Normal human knee articular cartilage. The optical coherence tomography (OCT) image (a) of the cartilage is relatively thick and uniform. The pronounced banding pattern on the OCT image is due to the birefringence of the highly organized structure of the collagen (red arrows). The alternating maximum and minimum intensities are due to changes in back scattering as light travels through the tissue while the plane of light polarization rotates. Previous work has shown that it is due to the presence of organized collagen that alters the polarization state of the light. Note: darker gray scale represents higher-intensity back scattering. The corresponding histology is shown in (b).