Although often considered a relatively modern invention, cathode-ray tubes are, historically, even older than more familiar electron tubes. Various types of cathode-ray discharge and display tubes were used extensively in physics laboratories and schoolrooms before the turn of the century, and as early as 1897 Karl Braun developed a cathode-ray display tube very similar to modern television tubes.
The “heart” of most present-day CRT’s is the electron gun. The gun is made up of a filament, an indirectly heated cathode, a disc-shaped control grid, and disc- or cylindrical-shaped focusing and accelerating grids (or anodes). Its purpose is to produce a sharp stream of accelerated electrons. The number of electrons in the stream (and hence its intensity, as well as the brightness of the spot it produces when it strikes a screen) is controlled by the voltage applied to the control grid. The beam’s sharpness of focus is determined by the voltage relationships between the focus and accelerating anodes.
Display Tubes. Direct descendants of the early Braun tube, display CRT’s are used extensively in TV receivers and monitors, oscilloscopes, radar equipment, and in a variety of test and research instruments. As the name implies, these tubes serve to display electrical phenomena on a fluorescent screen, either as a line, pattern, or reproduced picture.
In general, display tubes are made up of an electron gun assembly, a means for focusing (if not contained within the gun itself) and deflecting the electron beam, and a fluorescent screen. Manufactured in sizes ranging from tiny units with a 1″ -diameter screen to giant picture tubes with 30″ screens, they are usually funnel-shaped. The screen itself may be round, square, or rectangular. The envelopes or “funnels” are made either of metal or glass, or a combination of both.
Most display tubes are identified by a combination numeral-letter type number. The first number indicates the nominal size of the tube’s screen, the first letter (or letters) the particular tube, and the last letter and numeral the type of fluorescent material (or phosphor).
Phosphors. Typically, a type 5BP1 tube has a 5″ screen with a type “P1″ phosphor. Similarly, a type 20DP4 has a nominal 20″ screen and a “P4″ phosphor. Cathode-ray tubes used as TV picture tubes generally have rectangular screens and their size designation refers to a diagonal measurement across the face of the tube. In some cases, TV picture tubes are called kinescopes.
An arbitrary system is used for identifying the various phosphors used. A type P1 phosphor, for example, has green fluorescence and medium persistence; you’ll find this type in most oscilloscope tubes. Type P4 phosphors have white fluorescence and medium persistence, and are employed primarily in television tubes. Type P5 phosphors have a bluish-white fluorescence and very short persistence; tubes with this type of phosphor are used for high-speed photography of electrical phenomena having a short time duration. The P11 phosphor is similar to the P5 type, but has a slightly longer persistence.
Types P7 and P14 are both two-layer phosphors. The P7 type has a long persistence, first emitting a bluish light, then a greenish-yellow. The P14 type has medium persistence, first emitting a bluish, then an orange light which persists for over a minute. These two types of phosphor are useful in instruments employed to observe low-speed recurrent and non-recurrent phenomena. The last type of phosphor, P15, has a very short persistence in the near ultra-violet region, emitting a visible blue-green light afterwards; its principal application is in flying-spot scanner tubes.
Electrostatic and Electromagnetic. Electrostatic CRT’s are those which employ electrostatic fields to move the electron beam obtained from the gun assembly. Electron beams may also be deflected by magnetic as well as electrostatic fields, however. Most TV picture tubes are electromagnetic types.
