Note, that when the duct cross-section is circular, D h becomes the diameter of that circle. Since you know the dimensions of the experimental apparatus, you can deduce the relationship between pixels and millimeters. We need the value of this expression for the chosen vertical cross-section. The temperature T b is unknown. But, if we encouch T m in terms of T b , T b will disappear from the problem.
T m is the mean temperature along the vertical cross-section.