An Introduction to

UVB Fluorescent Tubes

How does a UVB Fluorescent Tube work?
All fluorescent tubes are sealed glass tubes containing an inert gas, usually argon, and a tiny drop of mercury.

The tube has two electrodes, one at each end, which are connected to an electrical circuit via a device known as a “controller” or “ballast”, which may be an older magnetic type with a starter tube, or a newer electronic device. The controller provides the initial kick-start for the lamp, then stabilises the current while the tube is alight.

Once lit, the current flowing through the electrodes vaporises the mercury in the tube and then excites the mercury atoms sufficiently to produce a steady flow of light, mostly ultraviolet, but with some blue and green visible light.

Everyday fluorescent lamps for human use are made of ordinary soda-lime glass which allows very little of that ultraviolet light through. They are also coated on the inside with fluorescent phosphor powder, which converts the energy from the ultraviolet light into visible light of a hue which depends upon its precise chemical makeup.

Lamps designed to emit ultraviolet light must therefore have less phosphor powder– so that less of the ultraviolet is converted into visible light – and must be made of glass which allows ultraviolet light through. Transparent fused silica “quartz” glass allows transmission of ultraviolet light, and is used for this reason, but it is expensive.

The UVB output of a fluorescent tube and the exact colour and intensity of visible light produced is thus determined by the precise quantity and blend of phosphors, and type of glass used.

The overall output of the tube is also determined by the wattage of the tube, which varies with tube length. Fluorescent tubes are produced in standard lengths, each with a corresponding wattage; typical examples being:

• 18 inches : 15 watts
• 24 inches : 18 or 20 watts
• 36 inches : 30 watts
• 48 inches : 40 watts

Some manufacturers only produce tubes with a diameter of 1 inch - "T8" size tubes. Others offer a choice between "T8" tubes and thicker ones with a diameter of 1.5" - "T12" size tubes.

The Output of UVB Fluorescent Tubes
All fluorescent tubes, unless fitted with a reflector of some type, emit light, including ultraviolet light, more or less evenly and symmetrically from their entire surface between the electrodes (situated about half an inch from the end of each tube). This radiation can be imagined as a cylinder, expanding and dissipating with increasing distance from the tube surface.

Manufacturers usually describe, for each product, the percentage of the total light output of the tube which is emitted as ultraviolet light. Hence a lamp may be described as producing "30% UVA and 5% UVB" and this would mean that the remaining 65% of output was emitted as visible light. This gives an indication of the balance between UVA, UVB and visible light but it does not indicate the intensity of ultraviolet illumination which can be expected; i.e., it does not distinguish between a dim, inefficient lamp and a bright, efficient one.

Our broadband UVB meters measure something different: the total UVB output in microwatts per square centimetre - the actual intensity of UVB illumination. As we have described earlier, this has advantages. We can measure the amount of UVB available to a reptile at any given distance from the tube; we can plot the rate of decay of a tube over time; and we can compare two tubes of the same brand and wattage and see at a glance whether their output is the same. 

We can also plot the shape and extent of the "cylinder" of radiation put out by a lamp, and use this to help decide upon placement of a tube within a vivarium.

However, we can only compare identical products. A higher reading does not necessarily indicate that the tube is better at promoting synthesis of vitamin D3. Different brands (with different phosphor blends and/or glass tubes, and therefore different proportions of UVB at various wavelengths) may have similar overall UVB output but vary in their ability to promote vitamin D3 synthesis. The results of new research should soon be available (see our Introduction to the Lighting Survey for details) but in the meantime we advise against comparing products against each other, based upon our test results here.

Types of Fluorescent Tube
The fluorescent tubes available for use in reptile vivaria fall into two categories:

1. Those designed to provide UVA plus UVB, to enable vitamin D3 synthesis in reptiles known to require it.
2. Those designed to simulate "daylight", sometimes known as "full spectrum" lamps, emitting UVA but only very low levels of UVB.

We have limited our survey (with one exception) to lamps stated by their manufacturers to be for the provision of UVB for vitamin D3 synthesis by reptiles. The exception was a single new Exo Terra Repti Glo 2.0 Daylight Terrarium Lamp which was included in a set of sample lamps donated to the project, and which we tested out of curiosity.

