These instruments would extract quantitative data from images of particles.

Optoelectronic instruments for real-time, in situ monitoring of particle fallout are undergoing development. Settings in which these instruments could prove useful include clean rooms for assembly of optical and electronic equipment, food-packaging facilities, and other industrial facilities in which one seeks to prevent contamination of products by airborne dust and fibers.

Heretofore, it has been common practice in particle-fallout monitoring to place initially clean witness plates in the affected work areas, expose them for suitable amounts of time, then take them to laboratories for analysis. Among the disadvantages of this practice are that it does not provide data in real time, and handling of the witness plates can alter the particle samples prior to analysis. Some optoelectronic instruments for real-time and post-exposure analysis of particle fallout have been developed previously, but none has offered the combination of features afforded by the present developmental instruments; namely, real-time operation, imaging of individual particles, and quantitative information on numbers and dimensions of particles.

A typical instrument of this type includes a witness plate mounted above a charge-coupled-device (CCD) or other video camera. The witness plate is illuminated to provide uniform omnidirectional lighting. The camera optics are adjusted to focus on the exposed surface of the witness plate, so that particles that have fallen onto the surface are imaged by the camera. The video output is digitized.

The resulting digital image data is processed by image-analysis software that detects particle edges, maps the particles, counts the particles, and determines principal dimensions and aspect ratios of the particles. The software uses aspect ratios to indicate distinctions between fibers (typical aspect ratios >10:1) and other particles. Instruments of this type can detect and measure particles with dimensions down to somewhat less than 10 µm.

This work was done by Paul A. Mogan of Kennedy Space Center and Christian J. Schwindt and Timothy R. Hodge of I-NET.

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