Dissolved Oxygen
Measuring Principle (Optical Determination)
Optical oxygen sensors are based on the very precise luminescence technology. While electrochemical sensors detect oxygen reduction at the electrode surface, the optical method is based on energy transfer between an excited luminophore and oxygen. 
What is dissolved oxygen?
Dissolved oxygen (DO) has the same structure as the oxygen we breathe in. The simple difference is that it is dissolved in water. This means that free oxygen molecules are available to be consumed by living organisms and play a very important role in many chemical processes.