US 3422298 A
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n- 1969 J. DE GIER 3,422,298
CATHODE-RAY TUBE HAVING A NON'DISCOLORING X'RAY ABSORPTIVE DISPLAY WINDOW Filed March 15. 1966 INVENTOR.
JOHAN NES DE GIER United States Patent 6504105 U.S. Cl. 31364 7 Claims Int. Cl. H01j 29/ 00, 31/00 ABSTRACT OF THE DISCLOSURE A cathode-ray tube having a display Window on the inner side of which a luminescent screen is disposed which luminesces under the impact of an electron beam. The window comprises of a first portion adjacent the screen of poor electrical conductivity and containing a relatively small percentage of readily reducible compounds so that it will not discolor under impact of the electron beam while a second portion remote from the luminescent screen consists of a glass composition which contains lead oxide and cerium oxide in amounts eifective to absorb X-rays generated in the tube and transmitted through the first portion without discoloring due to the X-rays being absorbed on the electron impact.
The invention relates to a cathode-ray tube provided with a luminescent screen disposed on a glass window, which window has a composition such that it has a poor electrical conductivity and/ or that it contains a low percentage of readily reducible compounds such as oxides of lead, antimony and arsenic.
It is known that the glass window of a cathode-ray tube operating at a high voltage, for example, above 15 kv. may exhibit a gradually increasing discolouration due to a prolonged electron bombardment. Such voltages are used, for example, in cathode-ray tubes for projection television, for radar, for flying spot scanning, for colour television and in large cathode-ray tubes (having a diameter of, for example, more than 42 cm.) for black-andwhite television. This discolouration may be accounted for by the reduction of oxides present in the glass due to charge transport. This discolouration is therefore counteracted by ensuring that the glass has a poor electrical conductivity or that it contains a low percentage of readily reducible compounds, while alternatively a combination of both steps may be used. Since the sodium oxide con tent is substantially exclusively determinative of the conductivity of a kind of glass, if a very poor electrical conductivity is required, the glass should not contain more than 1% by weight of sodium oxide. The content of readily reducible compounds does not exceed approximately 7.5% by weight in this case. In practice, it is less simple to prepare glass having a low content of sodium oxide, since such a kind of glass is excessively hard and its expansion coeflicient is excessively low 'while it is not readily meltable. It is therefore rather ensured that the glass contains a very low percentage of readily reducible compounds, i.e., not higher than 2.5% by weight, more particularly not higher than 1% by weight, in which 3,422,298 Patented Jan. 14, 1969 case the sodium oxide content does not exceed 10% by weight. The permissible quantity of readily reducible compounds depends upon the sodium oxide content, while these two magnitudes in turn depend upon the actual charge of the cathode-ray tube during operation. The said readily reducible compounds do not all contribute to the same extent to the discolouration of the glass. For example, a given content of lead oxide brings about a much stronger discolouration than the same quantity of antimony oxide, so that in a given case the permissible quantity of antimony oxide is greater than that of lead oxide.
In cathode-ray tubes operating at a high voltage of, for example, above 15 kv., due to the electron bombardment of the luminescent substance and the glass, also X- rays are produced which also bring about a discolouration especially in lead-containing glass, while at a very high voltage such a discolouration also occurs in the absence of lead if the glass contains, for example, antimony, arsenic, barium or aluminium, as the case may be in the form of impurities. As is known, this discolouration may be counteracted by adding cerium oxide to the glass. Consequently, more particularly the glass window then contains cerium oxide.
These X-rays the intensity of which strongly depends upon the actual charge of the cathode-ray tube, may penetrate through the glass window. In order to protect the environment, it should consequently be prevented that X-rays, if they surpass a given intensity, can reach the environment. X-rays are readily absorbed by glass containing oxides of heavy metals, more particularly leadcontainin'g glass, but as set out hereinbefore, the glass window should preferably contain a very low percentage of lead oxide in order to prevent this window from being discoloured due to the electron bombardment. In practice, it has been found to be difficult to avoid the discolouration of the glass at voltages upwards of 25 kv. and nevertheless to ensure that the X-rays produced are absorbed to a sufficient extent, inter alia also in the case in which the glass window contains 7.5 by weight of lead oxide, since at a high charge the window of a small tube does not have a suflicient thickness to absorb the X-rays to a sufficient extent. The invention provides a solution in which on the one hand the discolouration of the glass window due to the electron bombardment is counteracted, while on the other hand the environment is protected against X-rays.
According to the invention, the cathode-ray tube is provided on the outer side at the area of the glass window with a pane containing lead oxide and cerium oxide. The composition of the glass window prevents it from being discoloured due to the electron bombardment, while the X-rays produced in the cathode-ray tube are absorbed on the one hand in the pane as a result of the presence of the lead oxide, while on the other hand they do not bring about a discolouration in the pane owing to the presence of the cerium oxide. The thickness of the glass pane amounts, for example, to 3 to 6 ms. The intensity of the X-rays depends upon the actual charge of the cathoderay tube. In general, a pane containing from 5 to 15% by weight of lead oxide and up to 1% by weight of cerium oxide will prove satisfactory.
