Found some interesting details on
how light works in Minecraft.
Light on any given square is a number from 0 to 15, with 15 the highest and equivalent to direct sunlight. For every square adjacent to that square, light is one less, unless superseded by a higher light source. Each light value is 80% of the value above, such that for example 14 is 80% of direct sunlight and 13 is 64% of direct sunlight.
Fire, lanterns, lava and lightstone emit 15 light in their own square and 14 in every adjacent square. Torches emit 14 light in their own square. Portals emit 11 and redstone torches 7. Moonlight is 4.
At 0-7 light (16.78% sunlight or less), monsters will spawn. Saplings and flowers will not stay planted (night is ignored for this purpose). This is seven squares or more from a torch, or eight or more from a lantern.
At 9+ light (26.21% sunlight or greater), friendly animals will spawn on grass. This is within five squares of a torch or six of a lantern.
At 10+ light (32.77% sunlight or greater), snow and ice won't form. At 12+ (51.20 sunlight or greater), snow and ice will melt. Sunlight is ignored.
At 13+ light (64% sunlight or greater), trees will grow. A sapling needs a torch adjacent to grow underground.