There are a couple apps that purport to be lightmeters. I tried one, but it didn't even register any reading at all, but got me an error message claiming "the lightmeter timed out"... Whatever that means.
To be sure: it was a beta, and the developer went to all sorts of pains during installation to disclaim any liability for the app not working. He was prescient.
But even if the software would work, I doubt whether a phone has a sensitive and sophisticated enough light sensor. The prerequisite hardware.