Normalizing an imaage


Im going over this paper by Vogel A Direct Numerical Approach to Perspective Shape-from-Shading. I have a problem in the way they define the normalized image brightness I(x) as:

I(x) = E(x)/sigma

where sigma “depends on the albedo of the surface and the intensity of the light source”.

I am estimating the albedo with the formula given here. As the albedo is in the range of [0, 1], and the max light intensity is 255, I’m guessing that sigma must be atmost 255. However, in the paper the authors have taken a value of 1000.

It would be great if someone could explain why, and where I’m wrong.