title: Determine illuminance from RGB image
body: <p>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 <a href="http://en.wikipedia.org/wiki/Illuminance" rel="nofollow noreferrer">illuminance</a> value (in <a href="http://en.wikipedia.org/wiki/Lux" rel="nofollow noreferrer">lux</a>)?</p>
  
  <p>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?</p>