We're surprised it hasn't happened before, but now it has: UK light supremos Exposure have been working on a dynamo-powered system and they're showing the prototype at the London Bike Show at ExCel this weekend.

Obviously Exposure have a long and illustrious history of creating very good battery-powered bike lights; we've recently reviewed the Diablo, Joystick and Strada and all three received high praise. Now they're applying the same principles to a dynamo system running from a front hub.

The hub itself isn't the work of Exposure, they've sourced it from a company called Shutter Precision and they're using the ultra-small PD-8 disc-compatible model. Exposure didn't have a weight for the hub but it's listed variously as between 360g and 410g depending on where you look. Anyway, it's not heavy, and Exposure reckon it's about as efficient as they come. It wouldn't be unfair to say it bears more than a passing resemblance to the Supernova Infinity S.

The light itself is yet to be finalised. For the show Exposure had the gubbins running inside a Toro body but, since the light won't have a big battery, the actual casing won't be anything like as large as that. The dynamo system uses a 3-LED setup, of which two (lower power) LEDs remain on when you're at a standstill thanks to a backup battery within the light body. You can expect the production light to be about half the length of the Toro to accommodate the emitters and the battery, which will power the light for up to ten minutes at about half power, which will be plenty to read your map at a midnight junction or fix a puncture on a pitch-black trail.

Claimed output for the dynamo system is 450 lumens, and Exposure reckon you get all that power, flicker free, at a speed of only 5mph. They're aiming at both road and off-road markets and will offer the hub in 26", 29" and 700c wheel builds as well as separately. Design and beam patterns are still to be finalised and we're not sure whether there'll be different builds of the light to cater for the different beam-pattern needs of MTB marathoners and night-owl roadies, although we'd certainly expect that to be the case given their attention to detail through the rest of the range. They couldn't tell us how much it would cost, either, or exactly when it'll be available. But rest assured: it's coming.