Turbidity is the cloudiness or haziness of a fluid caused by individual particles ( suspended solids ) that are generally invisible to the naked eye , similar to smoke in air. The measurement of turbidity is a key test of water quality .
Turbidity measured this way uses an instrument called a nephelometer with the detector setup to the side of the light beam. More light reaches the detector if there is less particle in a sample. If there are lots of small particles, light beam will be scattered. So it will reach the detector in a less intensity. The units of turbidity from a calibrated nephelometer are called Nephelometric Turbidity Units (NTU).