Light intensity is heavily attenuated in tissue and falls off exponentially from the illumination point. The maximum achievable probing depth of NIRS is limited by the illumination strength – determined by the thermal damaging threshold – and the detection sensitivity. Imaging depth strongly depends on the tissue type and the application. Typical achievable transmission limits of NIRS are about 12 cm for breast tissue, and 6 cm on the arm or leg. For brain imaging, the probing depth of NIRS is about 3 cm.