I ran across a bit touting some new lamp, called GravityLight, which would give you “free” lighting — all you have to do is lift a weight. My first reaction was a flashback to the Gravia lamp, which I critiqued in the first few months of this blog. Turns out the Gravia was a concept piece that had over-reached the light output and duration that was possible — it was only capable of putting out a few tens of milliWatts to generate the light, well short of what was promised.

But the GravityLight appears to be the real deal, and it’s being funded on indiegogo as a replacement for kerosene lamps in areas without a power grid. The difference? It promises less light over a shorter time, plus a slight increase in efficiency of LEDs, and that’s all the difference you need. Instead of 800 lumens of light, sent in all directions, you get a beam from the LEDs. If you drop the output from 4*pi steradians (i.e. all directions) to, say, 1/8 of that, it’s the equivalent of just 100 lumens, and super-bright LEDs have a luminous efficacy of about 100 lumens per Watt, meaning you only need ~1 Watt output for such a light.

The previous system could only supply ~17 milliWatts of power, but that was because the energy was promised over a 4-hour period. If you release the same energy over a shorter time, the power is greater. A 20kg mass raised a little over a meter is 200 Joules, so that’s a Watt for 200 seconds. If you lower the light output, via dimming or making a narrower beam, or you use a greater mass and/or raise it higher, it will last longer.