I have a photograph in raw format taken by a Nikon D-90 camera of a white LED spotlight shining on a wall in an otherwise dark room. I need to compute the relative luminous intensity of the bright center of the spotlight which is much brighter than the diffuse area around it. The reason is that I am trying to compute the maximum candelas of this spotlight from its lumens rating for that bright center area. I would like to know what percentage of the total light emitted (530 lumens) is going to the bright spot in the center since from that I can compute the candelas by dividing the fraction of the lumens that go to the center by the steradians of the center bright spot. I have Photoshop CS5 extended to examine the photograph. How can I get an approximation of the luminosity ratio of the brightness level in the center to the brightness of the whole area illuminated to compute this?
Will the RGB values of the photograph be a linear scale or some kind or logarithmic scale? If I can figure the relative brightness at a particular location, I could use the area that has that brightness level