The present invention relates to a method and apparatus for using an imaging detector to image an optical pattern created by illumination of a viscous material that is applied to the skin. The viscous material forms a detectable pattern that can be imaged by the imaging detector. In some examples, the viscous material is reflective and allows reflection from an illumination source to create an optical pattern on the imaging detector. In other cases, the variation in thickness of an absorbing or scattering viscous substance can be imaged by the detector. Polarized illumination and detection can be used to enhance the response.