We propose to compute the joint histogram of irradiances at neighboring pixels, which counts how many times do a pair of irradiance values occur at neighboring pixels. We have found that the joint histograms are very similar across camera models, especially when computed at the extreme lens setting -- smallest focal length and largest f-number. So as a prior, we use the joint histograms computed from photographs from a single camera model. We use this prior to estimate the response function of specific camera models.
Here we show the log of the joint histograms of (a) red, (b) green, and (c) blue irradiances computed from 15,550 photographs captured by Canon S1IS cameras with the extreme lens setting -- smallest focal length (5.8 mm) and largest f-number (4.5). The inverse camera response functions used were normalized so that irradiance values were in the range (0,255).