If you don't play in the sun, your weapons will be weak, or possibly ineffective.
The light sensor could be a solar cell. Most likely you've seen solar cells in calculators. Basically they work by converting light into an electrical current. Any light source can produce energy for the cells, but even on a cloudy day, the light from the sun is much greater than that of a lightbulb.
It's a really interesting concept, and if the game is good, it could become a sleeper. Will i overcome my hate of the sun to experience this game? Probably.