Cathode-ray tube

#This invention relates to cathode-ray tubes, such as color television tubes, and more particularly to cathode-ray tubes having the physical property of absorbing X rays generated within the tubes during their operation at high voltages.

#A cathode-ray tube, such as a color television tube, which is subjected to high voltages during operation emits a small amount of X rays through its face plate and even through the walls of the funnel portion. X rays are formed when a stream of very fast high-energy electrons strikes matter and is slowed down. The sudden "braking" of the electrons produces electromagnetic radiations of very short wavelength which are called X rays. Television sets have 20,000 volts or more applied across the tube during operation and these voltages are sufficiently high to result in the production of X rays within the tube whenever a portion of the stream of electrons produced by the electron gun strikes solid surfaces. The X-ray emission peak from a color television set, for example, has a wavelength of approximately 0.7A at 25,000 volts, and a wavelength of approximately 0.5A at 35,000 volts. Whenever a television service technician adjusts the control to brighten the picture on the set, he may choose to increase the voltage across the tube. This action results in geometrically increasing the intensity of X rays which are produced. It is necessary to keep any such X ray emission through the glass envelope of the cathode-ray tube at a sufficiently low level that viewers, and adjacent nonviewers as well, of the images produced on the face plate during operation of the television set are not subjected to any harmful radiation. One way in which the art has attempted to solve the problem is to form the cathode-ray tube from a glass having lead oxide as one of the essential ingredients. While the X-ray absorption properties of such glass are improved, other disadvantages have arisen, including discoloration of the glass struck by the X rays and by the stream of electrons. This problem of browning of the glass is particularly serious with respect to the face plates of color television tubes since the quality and brightness of the color of the images seen by the viewer are adversely affected. To offset this browning or discoloration, it has been necessary to incorporate cerium oxide into the PbO-containing glass composition. Another disadvantage of the use of PbO, together with its accompanying cerium oxide, is the increase in density of the glass and the resulting increase in weight of the tube. It is estimated that for every 0.01 increase in density of the glass, the weight of a 25-inch color television tube increases by 1.33 ounces.

#Accordingly, it is an object of the present invention to provide a cathode-ray tube, such as a color television tube, which has an improved property of absorbing X rays formed within the tube during its operation and emitting a substantially lower and safer amount of such X rays into the viewing area of the tube.