Red and near-infrared light with wavelengths between 700 and 1550 nanometers can penetrate a few millimeters into the human body before it is either absorbed by the tissue or scattered (that is, reflected in a different direction). Scattering occurs at locations in the tissue where boundaries between materials, such as vessel walls, give rise to a change in the index of refraction. As a result, some of this light is backscattered (reflected 180°) and exits the tissue in the same place it entered. Optical coherence tomography (OCT) is a system for delivering light and collecting backscattered light to produce images with micrometer resolution.