[kamocat]

If you have the LabVIEW toolkit for NXT, it shouldn't be hard to make a data logging application.
I think the voltage out from the light sensor is directly proportional to the intensity of light in, but I wouldn't count on it. (I don't even know whether that's on a logarithmic scale or not) I do know that the value back from the "light sensor" VI is directly proportional to the voltage in.
Your best option really would be to make a jig and do repeatable tests.
(If you want to do some advanced stuff, you could even use the Direct Control VIs)