Fast Classification of Dust Particles from Shadows

A fast and versatile method for classifying dust particles dispersed in the air is presented. The method uses images captured by a simple imaging system composed of a photographic sensor array and of an illuminating source. Such a device is exposed to free particulate deposition from the environment, and its accumulation is measured by observing the shadows of the particles the air casts onto the photographic sensor. Particles are detected and classified in order to measure their density and to analyse their composition. To this purpose, the contour paths of particle shadows are traced. Then, distinctive features of single particles, such as dimension and morphology, are extracted by looking at corresponding features of the sequence of local orientation changes of contours. Discrimination between dust and fibre particles is efficiently done using the varimax norm of these orientation changes. It is shown through field examples that such a technique is very well suited for quantitative and qualitative dust analysis in real environments.