Imaging skin pathology with polarized light

Linearly polarized light that illuminates skin is backscattered by superficial layers and rapidly depolarized by birefringent collagen fibers. It is possible to distinguish such superficially backscattered light from the total diffusely reflected light that is dominated by light penetrating deeply into the dermis. The method involves acquisition of two images through an analyzing linear polarizer in front of the camera, one image (I(par)) acquired with the analyzer oriented parallel to the polarization of illumination and one image (I(per)) acquired with the analyzer oriented perpendicular to the illumination. An image based on the polarization ratio, Pol=(I(par)-I(per))/(I(par)+I(per)), is created. This paper compares normal light images, represented by I(per), and Pol images of various skin pathologies in a pilot clinical study using incoherent visible-spectrum light. Images include pigmented skin sites (freckle, tattoo, pigmented nevi) and unpigmented skin sites [nonpigmented intradermal nevus, neurofibroma, actinic keratosis, malignant basal cell carcinoma, squamous cell carcinoma, vascular abnormality (venous lake), burn scar]. Images of a shadow cast from a razor blade onto a forearm skin site illustrate the behavior of Pol values near the shadow edge. Near the shadow edge, Pol approximately doubles in value because no I(per) photons are superficially scattered into the shadow-edge pixels by the shadow region while I(par) photons are directly backscattered from the superficial layer of these pixels. This result suggests that the point spread function in skin for cross-talk between Pol pixels has a half-width-half-max of about 390 microm.