sheet less reflective.
The invention also provides a cathode ray tube for use in visual display units and the like in which the display screen includes over some or all of the display area an optical filter for filtering out ultra violet and other harmful radiation which may be emitted from the screen during use and which is combined with a conductive film or grid of conductive elements so that any electrical charge which might build up on the surface of the screen during use can be dissipated by electrical conduction to earth or to a discharging electrode or the like.
It will be seen from the foregoing that where a grid of lines is employed, the material forming the lines need not be transparent provided the line spacing is such as to allow sufficient light through from the display and does not interfere with the reading of the information in the display.
According to a particularly preferred aspect of the invention the film of transparent electroconductive material or the grid of conductive lines may be formed by metallising the surface of the CRT or a sheet of glass or plastics material to be positioned in front of the CR_T. The metallising may be achieved using a vacuum deposition process or the like.
It has been noted that if a colour television screen is viewed at close range, as is often the case where a colour television monitor is being used as a computer terminal, the individual triads of colour can be readily distinguishable. This is clearly undesirable and can be avoided by utilising a diffusing screen between the phosphorescent surface which produces the colour triads on impingement by an electron beam and the viewer.
The diffusing screen may be formed integrally with the glass screen forming the end of the cathode ray tube or may be a separate sheet member located in front of a conventional cathode ray tube screen.
One convenient form of diffusing screen comprises a sheet of plastic or glass one side of which is slightly "frosted" by having formed thereon a large number of dimples or pyramidal projections. The utilisation of a diffusing screen of this type whether formed integrally with, or in front of the screen of the monitor produces a more favourable colour rendering.
According to a further feature of the invention, it has been found that a protective screen for reducing both glare and the electrostatic field emanating from a cathode ray tube screen may be formed from an open-weave mesh of nylon or similar plastics thread coated by a suitable electrically conductive material, with electrical connection means being provided for dissipating the electrostatic field, and mounting means being provided to effect the mounting of the mesh in front of the screen.
If the thread from which the mesh is woven is itself a dark colour and the conductive film on the thread is also dark in colour, the screen not only possesses the necessary characteristics to dissipate an electric field in front of the CRT, but also possesses all the characteristics needed to reduce the glare from the screen and also reduces reflections from the screen due to light incident thereon.
In a preferred embodiment of this aspect of the invention, a nylon thread is woven into an open weave mesh and is treated with a conductive resin, or is metallised so as to render the mesh electrically conductive.
The conductive open weave mesh is preferably supported in a frame which corresponds to the opening containing the cathode ray tube which is to be viewed and which is adapted to be secured to a housing or other member which surrounds the cathode ray tube screen.
In one form, the frame which bounds and supports the conductive mesh is provided with impact adhesive in one or more regions around the rear face thereof so that the frame will remain in position on the housing after it has been applied thereto and lightly pressed into place.
In such embodiment, the impact adhesive region is preferably protected by means of a peel-off strip material until the device is ready to be applied to a television monitor.
It has been found that where printed closely spaced lines or dots which are inevitably arranged in straight lines, are placed in front of a television monitor, moire fringe effects are obtained when the screen is viewed which are quite unacceptable for normal usage.
According to a still further aspect of the present invention, the moire effects can be eliminated by arranging that the parallel lines making up the grid of lines or the lines of dots or the warp or weft of the woven mesh lies at an angle in the range 10 to 80 degrees relative to the line scan direction of the scanning spot making up the raster in the cathode ray tube.
A preferred single inclination between the lines, lines of dots or warp or weft is 30 degrees to the line scan direction.
When the mask is prepared by printing conductive ink onto a transparent sheet by an intaglio process, a plate is first prepared by etching a rectilinear array of fine grooves onto the plate surface. The plate is then inked so as to fill the grooves; the surface is wiped so that ink just remains in the grooves, and the transparent sheet (eg _PVC) is pressed into contact with the plate so as to become inked.
This mask can then be mounted on a backing sheet of paper.

Claims

1. A visual display unit having an electro conductive material located in front of the visual display unit cathode ray tube screen, the electroconductive material being connected to charge dissipating means and being arranged relative to the screen so that electrical charge appearing on the screen is conducted to the dissipating means.

2. A unit as claimed in claim 1, wherein the electroconductive material is formed on the surface of the screen.

3. A unit as claimed in claim 1, wherein the electroconductive material itself forms, or is applied to a transparent substrate, and the substrate is located in front of the screen.

4. A mask for the cathode ray tube screen of a visual display unit, the mask having electroconductive properties and being substantially transparent so as not to impair significantly the readability of a screen when it is located in front of the screen.

5. A mask as claimed in claim 4, wherein the mask includes electrical connection means through which the electroconductive mask can be connected to means for dissipating electrical charge.

6. A mask as claimed in claim 4 or claim 5, wherein the electroconductive properties are provided by an electroconductive mesh.

7. A mask as claimed in claim 6, wherein the mesh is of metal wire.

8. A mask as claimed in claim 7, wherein the wire has a surface with a matt finish.

9. A mask as claimed in claim 8, wherein the metal is stainless steel which has been treated chemically to produce a matt black finish.

10. A mask as claimed in any one of claims 6 to 9, wherein the mesh is supported on a transparent sheet member.

11. A mask as claimed in claim 4 or claim 5, wherein the electroconductive properties are provided by conductive ink printed onto a transparent sheet member.

12. A mask as claimed in claim 11, wherein the ink is printed onto the sheet member by an intaglio process.

13. A mask as claimed in claim 10 to 12, wherein the sheet member has radiation filtering properties.

14. A mask as claimed in any one of claims 10 to 13, wherein the transparent sheet member is of flexible plastics material and will adhere to glass without the use of any separate adhesive.

15. A mask as claimed in claim 13, wherein the ink is printed in the form of a grid of interconnected lines.

16. A kit for applying a mask to the screen of a visual display unit, the kit comprising a mask as claimed in any one of claims 4 to 15 and of overall dimensions as large as the largest screen size to be fitted, and a template having marked thereon outlines of typical screen sizes to facilitate marking and subsequently cutting of the mask to fit a particular screen.