Is there a way to use camera (web camera or phone camera) as a light meter? Phone camera can grab image and get data in RGB format, but has anyone tried to determine illuminance value (in lux)?
I am aware that there is no formula to do this (correct me if I am wrong), but is it possible to model it, and get an approximate value of illuminance?
The problem you would have, as all reflective light meters do, is that you can't tell the difference between a bright light (high lux) shining on a dark object and a dim light (low lux) shining on a light object. In other words, you get luminance, not illumination.
The second problem is that you need to know the amplifier gain, exposure settings (f-stop, exposure time), transfer function (gamma), and other information to be able to get a correct photometric interpretation of the pixels in the image. If you have that information, though, you can just ignore the pixel data and just use the exposure information as a light meter.
The EXIF data in an image will tell you the exposure values which you can add up to get the luminance. See http://en.wikipedia.org/wiki/APEX_system for more details.