Optical DO Sensors emit blue light from an LED, which serves to agitate certain luminescent dyes. A photodetector measures the luminescence after certain processes take place, and calculates the concentration of dissolved oxygen. Optical DO Sensors are effective at measuring low concentrations of dissolved oxygen and require minimal maintenance. They only need to be calibrated every couple of months. However, they do take more power than electrochemical sensors and they take longer to acquire a reading.