Light function.

Any suggestions for a light function, in terms of [(distance to point)^2] and [intensity]? I''m using the [(distance to point)^2] as a square value to avoid expensive square roots. I have come up with 2 possibles: 1) [intensity]/[(distance to point)^2] 2) ([intensity]^2-[(distance to point)^2])/([intensity]^2) I''m currently working on a point light source, not a spotlight or similar. Any suggestions on which is better, or any other (resonably fast) functions that are more realistic?