OCT utilizes low coherence of broadband light to suppress background noise. It obtains resolution which is 1~2 magnitudes higher than conventional ultrasound imaging (ym), and very high signal-to-noise ratio (over 100 dB). OCT employs near-inferred light source. It does no harm to human body. Both transparent and nontransparent tissue can be imaged. It is non-contacting and non-invasive. In vivo test under natural condition can be done and test result is unrelated to thermal and mechanical properties of the tested object. Systems for OCT are usually compact and easy-to-operate , which makes this technique especially promising for clinical application.