[AIVC, Air infiltration and ventilation glossary, 1992]:solid particles of aerosol or bubbles used as a tracer for measuring the rate of air movement. These particles usually have diameters of 2 to 3 microns, and can be detected by using either a) a fluorescent light scattering detector; b) a photomultiplier (P-M) detector, or c)a phosphorescence with a P-M detector.