How It Works

Laser-light scattering photometry is a way of measuring particles in air based on the scattering of light by those particles.A thin beam of polarised light is directed into a small area through which air-containing particles passes.

Particles pass through the light beam and scatter the light.

The direction and amount of scattered light is dependent on the wavelength of light and properties of the particles such as their size and shape.

We measure the amount of light at an angle offset to the original beam and the intensity of that scattered light is proportional to the amount of particles in the sampled air.

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The relationship between light and particles was initially established by Gustav Mie (1) in 1908, and is similar to the reason why the sky is blue.

It was not until 1967 this theory was put into practical application with the design and development of an instrument (2) capable of measuring the ‘haze’ present in ambient atmospheres (3) The intensity of scattered light measured at a particular angle is related to the mass of aerosol passing through the light beam. Instruments can be calibrated with a test aerosol with known density, refractive index and size distribution, similar to that expected to be encountered in the field.

This calibration establishes a “response factor” from which instrument parameters are calculated to produce a mass measurement of particles.

Commercially available LLSP instruments are capable of producing measurements of mass each second with data logging capabilities.

Limitations of LLSP instruments are primarily centred on the response of the instrument to a calibrated aerosol.

Significant changes from the calibration aerosol to that encountered in the field may lead to under or over estimation of mass concentrations.

If uncontrolled, humidity and particle contamination can also adversely affect the performance of LLSP instruments.

A review of light scattering photometers for use in measuring reparable dusts in occupational settings was conducted in 1998 by NIOSH (1)

It concluded their primary benefit was in providing real time data, logging capabilities and relative concentration changes.

Further work in the use of LLSP instruments in underground applications (2) showed them to be capable of qualitatively tracking DPM mass, but acknowledged the issues around non-DPM particles and effective calibration using a mine specific aerosol.

[1] Mie, Gustav (1908). “Beiträge zur Optik truber Medien, speziell kolloidaler Metallösungen”. Annalen der Physik 330 (3): 377–445.

[2] Ahlquist, N.C.; Charlson, R.J. A new instrument for measuring the visual quality of air. JAPCA 1967, 17, 467-469.

[3] Charlson, R.J.; Ahlquist, N.C.; Horvath, H.

On the generality of correlation of atmospheric aerosol mass concentration and light scatter.

Atmos. Environ. 1968, 2, 455-464.


Prior to the advent of instruments that could measure particle concentrations in air every minute or every second, air was sampled through various media. Air was pumped through filters or glass bottles containing a liquid for at least 15 minutes and the particles collected for later analysis in a laboratory.

Many methods still exist using these techniques, especially where the particles are difficult to analyse or contain other substances of interest.

This approach generates an “average” concentration over the time of collection, but cannot show the change in concentration over that time period.

The graph shown is a real time measurement of particle concentrations in a mine. The green trace shows the conditions improving over time; the red trace illustrates conditions deteriorating over time.

Both results have the same “average” concentration of 0.148 milligrams per cubic metre of air.

[1] United States National Institute of Occupational Safety and Health (NIOSH) (1998) Aerosol Photometers For Respirable Dust Measurements, Paul A. Baron, Ph.D.

[2] Watts WF, DD Gladis, MF Schumacher, AC Ragatz, DB Kittelson. Evaluation Of A Portable Photometer For Estimating

Diesel Particulate Matter Concentrations In An Underground Limestone Mine Ann. Occup. Hyg., Vol. 54, No. 5, Pp. 566–574, 2010


There are a range of methods and instruments available to measure particles in real time, some are based on the amount of light a particle will absorb, others use the electrical properties a particle to measure its size and quantity, other techniques use the amount of laser light scattered by particles for measurement.

There are sophisticated instruments that weigh a tiny amount of dust deposited on vibrating crystals or measure the slightest change in pressure or temperature as particles are irradiated with lasers to calculate the concentrations of particles in air.

All of these methods are capable of measuring particle concentrations down to very low levels (micrograms per cubic metre of air) (1 microgram = 1 millionth of a gram) and can often “see” particles as small as 20-50nm (nanometers).

Each technique has its pros and cons, but more often it comes down to the devices usability in underground environments and the maintenance requirements needed.