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The EB Mega-Ray lamp is a PAR-38 reflector lamp with a moulded spreader lens on the surface and a slightly milky inner coating. Earlier models (2004-5) had short glass necks; newer lamps have a longer neck with a brass collar (Fig. 3.)
The EB lamp and ballast box are purchased as a kit; replacement lamps may then be purchased separately. At present these kits are only available in an 110volt version (the USA line voltage) and to operate them in the UK, with our 220-240volt mains electricity, we used a step-down transformer.
We found that some of the ceramic lamp fixtures we tried would not accommodate the bulbs with the brass collars, because the lamp would not screw far enough into the fitment. Bending the contacts within the fixture down towards the lamp solved this problem in most cases.
As with all externally ballasted mercury vapour lamps, when first switched on, the lamp produced a faint light from the arc tube which developed over a few minutes into a very bright light with (to the human eye) a purplish-blue tint and a directional beam.
The SB Mega-Ray lamp is almost identical in appearance except for the tungsten filament, which can be seen inside the lamp. A self-ballasted lamp, when first switched on it emitted a very bright yellow-white light from the tungsten filament which changed in tone to a bluer tint as the arc tube lit up and the beam developed.
The Mega-Ray Zoo EB Lamp is identical in appearance to the EB Mega-Ray lamp except that there is no milky coating on the inner surface of the lens; the glass appears completely clear.
1. Direct Readings
Because of the physical movement of the arc within the tube, and the way the radiation is generated, the meter readings from mercury vapour arc tubes are rarely steady at close range, and in fact the visible light can often be seen to "dance" as well, casting flickering shadows at the edge of the beam. For consistency, the highest reading seen three times at each distance is the one recorded, as it is assumed that this reading represents the most accurate alignment of the hand-held meter with the beam.
New lamps were burned-in for at least 90 hours before these recordings were made. All readings were taken after the lamps had been switched on for at least 30 - 60 mins, to ensure they were fully warmed up and the output stable.
(a) UVB recordings.
A series of direct readings were taken with the Solarmeter 6.2 UVB meter at increasing distances from directly beneath the centre of the lamp face. Figure 4 shows the output of all ten Mega-Ray lamps which were included in the trial.
The results show that these lamps do indeed emit UVB at the levels specified. There was wide variation between individual lamps; this has proven to be a feature of every brand of mercury vapour lamp we have tested so far. In this case, this was expected, since the lamps were labelled with their output as recorded prior to dispatch.
Readings at close range were higher than seen in nature, but at 18" (manufacturers recommended minimum distance) the four specially-requested "high-output" lamps produced levels of UVB resembling that found in natural sunlight they emitted between 127 and 228 uW/cm² after burning-in.
At 12" (manufacturers recommended minimum distance) the four "standard-output" lamps produced similar levels, between 122 and 202 uW/cm². The "Mega-Ray Low" was emitting 71 uW/cm².
Figure 5 shows the output of the EB Mega-Ray Zoo Lamp after burning-in.
This lamp, designed for use only in very large enclosures, has a vastly higher UVB output. The recommended minimum basking distance is 36 inches; at this distance the output was 259 uW/cm². At 72" the lamp was still emitting 70 uW/cm². At all distances closer than 26", readings were higher than seen in nature even in the tropics.
(b) UV Index recordings.
In many countries, especially in the summer, the Solar UV Index is reported along with the weather forecast in newspapers, on TV and on the radio. The colour-coded Index is easily understood and is becoming widely recognised. To quote the World Health Organisation (from their online booklet, Global UVI ):-
The Global Solar UV Index (UVI) is a simple measure of the UV radiation level at the Earths surface and an indicator of the potential for skin damage. It serves as an important vehicle to raise public awareness and to alert people about the need to adopt protective measures when exposed to UV radiation.
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