Quick answer
A replacement UV lamp has to match your system on physical fit, electrical fit, and spectral type — a lamp that merely looks the same can fail to start, run at the wrong power, or emit the wrong wavelength. A practical illustration of how far appearance can mislead: an aging germicidal lamp can keep glowing visibly long after its UV-C output has dropped by over 90%, because the visible blue light you see has no germicidal function — only the invisible UV-C does (American Aquarium Products; Coospider).
To get the right lamp, read the markings on your current lamp and its housing, write down the parameters in the table below, and compare them against any candidate replacement. When a marking is missing, measure the lamp end to end (including pins), photograph both ends, and note the ballast's rated wattage. See the deep sections for what each parameter means and why it matters.
Step 1 — Read what is already in your hand
Before searching for a replacement, gather the evidence on the failed lamp and its system. The lamp itself is usually printed with a part or type number; the ballast or housing label carries the electrical rating; and the original system manual lists the OEM lamp code. Suppliers consistently advise that you identify the base, locate the wattage, and measure the lamp end to end before ordering — a visual guess is not enough (The Pond Guy; ProLampSales).
If the printed code is legible, that is the fastest path: a same-code lamp from a reputable source is by definition a fit. If the print has burned off (common near the lamp ends), fall back to measured parameters.
Step 2 — The parameters that must match
Arc length and overall length
UV lamps are specified by two lengths. Overall length is the full physical span including the bases/pins — it determines whether the lamp physically fits the chamber. Arc length is the active light-producing section between the electrodes — it determines how much of the chamber is actually irradiated. Suppliers measure and list arc length separately from overall length precisely because the two are not interchangeable (ProLampSales). UV-C lamp lengths are often quoted in millimetres rather than inches, so confirm the unit when comparing.
Wattage
Wattage, read together with length, tells you whether the lamp is a standard-output or high-output type (ProLampSales). Wattage is not a free choice: the lamp wattage must match what the ballast is built to drive (see Step 3).
Base / connector type
The base is one of the most error-prone parameters because there are many incompatible families: two pins on each end, four pins on one end, a single pin on each end, recessed single-contact, or proprietary connectors. When in doubt, a clear photograph of each lamp end is the most reliable way to identify the base (ProLampSales).
Single-ended vs double-ended
A single-ended lamp has its electrical connections on one end only (for example a 4-pin lamp where both electrodes are wired to one base). A double-ended lamp has a connection at each end. This determines the holder geometry of the fixture — a single-ended lamp cannot be wired into a double-ended fixture or vice versa. Replacement catalogues list this as an explicit attribute for exactly this reason (CureUV).
Low-pressure vs amalgam (high-output)
Both are low-pressure mercury technologies, but they are not drop-in equivalents. A standard low-pressure lamp has an optimal operating temperature near 40 °C; an amalgam lamp uses a mercury alloy (with a metal such as indium or gallium) that regulates mercury vapour pressure and keeps UV output stable at ~100–120 °C, allowing it to be driven harder. As a result, amalgam lamps deliver up to roughly 3× the UV-C output of a standard low-pressure lamp of comparable size (Opsytec; Infralight). An amalgam lamp generally needs a ballast designed for its higher drive current — substituting one for a standard lamp is an electrical mismatch, not just a brightness upgrade.
Ozone vs ozone-free
Germicidal lamps differ in the quartz used for the envelope. Ozone-free ("low ozone", "L"-type) lamps use quartz doped with titanium that blocks the 185 nm line while still transmitting the 253.7 nm germicidal line — such lamps transmit up to ~90% of their energy at 254 nm and cannot generate ozone. Ozone-generating ("VH" / very-high-ozone) lamps use clear fused quartz that also passes 185 nm, which reacts with oxygen to produce ozone (Light Sources – 254 nm lamps; Light Sources – 185 nm lamps). Swapping an ozone-free lamp for an ozone-generating one (or the reverse) changes the chemistry of the whole installation — a critical mismatch for occupied spaces and for systems not designed to handle ozone.
Step 3 — Why a visual match is not enough
Two lamps with identical glass diameter, length and base can still be electrically incompatible. UV lamps require a ballast that supplies the correct starting voltage and operating current, and lamp and ballast must be matched:
- For magnetic ballasts, the replacement should sit within ±10% of the lamp's rated wattage. Running a 40 W lamp on a 60 W ballast damages the lamp quickly (All About Circuits forum).
