nephelometer

A nephelometer is an instrument that measures the intensity of light scattered by a sample as a function of angle, typically to determine turbidity or particle concentration. It embodies the principle of nephelometry, which analyzes scattered light from suspensions or aerosols, and is distinct from devices that assess transmitted light.

In operation, a collimated light source such as a laser or LED illuminates the sample. A photodetector positioned at a fixed scattering angle collects the light that has been scattered by particles in the sample. The detected signal, after appropriate calibration, is related to particle concentration and the optical properties of the suspension. In liquids, measurements are commonly made at or near 90 degrees to minimize unscattered light.

There are two main categories of nephelometers. Liquid nephelometers are used to assess turbidity in water and process streams, typically reporting turbidity in nephelometric turbidity units (NTU) or Formazin Nephelometric Units (FNU). Aerosol nephelometers are used in atmospheric science to determine the scattering coefficient of airborne particles, often reporting b_scat at visible wavelengths and sometimes at multiple angles.

Calibration and standards are essential for quantitative results. Formazin-based standards are commonly used to establish NTU/FNU scales in liquids, while aerosol instruments are calibrated against reference aerosols and may require corrections for wavelength, angle, and sample composition.

Applications include drinking water and wastewater monitoring, beverage and pharmaceutical quality control, industrial process monitoring, and atmospheric science. Limitations include sensitivity to sample color, multiple scattering at high turbidity, and dependence on particle size distribution and refractive index.