![]() The beams strike the screen at discrete angles, activating the appropriate phosphors.) A CRT from a television with its electron gun and coils visible. Needless to say, the electrons are not different colors. (For color, three such electron guns are required. Its intensity is modulated and it is directed toward the center of the screen. This cloud of electrons is focussed into a narrow beam. This glowing filament heats the negatively biased cathode, which ejects electrons. They heated the cathode, stimulating more intense electron emission. ![]() The first CRTs had cold cathodes, but by the 1920s incandescent filaments were added. And second, a vacuum is required for the beam of electrons, generated at the narrow end of the tube, to travel to the screen at the large end where a phosphor layer resides. The glowing filament, as in an incandescent light bulb, would burn out instantly in the presence of oxygen. In the event of breakage, a substantial implosion sends glass shards flying everywhere. Sealed in a large, funnel-shaped glass envelope, the CRT as used in old-style TVs is of necessity fairly heavy because the glass envelope contains a vacuum. The one advantage of CRT technology is that it is easier to understand. All agree that it is a great improvement. The flat screen is light, thin, consumes far less energy, and is less expensive to manufacture. Modern digital oscilloscopes universally have flat screens. Prior to the digital revolution in electronic instrumentation, oscilloscopes displayed waveforms on cathode ray tube screens.
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