- Underpowering or overpowering causes lamps that fail to start, run dim, or burn out early (All About Circuits forum).
- Medium-pressure lamps cannot be operated on low-pressure ballasts (All About Circuits forum).
The spectral type is also invisible to the eye: an ozone-free and an ozone-generating lamp can look identical but behave completely differently. And lamp ageing is deceptive in the other direction — a lamp at end of life keeps producing visible light while its germicidal output has collapsed. A 254 nm line typically falls to about 70% of its initial value after roughly 7,000 hours, and effective germicidal life is often quoted at around 9,000 hours (Coospider). "It still lights up" is therefore not evidence that a lamp is still doing its job — and not a reason to delay replacement.
Step 4 — OEM vs compatible / aftermarket lamps
You will usually have a choice between the OEM lamp (sold under the system builder's brand) and compatible / aftermarket lamps. An honest view of the trade-offs:
- A UV-system supplier states that many germicidal lamps on the market — including OEM-branded ones — are physically produced by specialist lamp manufacturers rather than by the system OEM itself (UV Superstore). Treat this as a supplier's industry observation, not a universal rule — but it explains why a reputable compatible lamp can match OEM performance.
- The same source warns that inferior lamps also exist: they look similar but use lower-grade glass and fabrication, lose UV intensity faster, and burn out sooner. Most lamps perform similarly for the first ~100 hours; the difference shows up afterwards (UV Superstore).
- OEM lamps offer guaranteed parameter fit and a clear warranty path. A good compatible lamp can save cost; a bad one is a false economy.
Practical rule: the OEM-vs-compatible question is secondary to getting the parameters right. Decide on the required arc/overall length, wattage, base, single/double-ended, low-pressure/amalgam and ozone type first — then choose between OEM and compatible suppliers, preferring sources that publish full specifications rather than just a price.
Parameter checklist — what must match
| Parameter | What to record | Why it must match |
|---|---|---|
| Overall length | Full length incl. pins (mm or in) | Physical fit in the chamber/fixture |
| Arc length | Active electrode-to-electrode span | Determines irradiated zone length |
| Wattage | Lamp rating; ballast rating | Must be within ballast tolerance (±10% magnetic) |
| Base / connector | Pin count + layout; photo of each end | Wrong base will not seat or wire up |
| Single- vs double-ended | Connections on one end or both | Fixture holder geometry |
| Low-pressure vs amalgam | Standard-output or high-output | Different ballast drive; ~3× output difference |
| Ozone vs ozone-free | "L"/low-ozone vs "VH"/ozone-generating | Changes chemistry of the installation |
| Diameter | Tube outer diameter | Fit inside quartz sleeve/holder |
Cross-references
- How UV lamp technology works — background on low-pressure mercury and amalgam discharge lamps.
- UV lamp anatomy — electrodes, fill gas, quartz envelope and bases explained.
- Wavelengths and action spectra — why 254 nm vs 185 nm matters for germicidal effect and ozone.
- UV-LED lifetime and degradation — the equivalent ageing discussion for LED-based systems.
- UV economics and ROI — cost framing for OEM vs compatible lamp decisions over a system's life.
- How to read a UV datasheet — decoding the spec sheet of a candidate replacement (coming).
- Ballast / driver replacement guide — when the ballast, not the lamp, is the failed part (coming).
- LED vs mercury decision guide — whether to replace a mercury lamp like-for-like or switch technology (coming).
Sources
- ProLampSales — Replacement Ultraviolet Bulbs & Sleeves — manufacturer/supplier guidance on identifying arc vs overall length, wattage, and base type before ordering.
- The Pond Guy — Pond UV Light Replacement Bulbs — practitioner-facing guidance: identify base, wattage and measured length rather than matching by appearance.
- Light Sources — Standard Quartz Germicidal 254 nm UV Lamps — quartz doping, ozone-free "L"-type vs ozone-generating "VH"-type.
- Opsytec — Optimization and cooling of mercury low-pressure lamps — operating-temperature optima for low-pressure vs amalgam lamps.
- Infralight — Amalgam Ultraviolet Lamps: 3 Times The Output — amalgam vs standard low-pressure output and mechanism.
- Coospider — Lifespan and Replacement of UV Disinfection Lamps — output decline over hours; visible glow vs germicidal output.
Further practitioner and electrical-compatibility material was reviewed (American Aquarium Products UV troubleshooting; All About Circuits ballast-compatibility threads; UV Superstore lamp-purchasing guide) and is cited inline where load-bearing.