Parts of a Fluorescent Light – Fixture and Tube Labeled

All 10 parts on the diagram

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Fixture 5

Tube The long glass lamp that gives the light. Its pins fit the lampholders. Also called a fluorescent lamp.
Ballast It limits the current through the tube. Without it the current would keep growing.
Housing The long metal channel of the fixture. It holds the ballast and wiring and carries the lampholders.
Lampholder The socket on each end of the tube. It grips the two pins and feeds them power. Often called a tombstone.
Starter A small plug-in can on older fixtures. It preheats the cathodes so the tube can strike.

Inside the tube 5

Glass tube The sealed glass wall of the lamp. It keeps the gas in and the air out.
Bi-pin base The metal cap on each end, with two pins. The pins fit the lampholder and carry current to the cathode.
Cathode A small coil of wire at each end. Heated, it gives off the electrons that carry the current. Also called an electrode.
Phosphor coating A powder lining on the inside of the glass. It turns ultraviolet light into visible light.
Mercury vapor A trace of mercury as a gas, mixed with argon. Current through it gives off ultraviolet light.

How this list was compiled: Part names checked on 2026-10-01; a name is kept only when two independent sources use it. (1) Wikipedia, English edition, Fluorescent lamp and Fluorescent-lamp formats, https://en.wikipedia.org/wiki/Fluorescent_lamp and https://en.wikipedia.org/wiki/Fluorescent-lamp_formats (Glass tube, mercury vapor and argon, phosphor coating, cathodes (coiled filaments), ballast to regulate the current, starter, bi-pin base, the tombstone lampholder.) (2) The Fluorescent Lamp, a Physics of Technology teaching module, https://archive.org/details/TheFluorescentLamp (Its figure of a practical fluorescent lamp: phosphor coating, mercury vapor, argon, cathode; the ballast as a current-limiting device; the starter.) (3) Fluorescent Lighting, a reference book on lamps and fittings, https://archive.org/details/dli.ernet.4876 (Tube, electrodes and cathodes, mercury and argon, starter, lampholders, housing, reflectors.) (4) A lamp maker's practical guide to fluorescent lighting equipment, https://archive.org/details/WestinghouseFluorescentLightingEquipmentAPracticalGuideToThe (Ballast, starter, lamp holders, bi-pin bases, electrodes, phosphor, mercury, argon.) (5) A fixture maker's specification sheet for a fluorescent strip light, https://archive.org/details/manualsbase-id-181567 (Housing, channel, ballast, ballast cover, lampholders.) No maker is named. Licence: own work

The drawing: Drawn for this site on 2026-10-01 from plain geometry; own work, no maker's model copied. Licence: own work

How it works

A fluorescent light is two things: the fixture on the ceiling and the tube that glows in it. The first view is a strip fixture from the side, cut open over its ballast. The second is one end of the tube, enlarged and cut open.

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The fixture

The housing is the long metal channel that holds everything. Inside it sits the ballast, which limits the current through the tube. A lampholder at each end grips the tube by its pins and feeds it power. On an older fixture a starter, a small can beside one lampholder, preheats the tube so it can light.

Inside the tube

Each end of the glass tube has a bi-pin base and, behind it, a cathode: a small coil that gives off electrons. The tube holds a little mercury vapor in a gas such as argon. Current through that vapor makes ultraviolet light, and the phosphor coating on the inside of the glass turns it into the light you see.

The lampholder at the end of a tube is often called a tombstone, after its shape.

Questions and answers

What are the parts of a fluorescent light fixture?

A housing, a ballast inside it, a lampholder at each end and the tube between them. Older fixtures also have a starter. The tube itself has a bi-pin base, a cathode, a phosphor coating and a fill of mercury vapor.

What does the ballast do in a fluorescent light?

It limits the current. Once a fluorescent tube strikes, the current through it would keep growing; the ballast in the circuit holds it to a safe, steady value.

What is the white coating inside a fluorescent tube?

A phosphor. The mercury vapor in the tube gives off ultraviolet light, which the eye cannot see, and the phosphor coating absorbs it and glows with visible light.

What is the starter on a fluorescent light?

A small plug-in can found on older, preheat fixtures. It lets current warm the two cathodes for a moment before the tube lights. Many newer fixtures have ballasts that need no starter.