Boktai: The Sun Is in Your Hand (ボクらの太陽, Bokura no Taiyō?, "Our Sun" in Japan), is a video game released on the Game Boy Advance in 2003 by Konami. Produced by Hideo Kojima, Boktai's game cartridge includes a photometric light sensor that measures the amount of sunlight/uv rays exposed to it.
The player takes the role of Django, a vampire hunter, who uses a weapon called the "Gun Del Sol" (Solar Gun), that fires bolts of sunlight at enemies. In order to charge the weapon, the player must actually take the game outside in the daytime (as verified by the light sensor). If the player's gun runs out of light reserves and there is no sunlight available, then the player must have to avoid enemies.
Source: Wikipedia, Boktai: The Sun Is in Your Hand , available under the CC-BY-SA License.