SRD:Control Light

By manipulating the ambient light level, you can decrease or increase the illumination of an area. The change in illumination can be gradual (taking as long as 1 minute) or sudden (occurring immediately when you manifest this power). You can alter the level of illumination from its original level at any time during the power’s duration.

Decrease: You can decrease the illumination of an area by as little as 5% (barely perceptible) or as much as 100% (total darkness). If you decrease the light by 50% or more, the visual ability of creatures that depend on light to see declines accordingly.If you decrease the ambient light in an area by 100%, even those with lowlight vision are unable to see within the affected area. For each 25% decrease in ambient light, characters in the area gain a cumulative +1 circumstance bonus on Hide checks (to a maximum of +4 when all the light is gone).

Increase: You can increase the illumination of an area by as little as 5% (barely perceptible) or as much as 100%. If you increase the light by 50% or more, the visual ability of creatures that depend on light to see improves accordingly.

You can use this power to increase the illumination of an area by 200% (improving visual abilities accordingly), but in such a case the power’s duration is only 1 round.