Cathode-ray tubes provided with a pane are known per se. In given cases, a pane is used for protection 3 4 against implosion. 'Ihis pane then has, for example, the under IV and V a few compositions of the pane 6. These following composition: compositions are stated in percentages by weight.
Percent by weight TABLE S10 68.1 Lizo 049 r I II III IV V Na O a K20 Rb O BaO PbO A1203 ---i L3 Sb O 0.52 Remainder 0.34 9- 2 0. Furthermore, especially in cathode-ray tubes for repro- Q34 ducing colour television images, a pane is used which is generally frosted in order to reduce the reflection of the What is claimed is: light in the environment. This pane has, for example, the A Cathode'ray tube havlhg a WlhdOW dlsplay foll wi i i of an image produced by impact of an electron beam on a percent b i h luminescent screen disposed on the lnternal surface of said s o 75 window, said window comprising a first portion adjacent 0 172 to the luminescent screen and consisting of a glass como ()2 position containing a relatively low percentage by weight A1203 of readily reducible compounds and having a relatively c o 7 g poor electrical conductivity whereby said portion does not o 3 discolor under impact thereon of the electron beam, and Remainder Q3 a second portion remote from said screen consisting of a glass composition containing lead oxide and cerium oxide The Procfiss of manufacturing the P y Start from in amounts which are effective to absorb X-rays generated a fiat glass Plate which is allowed to bulge during heating in said tube and passing through said first portion without so that the concave shape corresponds with that of the b i di l d h b or h l t o b Window of the Cathode-fay tube, whereupon the P is 2. A cathode-ray tube as claimed in claim 1, in which adhered t0 the windowthe first window portion does not contain more than 10% In another method, the pane is moulded into the deby weight f Sodium id sired shape, whereupon it is adhered to the window of the 3 A cathode-ray b as l i d i l i 2, in hich tube. the first window portion does not contain more than 1% The lnvention further relates to an apparatus comb i h of di id prising a cathode-ray tube provided with a luminescent 4 A cathode-ray t b as l i d i l i 2, i whi h Screen disposed on a glass Window, which Window has a the first window portion does not contain more than 2.5% composition such that it has a poor electrical conducb i h of dil d ibl id tivity and/or that it contains a low percentage of readily 5 A h d b as l i d i l i 3 i hi h reducible compounds such as oxides of lead, antimony thg fi t Window portion does not contain more than and arsenic. According to the invention, in the apparatus by Weight f readily reducible i on the front slde of the cathode-ray tube a pane 15 P 6. A cathode-ray tube as claimed in claim 1, in which vided which contains lead oxlde and cerium oxide. The the first Window pOrtion contains cerium Oxide composition of the glass window prevents it from being 7. A cathode ray tube as claimed in claim 1, in which discoloured due to the electron bombardment, while the the Second Window Portion contains f 5 to 15% by X-rays produced in the cathode-ray tube are absorbed on Wdght of lead oxide and up to 1% by Weight of cerium the one hand in the pane provided in the apparatus due to oxide the presence of the lead oxide, while on the other hand References Cited they do not bring about a discolouration in the pane due to the presence of the cerium oxide. Also in this case, a pane UNITED STATES PATENTS containing from 5 to 15% by weight of lead oxide and 2,293,529 8/1942 Bedford 313-64 X up to 1% by weight of cerium oxide will generally prove 2,579,709 12/ 1951 Smith et a1. 10653 satisfactory. 2,676,109 4/ 1954 Barnes et a1 313-64 X Apparatus provided with a pane on the front side of 2,747,105 5/1954 Fitzgerald et a1. 106-53 the cathode-ray tube are known per se. Such a pane 2,856,303 10/1958 Armistead 106--53 serves, for example, for protection against implosion. 3,209,191 9/1965 Hamilton 3131l0 The invention will now be described more fully with 3,356,579 12/1967 Harrington 106-53 reference to a drawing the figure of which is a partially diagrammatic sectional view of a cathode-ray tube. The 00 FOREIGN PATENTS envelope of the tube consists of the window 1, the cone 2 8587667 5/1940 France and the neck 3. Inside the tube a luminescent screen 4 2 Provided 11 the Windovg i ghile 3 3 8;" th ii JOHN W. HUCKERT, Primary Examiner.
lagrammatica y is arrange in e nec or e sa e of clarity, the remaining parts of the interior of the tube 65 JAMES Assmam Exammer' are not shown. The pane 6 is adhered to the window 1 US Cl XR of the tube. The table indicates, by way of example,
under I, II and III a few compositions of the window and 106-63; 2202.1; 313-317