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Laser Obscuration Time
The
Ankersmid “EyeTech”
Particle Size & Shape systems (Lab Systems, On-Line/In-Situ Probes) are both
based on a well-founded principle of Light Obscuration, also known as the Laser
Obscuration Time (LOT) technology. Stationary or
moving particles can be measured by a scanning and rotating laser beam (= PSA, Particle Size
Analysis) and/or
Characterized/visualized (optical) (= DSA/DSC, Dynamic Shape
Analysis/Characterization) using a CCD (video)
camera. All controlled and automated using a
powerful 21 CFR -11 (image) analysis software, which entails capabilities like
pre-processing, filter options, re-run analysis data, etc.
A He-Ne laser beam
passes through a rotating wedge prism and is focused down to a small spot. The
result is a circular movement of a focused laser beam. The laser beam scans the
individual particles in the measurement zone. As the particles within the sample
volume are individually bisected by the laser spot, interaction signals are
generated. These signals are then detected by a photodiode.
Since the beam
rotates at a constant speed, the duration of interaction (obscuration) provides
a direct measurement of the individual particle''''s size. The interaction signals
are collected and analyzed in 600 discrete size intervals.
Sophisticated pulse
analysis algorithms are employed to reject out-of-focus and off-center
interactions.
The strength of the Laser Obscuration Time principle is that
it relates solely and directly to particle size, rather than to secondary
properties from which size may be inferred. This eliminates inconsistencies due
to sample refractive index, viscosity variations, Brownian Motion and thermal
convection.
Routine calibration
is not required, and to a large degree, results are not dependent on the optical
properties of the particulates or the medium. Problems of coincidence and
orifice clogging are avoided by using interaction pulse analysis to provide an
optically defined measurement zone, rather than a mechanically defined
orifice.
Simultaneously
offering high resolution and wide dynamic range, the Laser Obscuration Time
analysis generates the most accurate one-dimensional sizing of particles.
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