So, check the chunk first for torches, glowstone, sky, whatever gives off a light source. Give those blocks their respective light levels. Then you want to make the blocks around that light source have a decreasing light value, as seen below.

Air = a, Torch = t, Number is light level (this chart might be a little hard on the eyes)
a,4 | a,5 | a,5 | a,5 | a,4
----+-----+-----+-----+-----
a,5 | a,5 | a,6 | a,5 | a,5
----+-----+-----+-----+-----
a,5 | a,6 | t,7 | a,6 | a,5
----+-----+-----+-----+-----
a,5 | a,5 | a,6 | a,5 | a,5
----+-----+-----+-----+-----
a,4 | a,5 | a,5 | a,5 | a,4

Now, make it so a solid block (like stone) stops light completely. (s = Stone)
a,4 | a,5 | a,5 | a,5 | a,4
----+-----+-----+-----+-----
a,5 | a,5 | a,6 | a,5 | a,5
----+-----+-----+-----+-----
a,5 | a,6 | t,7 | a,6 | a,5
----+-----+-----+-----+-----
s,0 | s,0 | s,0 | s,0 | s,0
----+-----+-----+-----+-----
a,0 | a,0 | a,0 | a,0 | a,0

This essentially means that the light level decreases by one for each block away from a light source.
t,7 | a,6 | a,5 | a,4 | a,3 | a,2 | a,1 | a,0

It also means, when a person breaks a block, you only have to check whether the sky is now shining on it, or if there are light sources within 15 blocks of it, then adjust the block's (air) light level accordingly.

Of course, I could be rambling nonsense, I don't know much about 3d =)