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.
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.
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.
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.
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.
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
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:
inches : 15 watts
inches : 18 or 20 watts
inches : 30 watts
inches : 40 watts
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.
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.
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.
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.
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.
of Fluorescent Tube
The fluorescent tubes available for use in reptile vivaria
fall into two categories:
designed to provide UVA plus UVB,
to enable vitamin D3 synthesis in reptiles known to require
designed to simulate "daylight",
sometimes known as "full spectrum" lamps, emitting
UVA but only very low levels of UVB.
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.
"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.
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.
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.
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.
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.
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.
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.
conducting our own tests on some of the brands commonly
used here in the UK and hope to report our findings shortly.
UK, most keepers use older style magnetic
ballasts familiar to aquarists. Examples include:
particularly convenient for use with short tubes in small
Arcadia Fluorescent Lighting Controller
Convertagear Aquarium Lighting Starter Unit
C. Hagen (UK) Ltd Glomat 1 Fluorescent Lighting Control
The ballast box, placed outside of the vivarium, is supplied
with cable connectors with end caps which fit over the terminals
of the fluorescent tube. The tube is then suspended over the
vivarium, or held in place under the vivarium roof by tube
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.
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.
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
light is hazardous.
Excessive exposure is harmful to human eyes and skin,
and excessive exposure is likely to be harmful to
reptiles as well.
follow the manufacturers instructions, in particular
take note of all safety warnings and follow minimum
use eye protection when checking UVB lamps.
We would strongly recommend that even “sun-worshipping”
desert reptiles are never exposed to levels of UVB
higher than that found in natural sunlight.
All reptiles must be able to shelter from UVB light;
a UVB gradient, similar to the heat gradient with
which we are familiar, is necessary in the vivarium.
would also recommend that where possible, lights
of any type, including UVB lamps, be positioned
overhead in the vivarium, so that reptiles are never
forced to endure unwanted glare.
check basking temperatures underneath a mercury