A nephelometer, also known as an aerosol photometer, is a device that measures the concentration of colloidal matter in a liquid or gas. Learn in detail about the nephelometer and its industrial applications.

A nephelometer is used for continuous analysis of air and water pollution. Nephelometry studies the level of pollution in water, as well as smoke and dust in the air. Nephelometry also assists in making food and beverages and in measuring the molar weights of high polymers that fall to the ground from industries. 

The intensity of a light flux dispersed by solid particles suspended in a fluid is measured using a nephelometer. The nephelometer monitors the weakening of a light flux when it goes through a solution containing particles in a suspension. The intensity lowers due to light absorption and dispersion.

Principle and Theory of Nephelometers  

The working of a nephelometer is based on the scattering of colloidal particles, absorption spectra scattering or absorption spectra of colloidal particles in suspended solutions. Some incident light is lost because of absorbance, reflections, and interactions, while the rest is transmitted.  Soluble molecules are tiny and light, and they disperse into solvents when exposed to light. On the other hand, residual and complex compounds are much larger and produce a forward-angled dispersion. The purpose of nephelometric detection is to quantify the forward scattering of incident light.

Instrumentation of a Nephelometer 

A nephelometer’s instrumentation is as follows: 

1.  Source:

Monochromatic radiation is used in nephelometers. A mercury arc with special filter configurations separating one of its emission lines is typically the best choice. A tungsten lamp is used to determine the unknown concentration of a particular medication. In this case, the blue spectral region delivers the most significant results.

2. Cell:

The study is usually conducted in a rectangular cross-section cell. Existing beams can also be entered and passed using cylindrical cells with flat sides. Due to the octagonal faces, measurements may be conducted at angles of 135°, 90°, 45°, or 0° to the main beam. It is forbidden for light beams to pass through the walls. 

3. Detectors:

Photo-multiplier tubes are used as detectors in nephelometers because the intensity of scattered radiation is typically low. Usually, the detector is angled at 90 degrees to the main beam. However, the detector angle changes according to light for optimum sensitivity. The detector on certain nephelometers is mounted on a circular disc, allowing observations at different angles ranging from 0° to 135°.

Experimental Techniques of Nephelometers

Intense light from an electric lamp falls on a glass plate after passing through a filter only when the instrument is being used for luminescence research. Some of the beams are deflected and land on the glass attenuator, while others penetrate the cell holding the solution being studied.

The light beam going through the cell has now been quenched in the light trap. The light flux reflected by particles in the solution passes via a lens, a customisable diaphragm, and a filter before being directed into the eyepiece by a rhombic prism. It lights just one-half of the optical field.

The light flux from the attenuator travels through the lens, adjustable diaphragm, lens, rhombic prism, and filter similarly. It passes through the eyepiece and illuminates the remaining half of the optical field. The light fluxes may be equalised by altering the slit width of the adjustable diaphragm, achieving optimal equilibrium.

The study solution should be placed in the cell, the dials of the adjustable diaphragm should be set to zero, and the intensities of light fluxes should be made close by inserting replaceable attenuators while utilising this instrument. Adjustable diaphragms are then used to balance the light fluxes.

These tests are carried out on a series of solutions containing the exact quantities of the investigated substance. The adjustable diaphragm is correlated to the solution concentration. This calibration curve from the diaphragm readings may be used to compute the unknown concentration.


The formula for determining the concentration of metals or ions in a colloidal state is:

C1 = C0 (L1/L2)

Where C0 and C1 are standard and unknown solutions concentrations, and L1 and L2 are the layer thicknesses in nephelometer cells.

Uses of Nephelometers 

The significant uses of nephelometers are measuring air quality for pollution monitoring, climate monitoring, and visibility. Besides this, nephelometers have many applications in medicinal chemistry for drug analysis.

Sheath air is clean and filtered air surrounding the aerosol flow to prevent particles from circulating the optic chamber. Before beginning the sample, the nephelometer generates sheath air by passing air through a zero filter.

Nephelometers are also employed in global warming research to determine the global radiation balance. The quantity of solar energy reflected into space by dust and particulate matter may be measured using nephelometers equipped with backscatter shutters.

Nephelometer also helps in many chemistry fields like inorganic analysis, drug analysis etc.

It helps in the study of water and air pollution.


A nephelometer is a device used to determine the concentration of colloidal particles in a liquid or gas colloid. It is based on the suspended phase’s forward scattering of incident radiation. Three different parts make up a complete nephelometer machine. The nephelometer is used in air quality measurement and pharmaceutical companies for drug analysis, quantitative analysis, and identification of various colloidal compounds in the chemicals sector.