Jacques Steven L, Ramella-Roman Jessica C, Lee Ken
Oregon Health and Science University, Department of Dermatology, Portland, Oregon and Providence St. Vincent Medical Center, Oregon Medical Laser Center, Portland, Oregon, USA.
J Biomed Opt. 2002 Jul;7(3):329-40. doi: 10.1117/1.1484498.
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.
照射皮肤的线偏振光会被表层反向散射，并被双折射胶原纤维迅速去偏振。将这种表面反向散射光与主要由深入真皮的光主导的全漫反射光区分开来是有可能的。该方法包括通过相机前的分析线性偏振器获取两幅图像，一幅图像（I(par)）是在分析器与照明偏振方向平行时获取的，另一幅图像（I(per)）是在分析器与照明方向垂直时获取的。创建基于偏振比的图像，即Pol=(I(par)-I(per))/(I(par)+I(per))。在一项使用非相干可见光谱光的初步临床研究中，本文比较了以I(per)表示的正常光图像和各种皮肤病变的Pol图像。图像包括色素沉着皮肤部位（雀斑、纹身、色素痣）和非色素沉着皮肤部位[非色素性皮内痣、神经纤维瘤、光化性角化病、恶性基底细胞癌、鳞状细胞癌、血管异常（静脉湖）、烧伤瘢痕]。从剃须刀片投射到前臂皮肤部位的阴影图像说明了阴影边缘附近Pol值的行为。在阴影边缘附近，Pol值大约翻倍，因为阴影区域没有将I(per)光子表面散射到阴影边缘像素中，而I(par)光子是从这些像素的表层直接反向散射的。这一结果表明，皮肤中Pol像素间串扰的点扩散函数半高宽约为390微米。
