There are marking on the bag in fluid ounces and litres showing a maximum of three litres can be held comfortably, with room to fold over the top. Given that it folds up very small - to the size of a pack of cards, and weighs 40g - then it could be a very useful addition when water is scarce. The bag is rounded at the bottom, and necessarily flat across the seal, which also means it pours very well - even well full.

The plastic material used, is significantly thinner than many canoeing dry bags, but has some give, and shouldn't rip if used for a stuff sack for clothing (no tent pegs).

Now for the clever bit. The material is fairly transparent and designed expressly to diffuse light shone through it, from the outside, to create a lamp. The Ambient Dry Bag is sold alone, but is also bundled with the Mammut T-Trail headlamp (for a cheaper combination price). The Ambient Light Dry Bag is designed to fit most Mammut headlamps - those with the thick straps. They attach to a handle on the outside of the bottom of the bag, the light shining toward the top.

I also tried it with my Petzl Zipka plus headlamp, with a thin retractable cord, and although it didn' fit as well, shone the light through just as well. There's probably no need to buy the T-Trail light if you already have a headlamp - but its probably worth taking it to the shop and testing it.

So, how well does it work. Once attached, you'll need to blow up the bag and fold and seal the bag quickly. Once lit, it provides a warm glow for cooking, reading and, as Mammut explain, 'a romantic ambience of an evening under canvas'. (Sex sells, even when we're talking about dry bags. Apparently.) Although light enough to read with, it is significantly dimmer. Where it works very well is in a larger, tent with more people - to play cards by for example - or for cooking.

With water in it, it works even better, refracting even more light - enough to almost make it worth while adding a bit, but full it would be much too heavy to hang it from the roof.

SRP:  £45 [Bag on its own is £15; T-Trail head torch on its own £35].

Mammut’s Ambient Light Drybag is available from outdoor retailers nationwide.

Call 01625 508218 or visit www.mammut.ch