DBG: Lights for bat work

Imagine a bat survey walk. You are walking through a woodland track on a dark night. In one hand you have your bat detector, and you are also carrying other equipment - a notebook and pencil, thermometer, maybe a recorder ... you will need a light to see your way, and to check settings and make notes - ideally without losing your night vision, or flashing light in others eyes. A head torch is often useful here as its hands free.

Now you reach your survey point. You know there are bats there - but are they Daubentons or Pips? Spotting and identifying bats can be a problem as they are dark coloured, very small, fly and change direction very quickly, and are seen against a background of dark trees water or dark sky. Now you need a light that is: directional, powerful, reliable, robust, lightweight, cheap, and runs for ages on AA or rechargeable batteries. Modern LED torches can meet all these requirements.

And finally, when all else fails and you are lost in the woods at night, its good to have a spare!

What types of portable light are available?

Conventional torches using incandescent bulbs, even halogen bulbs, are heavy, run hot and drain batteries quickly. Torches using Light Emitting Diodes (LED's) are now usually the best choice. Torches using High Intensity Discharge lamps(HID's) produce enormous light outputs (see below) and are becoming economical - but are larger than LED torches, can be heavy, and drain batteries quickly. They also get VERY hot quickly so are not suited to being turned on for long periods.

Incandescent lamp torches and lanterns

These all use a bulb with a tungsten filament, which radiate most of their output in the infra-red.  To get better efficiency you need to run the filament hotter. But tungsten cant be used at much over 3000 degrees as it starts to evaporate, weakening and eventually destroying the filament. Tungsten-Halogen are the most efficient, adding Xenon makes them even better.

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Lanterns rated at over a million candlepower mostly use 55W car headlight bulbs ( shown above). The downside here is their limited burn time. This is down to the massive amount of heat they produce, and the size and weight of the battery needed to power them. Getting huge candlepower needs a powerful bulb, careful focussing, and a large reflector. This one uses a 6v 4AH battery, weighs 1.6kg
Lumens is a measurement of the total light output of the lamp, while candlepower is a measurement of the brightest part of a focused beam.

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LED torches

One very useful feature to look for in a torch is the ability to choose a lower light output. This FENIX L2D offers 12, 53, 107 and 180 lumens from a CREE Q5 LED and two AA cells. Power is selected by rotating the bezel switch (separate from the on/off switch). Some torches require you to cycle through the power levels - not much good if you are trying to preserve your dark adaptation! "Led lenser" torches and some of the other high-end torches use a focussing lens that is matched to the shape of the led to give a beam that can be focussed from spot to flood!

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This torch by XTAR uses a SSC P7 900 lumen LED (shown above) driven from a single 18650 Lithium rechargeable cell to give about 600 - 150 lumens output. The run time is about 2h on a fully charged battery, or 20h on the low setting. It comes in two models, the P7-C and P7-C2.
I chose the latter for a bit of "spill" - if a bat flies out of the beam this means I can see where its gone. It came in a kit with two batteries and a charger for about £20 delivered. Its really good value for money, and easily outperforms my big heavy lantern. I got mine from Ebay. Beware of torches that use AAA size batteries unless size is really important, as the capacity of these is small compared to AA's.

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There are some great deals (Ebay) for torches like these with CREE XM-L T6 LED's and including a 26650 Lithium cell and a charger for £10.00 inc p&p! The 26650 should be better suited to the high current demand (over 2 amps) and give decent battery life.
This Ultrafire is focussable and has high middle & low power settings. Dont use it on high for too long it WILL get hot! Some manufacturers are using cloned LED's and over-running them to get very high lumens ratings, at the expense of their useful life.

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For hands-free use a head torch is invaluable. Models keep changing - Noel recommends Petzl as a good make - but they are expensive. This one by Gelert uses 3 AAA cells, is available from Amazon for £5, and usefully has two white light levels and a red light to preserve night vision. Best advice - look in shops for a model that is well balanced on your head. Some have a flash setting for cycling - thats not helpful for our use!

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High Intensity Discharge (HID) torches

Recently introduced headlight systems for upmarket cars use HID lamps at up to 35W to produce a staggering 3000 lumens. This technology has been adapted to make torches with high light output but a more realistic burn time.

Usually powered by Lithium rechargeable batteries, they can still be rather heavy because of the electronics Back in 2007 these were only to dream about($7200!) but prices are coming down. They are currently available from £50 with free shipping, and comes with 2 * 18650 batteries to give a claimed 50 min on 35W and 75 min at 20W setting.

These lights include a sensor that turns off the light when the torch gets too hot - so dont expect to use them continuously!

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