Cathode ray tube

this diagram of an oscilliscope CRT shows how it differs from a television. The electron beam's intensity is constant. The deflecting coils draw the electron beam across the tube's phosphor surface. In an old analog television, the deflection coils cycle through driving the beam along every pixel on 480 rows on the tubes surface, with the image drawn by adjusting the intensity of the beam for where the image is light or dark.

In this image (1) are the deflecting elecromagents;

(2) is the electron gun;

(5) is the phosphor coated screen that lights up when hit by electrons.

the cathode ray tube of an oscilliscope

The Cathode ray tube is a vacuum-tube-based display technology that produces images the user can see on one face of the tube.

In electronics a cathode is the electrically negative pole in an electronic circuit. The anode is the electrically positive pole. When a user plugs an electric battery into an electrically powered device, they connect the battery's connectors to a cathode and anode. In 1876 Eugen Goldstein coined the term "cathode ray" for invisible electrically charged beams broadcast from cathodes. His name preceded the description of the electron, in 1891. We now understand that a cathode ray is a stream of negatively charged electrons.

It is a fundamental principle of electromagnetism that anything with an electrical charge is sensitive to a magnetic field. Cathode Ray tubes display a signal, like a television signal, in real time. Inside the tube the varying intensity of incoming signal directly controls the intensity of the beam of electrons the electron gun at the back of the tube produces. That beam next travels through an anode, with a hole in it, so only a thin beam of electrons emeger. Next that beam passes between two pairs of electromagnets. These electromagnets are used to precisely deflect the beam of electrons.

Prior to the development of television technology cathode ray tubes were the display elements of technology like radar sets, and oscilliscopes.

Both uses of cathode ray technology have an electron gun emit a narrow ray of electrons, and then use deflecting electromagnets point the electron beam to a screen with a coating that emits visible light, when hit with an electron. But they differ in which element of the design interprets the input signal.

In radar sets and oscilliscopes the beam is of constant intensity, and the input signal directly changes the intensity of the deflecting magnetic fields from the deflecting electromagnets. These tubes draw lines on the screen.

In analog televisions the signal controls the intensity of the electron ray. The deflecting electromagnets cycle a series of changes, like clockwork. Patterns of light and dark appear on the screen, at a rate of 30 frames per second. If the image requires a pixel to be light, the ray coming from the electron gun will be of high intensity, and if it is to be dark, the ray will be of low intensity.

In the late 20th century televisions, with advances in how many transistors could be fit on an electronic chip, and thus how cheap computer power became, manufacturers started to made televisions that accepted a digital signal. The digital signal carried an order of magnitude more information than an analog signal. Manufacturers continued to make cathode tube based computers, which interpreted a digital television signal, and used it to drive the analog electron gun.

When the manufacture of flat screen television became cheap, and it became practical to make them large, they largely replaced cathode ray tube televisions.