Anatomy of a UV Lamp — Structure & Components

"UV lamp" is a family, not one thing

There is no single "UV lamp." UV sources split first into conventional gas-discharge lamps and solid-state LEDs — and the discharge lamps split again into low- and medium-pressure mercury, amalgam and excimer. They differ along two independent axes: their physical form (a tube radiates in all directions and needs a reflector; an LED array points where you aim it) and their spectral character (a sharp line, a narrow band, or a broadband continuum). A lamp's shape does not decide its spectrum.

:::visual{name=uv-lamp-taxonomy caption="The UV-source family along two independent axes — form and spectral character."} :::

The most common germicidal type — and the one we dissect below — is the low-pressure mercury lamp: a sealed quartz tube with an electrode at each end, a noble gas and a trace of mercury, run by an external ballast. Apply voltage and the mercury vapour emits UV, primarily at 253.7 nm (germicidal) plus a 185 nm ozone line. An LED UV source reaches the same goal completely differently — a semiconductor (AlGaN) chip, no gas, no mercury.

Anatomy of a low-pressure mercury lamp

:::visual{name=lamp-anatomy caption="The four in-lamp components of a low-pressure mercury lamp; the ballast (5) sits outside."} :::

1 · Quartz-glass envelope

Ordinary borosilicate glass blocks UV below roughly 350 nm almost completely. The envelope is therefore fused quartz — transparent to the 254 nm and 185 nm the discharge produces. (Some low-pressure lamps use soft soda-lime glass when only the visible glow is needed.)

:::callout{variant=insight title="Why not ordinary glass?"} Glass would absorb the germicidal 254 nm and let almost nothing useful out. The envelope material is the gate that decides whether UV ever leaves the lamp at all. :::

Solarisation — an ageing mechanism. UV photons slowly degrade the quartz over the lamp's life; the envelope clouds and UV transmission drops. So part of a lamp's output decline comes from the envelope itself, not just the discharge. As a reference point, the 253.7 nm line of a typical low-pressure mercury lamp falls to roughly 70 % of its initial value after about 7,000 hours — the combined result of envelope ageing and electrode wear (Component 2).

2 · Electrodes

Two electrodes sit at the ends, usually tungsten coated with thorium, barium or calcium oxides. These oxides have a low work function: heated, they release electrons easily, which starts and stabilises the arc.

• Hot cathode (classic): the filament is pre-heated, so the lamp starts gently. Longer life (8,000–16,000 h), but a 0.5–2 s warm-up.
• Cold cathode: a thick-walled metal thimble instead of a filament. Instant-on and shock-resistant, but shorter life (~6,000 h). Preferred for mobile devices and frequent on/off cycling.

3 · Fill gas + mercury

• Noble gas: argon or an argon/neon mix (0.5–3 mbar) — the initial charge carriers that ignite the arc.
• Mercury: 10–100 mg per lamp, liquid or as a solid amalgam pellet. In operation it evaporates to about 0.001 mbar — the low-pressure point optimal for 254 nm and 185 nm emission.

How that mercury output is distributed across wavelengths is exactly what separates the lamp types — and it is a common myth that a low-pressure lamp is "monochromatic":

:::visual{name=hg-spectrum caption="Low-pressure mercury is line-dominant (254 nm); medium-pressure is broadband. Schematic — not to scale."} :::

:::detail{title="Engineering depth: amalgam vs. liquid mercury"} Run a traditional Hg lamp too hot (above ~40 °C) and its UV efficiency drops sharply — the Hg vapour pressure overshoots the optimum. An amalgam (an Hg-indium or Hg-gallium compound) holds the vapour pressure at the optimum across a wider temperature window. That is what enables high-output and compact, warm-running designs such as in-reactor water disinfection — at the cost of a slower warm-up to full output. (Amalgam tunes vapour pressure; it is not the same as the metal-halide doping used to tune medium-pressure curing lamps.) :::

4 · Base

The mechanical mount plus the electrical connection to the ballast. Common standards: G13 (bi-pin, T8 like classic fluorescent tubes), 2G11 (4-pin compact) and G23 / G24q (compact UV). High-output and medium-pressure lamps often use custom bases — they need higher currents and dedicated cooling. The base usually also carries the end-of-life (EOL) coding so the ballast can identify the lamp.

5 · Ballast (external)

UV lamps are negative-resistance devices: without a current limiter the current runs away and the lamp fails within seconds. The ballast is mandatory, not an optional accessory.

