It consists of three zones. The signal zone allows light to pass through unaltered. The dark zone allows no light to pass, and is used to measure the background noise of the instrument. The chopper rotates 23 times per second. The property that the nephelometer measures is the extinction of light over a distance.
Extinction is caused by both scattering and absorption of light by particles.
They are each divided into two components. The nephelometer measures b scat and b rg and subtracts to get b sp . The nephelometer counts photons using the photomultiplier tubes. The photon counts are converted to counting frequencies and then scattering coefficients usuing calibration constants. This is desribed in detail in the 'Signal Processing' section.