Light Propagation in Tissue

i just want to simulate the propagation of monochromatic light (800nm) in a 3 Layer tissue model 3D.

Does anyone know a working Comsol solution?

Problem:

to many elements with this wavelength.

Setup:

3 Layer Model --> Skin, Bone, Blood

Source: 6mW Power, 800 nm wavelength

Detector: 2mm^2 photodiode

Question: intensity of the reflected light at the detector.

The detector ist in the same plane at the skin surface.