WO1993011542A1

WO1993011542A1 - Shielding device

Info

Publication number: WO1993011542A1
Application number: PCT/SE1991/000799
Authority: WO
Inventors: Elisabet ÅKERBERG
Original assignee: Inventa
Priority date: 1990-05-23
Filing date: 1991-11-25
Publication date: 1993-06-10
Also published as: SE9001871L; SE467432B; SE9001871D0

Abstract

A device for shielding off magnetic fields, induction and electrostatic fields emanating from a display device or a terminal (1) comprises an electrically conductive cover (2) and a front piece (5) which have a common earth potential. The cover encloses the terminal, so that the display surface is located in a front opening (4) of the cover. The front piece is of tubular shape. A first end having a connection opening (6) is to be connected in form-fitting fashion to the cover in the area round the front opening (4). A second end having an operator's opening (7) is located in front of the operator's site.

Description

SHIELDING DEVICE

Background of the Invention

The present invention relates to a device for shield- 5 ing off fields and is especially adapted for use in asso¬ jj ciation with terminals, such as cathode ray tubes and plasma display panels.

In this context, the term "field" relates to magnetic flux density, so-called magnetic field B, measured in the 10 unit Tesla; the time derivative of the flux density, so- called induction dB/dt, measured in the unit Tesla/s; and electrostatic fields, measured in the unit V/m. For the sake of conformity, internationally approved testing labo¬ ratories indicate, instead of electrostatic fields, the 15 equivalent surface potential generating a specific elec¬ trostatic field, measured in volts. The testing authority SEMKO never indicates values below 2 mT/s for the induc¬ tion and below 10 nT for the magnetic field. The maximum desired values are 25 mT/s and 50.nT, measured at a dis- 20 tance of 0.5 m from the display surface. Objects of the Invention

The present invention aims at providing a device for shielding off fields. One object of the invention is to provide an inexpensive, simple and flexible device. 25 Another object is to provide a device which is convenient in terms of visual ergonomy. A further object is to pro¬ vide a device that can be used in association with old as well as new terminals. Summary of the Invention 30 These objects are achieved by a device as defined in the characterising clause of appended claim 1. Further embodiments and developments are defined in the appended subclaims.

The device has an electrically conductive cover 35 enclosing the terminal, so that only a display surface is exposed in a front opening of the cover. An electrically conductive, tubular front piece constitutes an extension piece of the cover at the display surface. The cover and the extension piece have a common earth potential. Drawings

Embodiments of the invention will be described below with reference to the accompanying drawings, in which

Fig. 1 is a schematic view of a first embodiment of the invention;

Fig. 2 is a schematic part section of the first embo¬ diment; and Figs 3a and 3b are schematic part sections of a second embodiment of the invention. Description of Preferred Embodiments

Figs 1 and 2 show a first exemplifying embodiment associated with a terminal 1, e.g. a cathode ray tube, which is to be placed in an electrically conductive cover 2. The cover encloses the terminal 1, so that only a dis¬ play surface 3 thereof is exposed in an opening 4 of the cover.

An electrically conductive, tubular front piece 5 can be connected to the cover as an extension piece thereof in the area round the display surface in the opening of the cover. The tubular front piece has a first end with a con¬ nection opening 6, and a second end with an operator's opening 7. The connection opening is adapted to fit the cover at the opening 4 in a joint 8. This joint is so tight as to allow no field leakage through it. The joint can be produced by means of glue, screw joints, soldering, snap-in joints, or hinges, or in any other way deemed suitable. The electrically conductive front piece and the elec¬ trically conductive cover are connected to a common earth potential. The material is galvanised sheet-metal.

At an operator's site 9, the operator views the ter¬ minal within a given field of vision 10 limited by imagi- nary lines traced between the operator and the outer frame of the display surface, and indicated by dash-dot lines in Fig.1. The operator's opening 7 faces the operator's site 9. As appears from Fig. 2, which illustrates the conical field of vision, the tubular front piece is conveniently designed, in a first embodiment, so as to taper conically by a flange portion 11 towards the operator's opening 7. Since the taper of the front piece is equal to or greater than the conical shape of the intended field of vision, the operator is not hindered in his work.

The length of the flange portion is chosen so as to result in the smallest possible field values. In one embo¬ diment, the flange portion projects about 10 cm from the display surface 3 for a display size of 30 cm.

However, the flange portion may be extended as far as the operator's site. If equipped with a lens system, the operator's opening may serve as an ocular.

Figs 3a and 3b schematically illustrate a second embodiment of the invention. This specific embodiment is conditioned by psychological and ergonomical aspects; the operator wishes to avoid looking directly at the display surface and instead sees a representation thereof. The tubular front piece 5 is equipped with a periscope-like mirror arrangement comprising two opposite mirrors 12 and 13. With this arrangement, the operator does not watch the display surface 3, but instead a double-mirrored represen- tation 3' thereof. The mirrors constitute two walls or at least parts thereof.

