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Suffering
from burn-out? The problem of decay.
When
a brand new tube is first switched on, it reaches maximum
brightness once the temperature within has stabilised -
in less than quarter of an hour. From that point on, however,
the lamp will begin a slow but inexorable decline in output,
both of ultraviolet and of visible light, throughout its
working life.
This
is due to complex chemical changes which occur between the
phosphor coating, the mercury, tungsten and other material
evaporating from the electrodes. The slow deterioration
in light output is well documented and referred to as the
"lumen maintenance curve"; it is said to fall by up to 5%
in an average household fluorescent lamp during the first
100 hours of use, and thereafter more slowly. The deterioration
in UV output may be greater than this, however.
Some
manufacturers used to recommend replacement of UVB tubes
every six months, even if there was little noticeable reduction
in visible light from the tube; most now suggest annual
replacement.
To investigate
the initial decay occurring in a new lamp, we recorded the
UVB output of all new tubes regularly throughout their first
100 hours of use - the "burning in" period. Graph
6 shows the typical pattern we observed in most tubes
we tested, illustrated in this case by a 24" Zoo Med Reptisun
5.0 tube. There is a loss of about 15% over the first few
hours but the initial rapid decay does not continue; by
the end of the test period, the rate of decay is slow enough
for the output to appear fairly stable from day to day.
To
investigate long-term decay, we installed several new tubes
in vivaria, and measured the output of these in situ, at
regular intervals over periods up to twelve months. We also
took readings from a range of older tubes in other vivaria
which had been in use for as long as two to three years
or more, to examine decay in output over long periods of
time. One lamp on test, an 18" ExoTerra Repti Glo 8.0, fitted
with a reflector, has been in constant use in a vivarium
for a year, for 10 - 12 hours a day depending upon the season.
Graph 7 shows the decay of
this lamp, over the first 4,000 hours of use. The output
of this tube after a year's use, in the reflector, is still
a respectable 28uW/cm² at 12" although this represents a
loss of almost 40% over the year. (Without the reflector,
the tube was only emitting 16uW/cm² at 12".)
Individual
differences
Our
initial tests revealed that tubes from different brands
did start off with different levels of output. (See Graph
1.) This might seem obvious from their brand names,
which indicate that a tube emits 5% or 8% or 10% UVB. However,
this figure has little to do with the actual amount of UVB
produced. A 5% UVB tube is one which is emitting 5% of its
output as UVB light; this does not tell you how powerful
the tube is, but only the proportion of UVB in the beam.
Hypothetically, a 5% UVB tube that was twice as radiant,
overall, as a 10% UVB tube would produce the same
amount of UVB.
In practice,
of course, one might expect tubes of the same size and wattage
to have somewhat similar overall output. Of the lamps
on test, 5 brands specify UVA output of about 30% and UVB
output of around 5-8%, indicating a visible light output
of about 62-65%. It is therefore surprising to find such
variation between the UVB output of these different brands.
We have not measured their UVA or visible light output,
however, so we cannot say whether the variation is due to
a difference in overall output (i.e., some tubes are radiating
more than others at all wavelengths) or that some tubes
are simply not meeting their specifications.
We were
interested to find that the initial output of individual
tubes of the same type from the same manufacturer may vary
somewhat. Individual tubes then seem to vary slightly in
their rate of decay, as well. We have several tubes on long-term
measurement and whereas all have decayed somewhere around
15 - 20% over the first three months, some lamps have decayed
steadily throughout, whereas others appear to have lost
most of this in the first 150 hours and thereafter have
decayed more slowly.
We have
found considerable variation in the output of older tubes
submitted for testing after many months of use. With some
brands, tubes that had been in use for a year were found
to be emitting as much, or more UVB than others of the same
type that were only five or six months old. We do not know
whether this is due mainly to a difference in output from
the beginning, or to different rates of decay. Graph
8 shows the output of a wide selection of tubes (all
24" in length, to make comparison within each brand
possible) that have been in use for different lengths of
time.
One
thing is clear: the UVB output does not cease at any point
during the tube's life, but apparently declines slowly throughout
the weeks and months, in a similar way to the visible light
output.
Some
tubes from brands with high initial output are still putting
out 15uW/cm² at 12" after a year or more. Others appear
to be decaying more rapidly and after six months some keepers
might consider the output low enough to warrant replacement.
Without using a UVB meter, however, it would not be possible
for anyone to determine whether any given lamp needed replacement,
or whether it would be perfectly serviceable for many more
months.
Meter
owners might be able to prolong the useful life of an older
tube by fitting a reflector and/or moving the tube down
closer to the basking area, using the meter to establish
the new position of the lamp in the vivarium. Obviously
this should not be taken to extremes, however, with an ancient
lamp hanging just inches over an animal's head!
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The
UVB spread chart
