Infrared light is directed at a cuvette containing the sample water. This light is scattered in all directions off the particles in the water. A detector, consisting of a photodiode, is placed at a 90° angle to the light source. The amount of light being scattered directly into the detector is measured in volts and translated into turbidity units. This style of turbidity sensor is called a nephelometer. A standard is used to calibrate the Turbidity Sensor in units of NTU, Nephelometric Turbidity Units. Other units such as JTU (Jackson Turbidity Units), and FTU (Formazin Turbidity Units), have values similar to NTU, but are not exactly the same.