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T-Rex
Active UV Heat Spot Lamp
 There
is only one spot lamp currently on sale in the UK: the T-Rex
Active UV Heat 100 watt Spot Lamp.
Until
the end of 2005, this was sold in distinctive packaging
with a picture of the Spot lamp on the side. (Fig.1) Since
the beginning of 2006, however, it has been packaged in
new boxes which actually picture the new Flood lamp but
carry details of both versions; the Spot lamp within is
unchanged in any way.
This
lamp is self-ballasted, and has a completely clear smooth
glass face. The results for direct readings from five lamps
are shown in Fig.2. (Below; click on image to view full
size version in new window)
The
output of the lamp increases as the distance increases from
the lamp’s surface, up to a distance of 8”, and then diminishes
again. The reason for this is that the “spot” lamp gathers
the light from the arc tube and focuses it into a more coherent
beam.
The
readings are very high; the weakest lamp tested, in use
for six months, was still producing 232uW/cm² at 12”. However,
this very high reading is only obtained in a very small
area at the centre of the visible beam. Careful alignment
of the meter was essential, to locate the peak reading at
each distance.
Fig.
2: Click on chart to view full size image
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This
lamp produces a narrow “spotlight” beam of UVB radiation
projected almost perfectly along the axis of the lamp,
as can be seen from the Spread Chart (fig.3., on
left). The narrow beam extends a long way. It
tapers gradually but at 4 feet from the lamp, 18 uW/cm²
can still be recorded at the centre point.
Fig.
4., (below) uses the Spread Chart to visualise
the UV beam. It shows one of these lamps in use in
a tall enclosure housing a group of chuckwallas (Sauromalus
obesus) where the one beam successfully provides
UVB at two levels, approximately 18” and 34” from
the lamp surface.
Burning
in a Spot Lamp
Successive
readings taken over the 75-hour initial burn of one
new lamp showed a very marked “burn-in” decay was
occurring. These results are shown in Fig.5. (Below:
Click on image to view full size version in new window)
The
first readings, at 30mins, indicated that at that
time, the lamp was emitting very high, even hazardous
levels of UVB radiation. All readings closer than
16” were higher than ever seen in nature, and at 12”
the lamp was emitting over 800 uW/cm². This is undoubtedly
why the manufacturer’s recommendations state that
the minimum distance that the lamp should be used
above the reptiles is 18”.
However,
extremely rapid decay over the first few hours of
use brought these readings down swiftly to safer levels.
After 15 hours the reading at 12” was under 400 uW/cm²
– a value typical of tropical sunlight – and after
45 hours burn, this had been further reduced to less
than 300 uW/cm².
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The
rate of decay usually slows after the first few days,
and this was indeed the case for this lamp. Between
45 and 75 hours burn, the precipitous fall in output
seen earlier had become a slow, fairly unremarkable
decline with signs that it was levelling off. Over
the entire burn-in period, however, the output dropped
dramatically: by 67%, for example, at 12”.
Heat
and Light from a Spot Lamp
The
spot lamp is a self-ballasted lamp, which, in addition
to UV light, emits additional heat and visible light
from the incandescent filament. The heat produced
by a 100watt spot lamp, however, would seem unlikely
to be sufficient to create a warm enough basking spot
at distances of several feet. Once again, when positioning
the lamp, careful measurement of temperatures beneath
the lamp would be necessary and in this case, thermostatically
controlled heating might well be required to supplement
that provided by the lamp.
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Fig.
5: Click on chart to view full size image
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