In the embodiment shown in Fig. 3a, the two mirrors are disposed as close as possible to the display surface 3, in practice in edge-to-edge relationship. This results in a mirror arrangement of maximum compactness. The angu¬ lar relations between the mirrors and the display surface, as well as the size of each mirror, are easily established by the expert, if taking into consideration that the field of vision 10 must remain unrestricted. Although the mirror arrangement in Fig. 3b has dif¬ ferent angles and measurements than that in Fig. 3a, the operator will perceive an identical representation 3 ' . In the embodiments shown, the front piece is coni- cally tapering towards the operator's opening, but this is not an absolute requirement for achieving the objects of the invention. The most effective and simplest mirror arrangement (Fig. 3a) achieves all the objects of the invention, but the expert will appreciate that several - modifications thereof are conceivable.

An electrically conductive, transparent piece of material, e.g. a sheet of glass 14, can be provided in the front piece at any suitable place, e.g. in the operator's opening 7 or even in the front opening 4 of the cover. If so, the flange portion can be restricted to a minimum. Lens systems, e.g. comprising Fresnel lenses, may improve the properties of the device even further and con- tribute to enhanced visual ergonomy.

The hood 2 and the front piece 5 need not be designed as two separate parts, but may be replaced with a single unit. The tubular shape of the front piece may be adjust¬ able, e.g. to different fields of vision and different viewing angles. The mirror arrangement itself can be adjustable so that the representation 3' falls on one side of, below or above the display surface 3. In one embodi¬ ment, the front piece is provided with special fastening means enabling it to be directly mounted on a terminal without any additional cover. Then, use is e.g. made of an area 15 round the display surface 3. The outer cover of the terminal and the front piece are then earthed jointly. In an embodiment not illustrated in the drawings, the tubular front piece has but one mirror re-directing the picture sideways. In this case, all text or other symbols shown on the display surface 3 have to be reversed.

The embodiments shown in Figs 2 and 3a have been tested for shielding effect by the Swedish Board for Testing and Approval of Electrical Equipment (SEMKO). The results from these tests are accounted for in the Table below. The equivalent surface potential was 22.3 kV for the display without shielding, and dropped to below 0.5 kV with shielding. All measurements were performed in accor¬ dance with the regulations of the National Swedish Statute Book on Measurements and Testings (MPFS).

TABLE

TEST OF TERMIMAL BY SEMKO

WITHOUT SHIELD- CONICAL FRONT MIRROR-

ING OFF FIELDS PIECE EQUIPPED FRONT PIECE dB/dt B ^• dB/dt B dB/dt B

H (mT/s) [nT] (mT/s) [nT] (mT/s) [nT]

R = distance of 650 mm from the centre of the magnet coils of the terminal which are disposed about 150 mm behind the display surface

H = angle in vertical plane; H=0 is straight ahead

A = angle in horizontal plane; A=0 is straight ahead from the display surface

* lowest recordable value = 2 mT/s lowest recordable value = 10 nT

Claims

1. A device for shielding off magnetic fields, induc- tion and electrostatic fields emanating from a display terminal (1) or the like, comprising a cover (2) enclosing the terminal in such a manner that only a display surface (3) thereof is exposed in a front opening (4) of the cover, and a tubular extension piece (5) provided at the display surface (3), c h a r a c t e r i s e d in that the extension piece (5) is conically tapering from a connection opening (6) at a first end thereof adjacent to the display surface (3), towards an operator's opening (7) at a second end thereof; and the cover and the extension piece are electrically conductive and have a common earth potential.

2. A device as set forth in claim 1, c h a r a c ¬ t e r i s e d in that the cover (2) and the extension piece (5) are made of galvanised sheet-metal.

3. A device as set forth in claim 1 or 2, c h a r ¬ a c t e r i s e d in that a periscope-like arrangement comprising two mirrors (12, 13) is provided in the exten¬ sion piece.

4. A device as set forth in claim 3, c h a r a c ¬ t e r i s e d in that the connection opening (6) is essentially parallel to the operator's opening (7).

5. A device as set forth in claim 3 or 4, c h a r ¬ a c t e r i s e d in that the periscope-like arrangement comprises a first mirror (13) which is arranged in edge- to-edge relationship with the display surface (3) at an angle thereto; and that a second mirror (12) is arranged at an angle to the first mirror, thereby making it pos¬ sible to observe the display surface through the opera- tor's opening (7).

6. A device as set forth in claim 1 or 2, c h a r ¬ a c t e r i s e d in that a hood-like arrangement com¬ prising a mirror is provided in the front piece, the con¬ nection opening (6) and the operator's opening (7) being transverse to one another.

7. A device as set forth- in any one of the preceding claims, c h a r a c t e r i s e d in that a Fresnel lens is provided in the extension piece.

8. A device as set forth in any one of the preceding claims, c h a r a c t e r i s e d in that an electrical¬ ly conductive transparent means (14) is provided in the extension piece (5) and has the same earth potential as the hood and the extension piece.