How do you measure Albedo?

Albedo is the ratio of reflected light/ incident light so to measure it one would have to measure these quantities. One way would be to use a light sensor or a light meter but I was wondering if it was possible to get some idea of albedo from a digital photo. After all the photo is simply a record of the reflected light so it should contain all the information. I use the programme paint.NET for manipulating images (it's free) so thought maybe I'd try using this.

Under the windows menu there is the colours option which opens a pallet. Click "more" and you get a display of the amount of Red Green and Blue in the colour as well as the HSV numbers. It is the V number (value) that we are interested in. This is related to the lightness of the colour, which is a measure of the amount of reflected light in a photograph. Under the tool options there is a colour picker that looks like a pipette, this can be used to sample the different areas in a photo. So If you open a photograph in Paint.NET you can get an idea of the amount of light reflected off different areas by sampling the colour and noting the V number. Perfectly white would have a V number of 100, so dividing the V number by 100 gives an idea of the albedo. it doesn't give the actual value since the V number is not proportional to the intensity of the reflected light but it gets the idea across. You have also got to be careful with shadows.