Minecraft wiki entry for History, associated text: The next 16,384 bytes contains the blocklight data. The format is identical to the previous skylight format. If a blocklight value is f, that means the block is emitting the max amount of light (like glowstone). The less light a block emits, the lower the value (like powered redstone). And when the blocklight value is 0, no light is being emitted from that block. Because the game automatically generates lighting, it is unimportant to modify this part of the file when generating a world via an outside program.