A turbidimeter is able to quantify clarity of a sample by passing light through it. 

loading a sample into the turbidimeter

Turbidity standards

Light passes through a sample to measure turbidity

A sample vial is carefully cleaned and loaded into the turbidimeter

Turbidity standards show the visual range of clarity with the associated NTU value
Once in the turbidimeter light is passed through the sample - light passes more directly through a clear sample, while light is refracted at varying levels through more turbid samples.

*NTU = Nephelometric Turbidity Units