Their low power consumption and long lifetime make them suitable as a light source for a variety of devices from bite alarms to digital cameras. The small amount of circuitry required means that they can be quite compact; in fact the battery power source is what normally defines the devices size.
I've just taken delivery of two torches made by Petzl where the normal bulb/circuitry has been replaced by 3 LEDs - they look superb - They are very light and compact and produce a very bright white light.
The two I have bought are marketed as the Zipka and Tikka and are almost identical in size and weight.
They both require 3 AAA LR03 1.5v batteries as a power source, the anticipated lifetime of these being in excess of 150 hours. The LED lifetime is quoted as several years in the pamphlet, which accompanied the torch.
The 2 torches are dome shaped in appearance (a bit like a mini bivvy). Their bases are about the size of a matchbox, height wise they are approx 35mm (Tikka) and 40mm (Zipka) at their apex.
The Tikka is the true head torch and comes supplied with a strong looking 25mm Headband which is elasticated and also adjustable. The Zipka comes with a self-contained retractable cord and can be worn just about anywhere. It can of course be worn on the head but the retractable cord is not as comfortable as the elasticated one. Although I have not tried yet - it should be able to loop around an umbrella pole and stay in position.
Both torches are incredibly light 64g & 70g (2.2 - 2.4oz) including the weight of the batteries.
The Tikka is the heavier of the two (not that you will notice 6 grams difference!), because the torch is so light you do not need to have the headband very tight around your head. The on/off switch is a ridged sliding switch at the top of the torch, it has a definite latching movement and it should not be possible to turn itself on during transportation, though this does not mean that the torch is awkward to turn on with one hand.
Access to the battery housing is by means of a recessed button. The button requires a modest amount of pressure to open, I found pressing it with a coin made the job easier - not that you should need to replace batteries often!
Once open, the torch is in two parts that are held together by 2 plastic straps that operate as a hinge. The battery recesses have the polarity marked to aid correct insertion. The two pieces fit back together securely but there is no rubber seal between the components so it's possible that water could ingress if submerged.
Reading the accompanying information it would appear that as the batteries are used the beams power degrades slowly. For the 1st 12 hours the beam has a range of 10meters, the next 12hrs at 5meters then 150hrs at 2meters!
The beam output is a very white light (you are advised not to look into the light), I can imagine these torches being the cause of some arguments due to dazzling anglers on the opposite banks if not used with consideration.
The beam does cause a degree of "light fringing" and a soft blue/purple glow may be noticed towards the edges of the main beam. (Similar to the effect of some modern car headlights)
A possible nice touch is the availability of different coloured lenses, The torches come with the normal transparent lens but a green and red lens is available from Petzl. The red lens may help reduce "torch rage" by allowing fish to be landed under a less intrusive light source when necessary.
These torches are of course designed for outdoor use - the Zipka is not recommended for sandy/muddy environments as this could clog the retractable cord mechanism. The instructions do recommend that if used in a damp or humid environment (or submerged!) then the batteries are removed & the torch allowed to dry out.
In comparison to a normal head torch these are slightly more expensive to purchase, although running costs should be minimal - one set of batteries per season.
I have seen these very torches appearing on the shelves on my local tackle shops, however I purchased mine online from Camping accessories shop Adventure Centre Limited http://www.cheaptents.com/ and saved myself nearly £10 per torch!
The Tikka retailing here at £21.25 & Zipka £25.50 (both prices include VAT - postage was a hefty £5 though)
Delivery took 5 days.