Polarized light reflectance and the sub-diffuse regime during optical imaging of skin

Polarized light imaging can gate photons reflected from a tissue such that superficially scattered photons from the sub-diffuse regime are distinguished from multiply scattered photons from the diffuse regime. Such polarization- gated sub-diffuse images can restrict the image contrast to the superficial epithelial (or epidermal) layer of a tissue where pathology often originates, and avoid being blinded by the much larger amount of diffuse reflectance from deeper tissue. A pair of co-polarized (HH) and cross-polarized (HV) images of a tissue are acquired and the difference image HQ = HH-HV subtracts the depolarized photons from the diffuse regime and yields an image using only the partially polarized photons from the superficial sub-diffuse regime. Using polarized Monte Carlo simulations (0.5-μm-dia. microspheres in water at 633 nm wavelength), this paper addresses the question, “How many scattering events are involved in the HQ image?” The simulations show that HQ(n) decays to HQ(1)e−1 or 0.37HQ(1) after 6 scatterings. Images with a polarization camera were acquired on reflectance standards and on skin of the cheek using a blue LED (405 nm) for illumination. Analysis yielded the total diffuse reflectance (Rd = 0.419), the sub-diffuse-regime reflectance (S = 0.013), and the diffuse-regime reflectance (D = 0.406).