Haitz's Law states that every decade, the price of LEDs have fallen by 10x while the performance (measured in flux per unit) has increased by 20x, for a given wavelength (color) of light.
REF:
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, thermometer ... 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.
And finally, when all else fails and you are lost in the woods at night, its good to have a spare!
Conventional torches using incandescent bulbs, even halogen bulbs, are heavy, run hot and drain batteries quickly.
Torches using Light Emitting Diodes (LED's) are improving very rapidly but we still need to make some compromises.
Torches using High Intensity Discharge lamps(HID's) produce enormous light outputs (see below) and are becoming economical.
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LEDs are continuously improving. In 1999 their efficiency
surpassed that of conventional incandescent bulbs, and in 2006 Cree
demonstrated white light LEDs with a better efficiency than
fluorescent tubes. Haitz's Law states that every decade, the price of LEDs have fallen by 10x while the performance (measured in flux per unit) has increased by 20x, for a given wavelength (color) of light. REF: |
| Seoul Semiconductor have recently (Feb 08) brought to market the Z-Power LED P7 series a single LED package
providing the worlds highest brightness of 900 lumens (90 lumens per watt)
and lower power (1W) leds in development are now achieving up to 160
lumens per watt. (lm/W) (Jan 2009) More recently Cree have extended their range to include the XM-L T6 range producing 1000 lm for a 10W input, and have demonstrated an LED which produces an incredible 230 lm at 1W. Luminus now (2011) produce "Big Chip" LED's which produce even more light by using a larger chip. The SST-50 is 5mm square, and produces about 1200 lm for 20W input, while the larger 9mm square SST-90 produces 2250 lm with an input of 35W. At lower drive currents (the "low power" setting) expect over 100 lm/W. These are already being used in flashlights - about £80 for the SST-50 and £110 or so for the cheapest SST-90 models. Many of these use 18650 batteries which will be hard pressed to deliver sufficient current for maximum light output (the SST-90 needs 9.0 Amps) By comparison our "low energy" light bulbs deliver 60 lm/W, and quartz-halogen headlight bulbs a puny 20 lm/W. (A lumen is the standard unit for measuring total light output, and a candle produces about 13 lumens) These developments mean LED's are now well suited to application for portable lighting. |
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One very advantageous 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 comes in a kit with two batteries and a charger for about £50 delivered. That might sound like a lot but its really good value for money, and easily outperforms my big heavy lantern. review here I got mine from Quality China Goods on Ebay |
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There are some great deals (Ebay) for torches like these with CREE Q4 LED's and including 18650 Lithium AA cells and a charger for £20 inc p&p! Many torches that use 18650 rechargeables also let you use CR123's as spares. It is reported that some manufacturers are using cloned LED's and over-running them to get very high lumens ratings, (over 220) at the expense of their useful life. 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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| This is a "Trustfire X6" flashlight using the new Luminus SST90 LED to produce a claimed max 2000lm with a run time of 90 min. The high power requirement is met by using THREE 18650 batteries. A bit inconvenient when the supplied charger only charges two at a time. To keep the LED cool the torch needs a large finned heat sink. At over a foot long its not very pocketable and weighs almost a kilo. Cost - about £100. This is NOT on my Christmas list! |
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| For hands-free use Noel recommends this Tikka Plus head torch from Petzl. (Amazon.co.uk £24) It offers three light levels and uses 3 AAA batteries. Don't use Lithium cells in this you will blow the LED's! |
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If you want to keep your existing torch but change to using LED's you
can get these Terralux LED bulbs which run from 2 - 9V and give 50
lumens. (£10) Also by Terralux LED converters to change any of the MAGLITE range to use high power LED's Check out http://www.thetorchsite.co.uk/index.html |
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| 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 or even 100W car headlight bulbs ( shown above). The downside here is
their limited burn time of about 20 min. 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 weighs 4kg
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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| This more "sensible" 100,000 cp type rechargeable lantern uses a 6V 4Ah gel cell and 6W flange type xenon lamp to give about 120 lm with a run time of about 4h continuous use. Its still bulky and the advantage of the bigger cell is reduced by the lower efficiency of the lamp as compared with the best LED's but this shape with a top handle is often useful, and it does have the advantage of being easy to charge, coming with both mains and in-car chargers. |
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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 needed. Back in 2007 these were only to dream about($7200!) but prices are coming down, this model is currently £141 with free shipping, and comes with a 6600mAh 12V battery to give 2.5 hours on 35W and 3 hours at 28W setting. It includes 3 1W LEDs which can run for up to 90hours in use!
More sensible ones are also produced using 10W lamps to give about 1000 lumens using a smaller battery which makes for a smaller, lighter torch - but they are dearer because of the bulk demand for car HID's. (£180)
The Great Internet Light Bulb Book http://freespace.virgin.net/tom.baldwin/bulbguide.html