Ballast topology. Magnetic (choke + starter) flickers at 50/60 Hz, is inefficient, and is only legacy stock in the UV field. Electronic (HF, 20–60 kHz) is the standard today: no flicker, longer service life, higher efficiency, often with a dimming function.

A practical rule of thumb: an electronic programmed-start ballast can extend lamp service life considerably versus a magnetic instant-start arrangement — especially with frequent on/off cycling. That matters because the largest lifetime cost in a UV installation is often not electricity but the lamp change and the system downtime around it.

Lamp types compared

:::callout{variant=info title="Comparing Hg and LED fairly"} Germicidal effectiveness is wavelength-dependent, peaking near 265 nm — the peak of the DNA absorption curve. A 265 nm UV-LED sits closer to that peak than a 254 nm mercury lamp, so it delivers more germicidal effect per unit of radiant power. Comparing devices by electrical watts alone therefore understates LEDs systematically; the correct basis is the wavelength-weighted germicidal effect (see the action-spectra article below). :::

Practical implications for operators

Recognising ageing

A UV lamp ages primarily through three mechanisms:

1. Electrode sputtering — emitter material deposits on the inner wall → visible black-grey "lamp blackening" at the ends; UV output drops.
2. Quartz solarisation — the envelope clouds and yellows → UV transmission drops.
3. Hg loss through wall adsorption — poorer arc stability; the lamp flickers or struggles to ignite.

As noted above, the 253.7 nm output of a typical low-pressure lamp falls to roughly 70 % of its initial value after several thousand hours. Validated installations (drinking water, pharmaceutical use) must verify this with measurement sensors — a visual inspection is not sufficient for compliance. The governing references here are AG LUV Guideline 100 and DIN 67506 for secondary-air devices.

Maintenance best practice

• Clean the quartz sleeve every 3–6 months (biofilm in water applications, dust in air). Even a lamp that has not aged can lose a substantial share of its delivered output when the sleeve is fouled or scaled.
• Replace the lamp per the manufacturer specification (typically annually for 24/7 operation) — do not wait until output visibly declines. The final portion of service life often comes with noticeably reduced germicidal effect.
• Avoid cold starts: lamps tolerate frequent on/off cycling poorly. Where possible, choose a programmed-start ballast and continuous operation.

:::callout{variant=safety title="UV-C is acutely harmful to eyes and skin"} Photokeratitis within minutes, DNA damage, long-term skin-cancer risk. Installations with exposed UV-C sources require: viewing windows of UV-blocking material (boroflint / Borofloat / doped acrylics); protective eyewear to EN 170; interlock switches against accidental opening during operation; and DGUV-compliant labelling plus regular staff training.

With Far-UVC (222 nm excimer) skin and eye penetration is markedly lower than at 254 nm — but only for the filtered 222 nm band. An unfiltered KrCl lamp also emits a longer-wavelength tail (around 254 nm) that does penetrate skin, so the "human-safe" claim depends on a short-pass optical filter. Irradiation of occupied spaces is under discussion, not yet finally settled in regulation (the EU and Germany are more conservative here than the USA). :::

Cross-references

These deeper-dive articles build on this structural understanding:

• Ballasts & Drivers — magnetic vs. electronic, start types, dimming
• Reflector Geometries — how the UV gets from the lamp into the reactor
• UV Lamp Technology — lamp-type families compared in depth
• Wavelengths & Action Spectra — how each wavelength acts on each organism
• LED Area Emitters — LED construction in detail (AlGaN chip + SMD/COB/DOB package + AlN substrate + secondary optics + driver). The LED counterpart of this structural topic.
• UV-LED Lifetime & Degradation — how LED components age thermally and electrically; what the anatomy implies for lifetime modelling.

Sources

• IUVA UV Disinfection Handbook (Bolton & Cotton, 3rd edition) — the standard reference text
• ScienceDirect — Mercury-Vapor Lamp overview
• WCP Online — Germicidal Lamp Instruction 101 — output decline and rated-life basis
• ISL Products — UV-C Ballast Start Types
• Crystal IS — LED vs. Lamp Output Comparison
• EDN — Considerations in the selection of UV LEDs for germicidal applications
• ams-osram OSLON UV — Product Specifications
• DIN 67506 (UV-C secondary-air devices) — AG LUV / DIN working group, 2022
• AG LUV Guideline 100 — minimum requirements for UV-C devices
• US EPA IMERC Fact Sheet — Mercury use in lighting

Status: May 2026. This article will be expanded as the planned deep dives (ballasts, reflector geometries, excimer) are published.