Io = illumination light
Tmel = in/out transmission through superficial melanin pigment
surface = reflectance of polarized light from superficial tissues
deep = reflectance of randomized light from dermis
The math
The depth of POL images varies from about 300 mm for skin, to 500 mm  for muscle, to 1.2 mm for liver, and depends on the birefringence of the tissue because birefringence scrambles linearly polarized light.