CRT Shield

The present invention relates to a CRT shield and particularly a shield for completely enclosing a cathode ray tube in metal while maintaining visibility thereof.

A cathode ray tube is capable of producing considerable emanations outside the visible spectrum which emanations might be received and decoded at a distance in order to reproduce the information displayed on the screen of the cathode ray tube. One prior art technique for minimizing such emanations is disclosed in U.S. Pat. No. 2,673,342 granted on Mar. 23, 1954 to J. C. Sims, Jr. et al. and comprises a tapered body for shielding the major portion of the cathode ray tube. A metallic mesh screen is soldered to a plate which is then mounted within the large end of the tapered shield in order to permit viewing of the screen and yet minimize the emanation of signals within the nonvisual electromagnetic spectrum. It is also well known to coat the implosion screen of the CRT with a conductive material that is also transparent in order to suppress X-radiation. However, modern signal sensing and receiving apparatus is so sensitive that even these prior art techniques cannot prevent unauthorized receipt of information concerning the data displayed on the face of the CRT.

It is an object of the present invention to more fully enclose a cathode ray tube in order to minimize non-visual emanations therefrom.

The present invention relates to minimizing nonvisual electromagnetic emanation from a CRT using a conductive collar bonded to a conductive screen and the collar projecting along the periphery of the face of the CRT towards the neck thereof with a conductive, tapered shield matching generally the shape of the tube and yoke and having a lip overlapping collar with an electrical interconnection between the tapered member and the collar.

A more complete understanding of the present invention may be had by referring to the following detailed description when considered in conjunction with the accompanying drawing wherein like reference numbers denote the same or similar parts throughout the several views in which:

FIG. 1 is an exploded view in perspective of a CRT with the components of the shielding; and

FIG. 2 is a detail of the shielding mounting hardware and potting thereof to the CRT.

Referring now to the drawings and more particularly to FIG. 1, there is shown within the shielding system, the CRT 10 to be shielded. The CRT comprises a face 12, a tapered part 13 and a neck 14 and is of thoroughly conventional construction. A conventional yoke structure (not shown) is also included on the neck of the CRT.

A very thin gasket 16 of adhesive-backed polyurethane foam material is put around the face 12 of the CRT 10. A brass collar 18 is placed over the periphery of the face 12 of the CRT 10 and engages the gasket 16 which separates the metal of the collar 18 from the glass of the CRT.

A phosphor bronze wire mesh screen 20 of very fine gage is seam welded (a series of spot welds) all around the periphery 22 of an opening in the collar 18 such that the screen 20 covers the entire viewing area of the face 12 of the CRT. The purpose of the mesh screen is to permit viewing of the images on the face 12 of the CRT while preventing the passage of nonvisual electromagnetic radiation that may emanate from the face 12.

The collar 18 has a plurality of perforations 24 around its rim which perforations are small enough in size and number to avoid the passage of significant amounts of radiation therethrough. However, these perforations are large enough and frequent enough around the periphery of the collar 18 to facilitate later bonding of the shield as will be explained in more detail below.

Another polyurethane foam gasket 26 separates the collar 18 from a conventional, tempered glass safety, implosion panel 28. If the screen 20 is strong enough and fine enough to retain glass particles in the event of an implosion of the CRT, the panel 28 would, of course, not be necessary. However, in order to facilitate the transmission of substantial quantities of light and energy, the screen 20 is preferably made of very small wire which may not provide sufficient structural strength to stop glass particles in the event of an implosion.

It will be well recognized that the implosion panel 28 can readily be placed between the face 12 and the screen 20 without impairing the operation of the implosion panel in protecting the viewer nor the operation of the screen 20 in trapping nonvisual emanations.

Still another thin polyurethane foam gasket 30 separates the implosion panel 28 from a mounting frame 32. The mounting frame 32 is equipped with mounting fixtures 34 and is formed preferably of molded thermoplastic such, for example, as polyphenylene oxide, with the mounting fixtures 34 formed integrally therewith.

All of the structural components from the cathode ray tube 10 through to the mounting frame 32 are sandwiched together to form a compact structure which is shown schematically and fragmentarily in cross section in FIG. 2. An epoxy adhesive 36 is then introduced into the region between the periphery of the tube 10 and the mounting frame 32 with the collar 18 positioned therebetween. The epoxy, as is well known in the art, forms a strong bond to the surface of the CRT 10 and also to the inner surface of the mounting frame 32 as well as to both sides of the collar 18. As can be readily seen in FIG. 2 the perforations 24 provide a direct link between the epoxy in contact with the CRT 10 with the epoxy in contact with the inner surface of the mounting frame 32 without depending significantly upon the bonding of the epoxy to the collar 18.

A length of beryllium-copper spring-metal finger stock 40 is seam welded to the inside of the collar nearest its periphery amd remote from the face 12 of the CRT. A tapered or dished portion 44 is made preferable of a magnetically-permeable material known generally as Mu-METAL and is composed of a major portion of nickel and minor portions of iron and copper. It may be desirable to copper-plate the Mu-METAL to increase its conductivity. The tapered end of the portion 44 extends over the tapered part 13 of the CRT and is of a size such that its periphery 46 frictionally engages the finger stock 40 that is bonded to the inside of the collar 18. Engagement with the finger stock 40 frictionally holds the tapered portion 44 in engagement with the collar 18. Such engagement also assures that a minimum of space exists between the collar 18 and the portion 44 so as to minimize the emanations passing through any gap between these two pieces of metal. Electrical continuity is also assured with a frictional engagement between the collar 18 and the portion 44, which friction is controlled by the nature of the finger stock 40.

A neck portion 54 of the tapered portion 44 can extend over the yoke and gun of the CRT in order to magnetically shield these magnetically active elements. In which case, an end cap 50 of conductive material such as aluminum can be used to complete the covering of the CRT. The end cap 50 has an opening therein only adequate to permit passage of the control cable 58.

Finger stock can be bonded to the inside of the end cap 50 for assuring the electrical and frictional attachment of the end cap 50 to the neck portion 54.

However, preferably, the neck portion 54 does not completely cover the gun and yoke of the CRT. In this way, the end cap 50 is a two-part structure comprising a Mu-metal tube covering the yoke and gun and engaging the neck portion 54 with the finger stock 52. The two-part end cap 50 also comprises an aluminum end that engages the Mu-metal tube using more finger stock and also accommodates the cable and plug 58. Such a two-part end cap 50 allows easier access to the yoke for servicing.

Although only one specific embodiment of the invention is shown in the drawings and described in the foregoing specification, it will be understood that invention is not limited to the specific embodiment described, but is capable of modification and rearrangement and substitution of parts and elements without departing from the spirit of the invention.