Can I measure light intensity using only .jpg pictures? I have two pictures. On first one can see a lamp shining straight into my smartphone's objective and on the second picture one can see the same lamp but covered with a piece of paper.
Can I use those two pictures to measure the absorption coefficient of the paper? Does it need sophisticated software, or simple graphics software is enough (eg. GIMP).
 A: No, you almost certainly can't get a useful brightness measurement this way.
The main reason is that your phone camera will adjust its exposure depending on the amount of light in the scene. So if you shine a bright light at it in one photo, and a dim light at it in another photo, it will adjust the photos so that both lights appear "medium bright" in the stored image.
Furthermore, it's very common when photographing a bright source with a dark background for the bright area of the scene to be fully saturated (pure white pixels) so that variations in brightness from pixel to pixel are lost.  
Conceivably you could get some information by reading the metadata in the image file to see what exposure it used in each case. So then you could guess that if the bright source image used a 1/1000 s "shutter speed" (actually just the sensor accumulation time) and the dim source image used 1/250 s, then you might guess that the bright source is about 4x brighter than the dim source. But the shutter speed is likely to depend on a lot of other details in the scene so that this measurement is likely to be so crude as to be nearly useless.
