Optoelectronic instruments are being developed for use in measuring the concentrations and sizes of microscopic particles suspended in air. The instruments could be used, for example, to detect smoke, explosive dust in grain elevators, or toxic dusts in industrial buildings. Like some older, laboratory-bench-style particulate monitors, these instruments are based on diffraction of light by particles. However, these instruments are much smaller; exploiting recent advances in optics, electronics, and packaging, they are miniaturized into compact, hand-held units.

The prototype instrument includes a miniature optical train targeted on a gas sample that contains particles to be detected. A light-emitting diode or laser diode generates a beam of light that is collimated by a first lens and passed through the gas sample. Forward-diffracted light produced by interaction of the beam with particles in the sample is collected by a second lens, which focuses the light onto one central circular and three concentric annular arrays of optical fibers. The fibers in each array carry the focused diffracted light to one of four photodiodes, so that the outputs of the photodiodes provide a coarse-resolution representation of the radial dependence of the diffraction pattern.

An aperture stop is placed in front of the second lens to attenuate the bright undiffracted beam. This aperture stop is an engineered low-reflectance, high absorbance optical element in the form of a thin-film absorber less than 0.2 µm thick. It transmits only about 0.2 percent of the power of the undiffracted beam through to the second lens, while reflecting only about 0.1 percent and absorbing the rest. In so doing, it (1) reduces flare light that might otherwise corrupt the dim diffraction pattern, (2) minimizes the undesirably strong spurious scattering of light that would occur if the undiffracted beam were allowed to be reflected back toward the sample at a substantial fraction of its original power, (3) minimizes spurious diffraction from stationary dirt particles, which eventually will come to adhere to the second lens, and (4) provides a lowpower proportional sample of the undiffracted beam for use as an intensity reference, as explained next.

The central array of optical fibers carries light from the attenuated undiffracted beam to one of the photodetectors. The output of each of the other photodetectors is divided by the output of this photodetector, so that the diffracted- light reading from each annulus is converted to a ratio that is independent of the intensity of illumination. This ratiometric readout technique normalizes out the effects of fouling of surfaces and of fluctuations in the source of light. Furthermore, each ratio is directly proportional to the density of particles in the sample gas.

The three ratios and a reading proportional to the intensity of the undiffracted beam are displayed on a liquid-crystal readout device. In principle, the sizes of the particles can be estimated from the diffraction pattern; that is, from the ratios. The prototype does not contain the microprocessor that will be needed in a complete, fully designed unit to control the acquisition of data and compute the sizes of particles. The algorithm needed to compute the sizes has not yet been fully developed either, though it is known that the algorithm will likely call for diffraction ratios at more than three annuli.

The wavelength of illumination in the prototype is 670 nm. This limits quantitative measurements to particles larger than about 1 µm. A shorter wavelength would, of course, enable the measurement of proportionally smaller particles.

This work was done by Elric W. Saaski of Research International, Inc., for Johnson Space Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Physical Sciences category.


Photonics Tech Briefs Magazine

This article first appeared in the January, 2002 issue of Photonics Tech Briefs Magazine.

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