Ferguson Zones, and the Lamp Distance They Imply

The zones describe behaviour, not a prescription

Ferguson and colleagues measured what wild reptiles were doing rather than deciding what they should have. Each zone carries two figures: the mean ultraviolet index across the microhabitats the species was found in, and the maximum index at which individuals were observed to bask voluntarily. Those two are not the same and the gap is large — zone 3 has a mean of 1.0 to 2.6 and a voluntary basking maximum of 2.9 to 7.4, which is the difference between where an animal lives and where it deliberately goes for twenty minutes.

That is why a terrarium is built as a gradient and not as a setting. The animal has to be able to reach the basking figure and to walk away from it entirely, which means a lit end, a shaded end and something to climb between them. A tank lit uniformly to the mean index of its zone offers neither the peak the animal seeks nor the shade it needs, and is worse than a bright lamp over one corner.

Mesh removes a third of it, and geometry decides the correction

A standard terrarium screen transmits about 65 per cent of the ultraviolet and a fine one about 55, and recovering that loss means bringing the lamp closer. How much closer depends on the lamp's geometry, because irradiance does not fall off the same way for both. A compact lamp or a mercury vapour flood behaves as a point source, so intensity falls as one over distance squared and recovering 65 per cent means moving to the square root of 0.65 — 81 per cent of the distance, nineteen per cent closer. A linear tube behaves closer to a line source, so it must move to 65 per cent of the distance, thirty-five per cent closer.

The practical consequence is that mesh hits tubes hardest, and tubes are the lamps most often mounted outside the screen. A 39 W T5 for zone 3 sits 66 to 107 cm above the basking site with no screen and 43 to 69 with a standard one — a difference of nearly forty centimetres at the far end, which on most vivaria is the difference between a lamp on the ceiling of the room and a lamp inside the enclosure. Mounting the tube inside removes the correction entirely and is the cleaner answer where the species and the enclosure allow it.

Where the sources disagree, the tool says so

Six of the twenty-eight species in the list carry two zones rather than one, because the published guidance does not agree: a ball python is given as zone 1 or 2, a bearded dragon as 3 or 4, a panther chameleon and a blue-tongue skink as 2 or 3. Picking one silently would hide a real disagreement behind a single number, so both are shown and the reader chooses — which in practice means aiming at the lower zone and letting the animal climb towards the higher one.

The lamp data carries the same honesty. The distance bands for the T5 tubes and the mercury vapour lamps come from the manufacturer's own per-lamp charts; the weaker families are read off a general colour chart and are flagged as such, because the two documents disagree by several inches where they overlap. A number sourced from a general chart and a number measured for one lamp are not the same kind of number, and the page does not present them as though they were.

Frequently asked questions

What Ferguson zone is a bearded dragon?

Three or four, depending on the source, and this is one of the six species where the published guidance disagrees. Aiming at zone 3 with a basking spot that reaches the upper end of it, and leaving a large shaded area, satisfies both readings — an animal offered a gradient will take the exposure it wants, which is exactly what the zone figures were measured from.

How often do I replace the lamp?

Every six to twelve months for fluorescent lamps and about a year for mercury vapour, and the interval is not about the lamp going out. Ultraviolet output decays long before visible light does, so a tube that still looks bright can be delivering a fraction of its original UV — the animal gets a lit enclosure and no ultraviolet, which is the failure mode that produces metabolic bone disease. A UV index meter is the only way to know, and it is the one instrument this hobby genuinely repays.

Can a lamp be too strong?

Yes, and the tool marks the distance below which it is. Each lamp has a published figure under which the chart shows a UV index above 7, which is the point at which no animal should be able to reach — 20 cm for a 15 W T5, 36 cm for a 39 or 54 W, 15 cm for a 100 W mercury vapour. Overexposure causes photokeratoconjunctivitis and skin damage rather than a slow deficiency, and it happens fastest when a lamp intended for a large enclosure is fitted to a small one.