Fig. 2　Blood-flow Sensor Device. A. Picture of the Bloodflow Sensor. This sensor measures 1 mm in height, 1.6 mm in length, and 3.2 mm in width. B. Principle of the Bloodflow Measurement. The device detects optical beat signals by analyzing the interference between reflected skin light (orange) and doppler-shifted light from red blood cells (green). RBC: Red Blood Cell