Definition
An electronic display device consisting of a sealed, evacuated glass tube in which a focused beam of electrons is fired from a cathode at one end onto a phosphor-coated screen at the other end. The beam is steered by electromagnetic or electrostatic deflection so that, where it strikes the phosphor, the screen glows and produces a visible image. In aviation it has been used for radar scopes, weather radar displays, and early electronic flight instrument systems (EFIS).
Plain English
A type of glass screen that draws a picture by firing a tiny beam of electrons at the inside of the screen, making it glow. The older big, deep TV monitors worked this way, and so did early cockpit displays before flat-panel screens.
Context Anchor
Seen in discussions of older aircraft instruments, radar displays, and electronic display systems that used tube-type screens before modern flat-panel displays.
Derivation
Cathode comes from the Greek kathodos, meaning 'a way down' — referring to the negative electrode where electrons leave. Ray refers to the beam of electrons fired across the tube. Tube describes the sealed glass vessel that contains it. Together: the tube where a ray is emitted from the cathode.
Why Pilots Care
Understanding legacy CRT instruments helps pilots and mechanics troubleshoot and maintain older aircraft systems still in service.
Analogy
Think of an old-style boxy television set — the deep, heavy kind with a curved glass front. That depth was needed to house the electron gun firing at the back of the screen. A CRT cockpit display works on the same principle.
Intuition Check
Do not picture a simple tube carrying visible rays. A cathode-ray tube is a complete electronic screen that uses an internal electron beam to make a visible display.
Example Sentence 1
The aircraft's original weather radar used a monochrome cathode-ray tube, which the owner replaced with a modern color LCD during the avionics upgrade.
Example Sentence 2
Mechanics replaced the cathode-ray tube in the attitude indicator after it lost brightness.