The "daylight" or "full spectrum" lights are not designed to enable vitamin D3 synthesis. However, several manufacturers suggest the use of these in conjunction with UVB-emitting tubes in order to improve the overall lighting quality inside the vivarium, increasing the UVA levels and the colour balance of the light.
This certainly does have this effect. This can also be achieved with incandescent light, including some of the so-called "basking lamps" with improved colour balance. If heat as well as light is required at a basking spot, incandescent lamps may be a good alternative since fluorescent tubes produce very little heat.

What are Fluorescent Tubes most suitable for?
Tubes supply a diffuse "glow" with low intensity visible light, very little heat, and a fairly uniform UVB gradient, resembling natural UVB "in the shade" on a sunny day. There are many species which appear to live in semi-shade in the wild, and rarely bask in sunshine, for which this type of gentle UVB illumination is ideal. (See our feature, What UV light do reptiles need?)
This type of set-up has also proven adequate for generations of sun-loving reptiles, as long as good quality tubes with high UVB output are used, but more recent innovations such as mercury vapour lamps have made other alternatives possible for sun-worshipping species in larger enclosures.

Fluorescent tubes are ideal for providing UVB to all species in small vivaria, however, since they produce little heat and are thus unlikely to cause overheating or disrupt the temperature gradient produced by the heat source in a small vivarium.

Since they are not particularly bright light sources, fluorescent tubes can be situated directly above a reptile at fairly close range without, apparently, causing distressing glare. However, some do emit high UVB at close range and traces of UVC (1-3 uW/cm²) can be detected at the surface of all tubes tested, hence a safe minimum distance of maybe 3 - 4 inches would seem advisable.

The diffuse glow from a fluorescent tube is not focused into a beam, and hence does not project a great distance from the lamp. To achieve UVB coverage over a wide area, it is advisable to use the longest tube which will comfortably fit inside the vivarium. Reflectors may be used to direct the beam further into the vivarium and increase the effective output, for species with higher UVB requirements.

Since fluorescent tubes produce very little heat, all reptiles which need warmth above room temperature will need, in addition, a heater of some type. Species known to bask, in the wild, in full sunshine will usually benefit from extra visible light as well. For these, an incandescent basking lamp may be the answer.

Types of Ballast
It is important to match the tube with a ballast of the correct wattage, or the tube may fail to operate correctly or have its life shortened.
There are several different types of ballast available in a range of wattages. There is some evidence from tests conducted in the USA that the type of ballast used, and even the brand used, has a significant effect upon the output of a tube.

We are conducting our own tests on some of the brands commonly used here in the UK and hope to report our findings shortly.

In the UK, most keepers use older style magnetic ballasts familiar to aquarists. Examples include:

• Arcadia Fluorescent Lighting Controller
• Interpet Convertagear Aquarium Lighting Starter Unit
• Rolf C. Hagen (UK) Ltd Glomat 1 Fluorescent Lighting Control Unit.

Although there are ballasts of this type available for all lengths of tube, keepers using longer tubes sometimes use conventional household fluorescent tube holders of the correct length and wattage. These have built-in ballast/starter units and may be affixed to the ceiling of larger enclosures, or suspended on chains or cables. The ballasts in some of these fixtures are electronic, rather than magnetic.

Electronic ballasts of the "aquarium" type with end caps are not yet in widespread use. Arcadia have recently brought out an electronic model, the Arcadia Electronic Controller, which operates two T8 fluorescent tubes. The Glomat 2 Double Fluorescent Lighting Control Unit is another new twin T8 tube electronic ballast, this time from Rolf C. Hagen (UK) Ltd.
Electronic ballasts are claimed to have several advantages over magnetic ones, including increased energy efficiency (as they produce less heat as a by-product) and reduced flicker as the 50Hz "strobe" effect typical of AC current is not generated by the tube, as it is when magnetic ballasts are used.

New products currently emerging abroad include dimmable ballasts, heralding an era in which keepers may be able to create an artificial "dawn" and "sunset". (Fluorescent tubes on the ballasts currently available in the UK cannot be dimmed.)

The Fluorescent Tubes on Test

The Fluorescent Tube Survey Results

THIS SECTION IS TEMPORARILY UNAVAILABLE - WE ARE CURRENTLY UPDATING THE SURVEY (OCTOBER- NOVEMBER 2010)