When to replace a reptile UVB bulb

Why the calendar rule exists

A fluorescent UVB lamp degrades in a way that is invisible. The phosphors that produce visible light are not the phosphors that produce UVB, and they do not decline at the same rate. The care guidance states the practical consequence directly: UVB bulbs must be replaced regularly to continue to function properly, and just because a lamp is still producing visible light does not mean it is still producing UVB.

The published interval for T5 fluorescent lamps is 12 months, and it is the same across every species page we cite. That interval is a conservative default for keepers without a meter.

What a declining bulb actually does to the animal

Nothing visible, for a long time. The animal continues basking under a lamp that looks identical. UVB exposure falls below the Ferguson zone target, vitamin D3 synthesis declines, and dietary calcium is progressively less well used. The first signs are slow and non-specific. This is precisely why the rule is a calendar rule rather than a symptom-based one: by the time there is something to see, the deficit has been running for months.

How to track it without thinking about it

1. Write the date on the tube. A permanent marker on the end cap, the day you install it. This single habit solves the problem for most keepers.
2. Put a 12-month reminder in your phone calendar at the same moment.
3. Buy the replacement before you need it if your lamp is a specialist line that is not stocked locally.
4. Replace the bulb and the fixture check together. Confirm the reflector is clean - dust on a reflector reduces output in exactly the way you are trying to avoid.

How a meter changes the answer

With a Solarmeter 6.5 the question stops being "has a year passed" and becomes "is the reading at the basking surface still inside the target". The guidance recommends investing in one for exactly this purpose: it tracks the UVI inside the enclosure and tells you whether the animal is getting too much or too little. In practice a meter will often tell you the lamp was out of range before 12 months, or that it is still in range at 13 - and either way you are working from the enclosure rather than from a general rule.

Which lamps this applies to

• T5 fluorescent (HO and SO): 12 months, every time.
• Compact coil: we do not recommend these as a primary UVB source at all. See the bulb roundup.
• LED UVB: not recommended. Reptile lighting researchers have concluded the technology is not yet ready for safe reptile use, and nonsolar UV spectra carry risks that are still being characterized.