I mine on bedrock level with long 2x1 corridors, and place torches 14 blocks apart (13 blocks in between the 2 torches). From what i've read, torches produce 15 light, and the light degrades by 1 step per block. Monsters are only supposed to spawn in light of 7 and less. When you place torches 15 blocks apart, the light in the middle should give a light level of 8, preventing monsters from spawning. This doesn't work.