The Broadsensor optical DO sensor is coated with a luminescent material. Blue Light from an LED strikes the luminescent chemical on the sensor. The luminescent chemical instantly becomes excited and then as the excited chemical relaxes, it emit red light. The red light is detected by a photo diode. The time it takes for the chemical to return to a relaxed state is measured by referring an independent red led light. Oxygen has a dampening effect on this reaction. The oxygen concentration is proportional to the time it takes for the luminescent material to return to a relaxed state.