The photometer used in our Topas, Osiris and Dustmate instruments gives a continuous and simultaneous indication of the PM1, PM2.5, PM10 and TSP mass fractions. The light scattered by airborne particles can be thought of as consisting of three components. Light reflected from the surface of the particle, light refracted through the particle and light which is diffracted from its original path by the presence of the particle. The intensity of the light scattered by reflection or refraction strongly depends on

the type of particle. Thus a white limestone particle will reflect much more light than a black diesel fume particle of the same size. On the other hand the diffracted component depends only on the size of the particle and is independent of its material composition.

For irregularly shaped particles, light which is reflected and refracted tends to be scattered over all possible directions. The diffracted component, however, tends to be scattered only through very small angles. For example, for a 5 micron diameter particle, 90% of the diffracted light is scattered by less than 10 degrees from the original direction of the light beam.

Turnkey's photometers analyse only the light scattered through 10 degrees or less. That is they respond only to the diffracted component and have a virtually constant response whether the particles are black or white. Other commercially available photometers detect light scattered through much wider angles or even at 90 degrees to the light beam.

Vrins Luchtonderzoek has an Osisris sampler available for rent and also performs windtunnel calibrations.
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