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The
method of single-particle optical sensing (SPOS), also called optical
particle counting (OPC), was originally developed for the field of particulate
contamination analysis. Particles in gas or liquid suspension flow through
a small "photozone" -- a narrow, slab-like region of uniform illumination,
produced by light from a laser diode or incandescent bulb. The particle
suspension is sufficiently dilute when the particles pass, one at a
time, through the illuminated region, avoiding coincidences. The passage
of a particle through the sensing zone causes a detected pulse, the
magnitude of which depends on the mean diameter of the particle and
the physical principle of detection -- light scattering or obscuration
(blockage).
The
illumination/detection system in the sensor is designed to provide
a monotonic increase in pulse height with increasing particle diameter.
A particle size distribution (PSD) is constructed one particle at
a time, by comparing the detected pulse heights with a standard calibration
curve, obtained from a set of uniform particles of known diameters.
The SPOS technique possesses several characteristics that make it
ideal for measuring the PSD of a wide variety of particle suspensions,
emulsions and dispersions. These include exceptional resolution, wide
dynamic range and high sensitivity. The AccuSizer 780 Optical Particle
Sizer has refined the SPOS technique for general use in particle size
analysis. Through the addition of a proprietary dilution method, called
Autodilution. and significant innovations in hardware and software
design, this important technology is attractive for a variety of applications.
Samples of arbitrary concentration can be measured quickly and automatically,
without sacrificing any of the essential advantages inherent in the
SPOS method.
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