Shadow Doppler Particle Analyzer ( SDPA )
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System Overview
The Shadow Doppler Particle Analyzer (SDPA) is designed for simultaneous measurement of velocity and size of arbitrary shaped particles. The SDPA system combines an imaging technique with a conventional laser Doppler velocimeter (LDV). In this system, a pair of LDV beams projects images of particles. They are processed and analyzed at lightning speed to find velocity, shape, orientation, and size of particles, regardless of their shape (spherical or non-spherical) and optical property such as refractive index and particle emissivity.
Picture: SDPA in Operation
The SDPA uses LD-pumped 100-mW YAG laser as the source of light. A green-color laser beam (l=532nm) from the source is split into a pair of parallel beams through the Acoustic-Optic Modulator (AOM), and they form a measuring volume at the focal point of the lens where the two beams meet.
Shadow signals required to analyze particle shape is obtained when the image of the measuring volume is magnified by an objective lens and projected onto an array of 64 optical fibers. The fibers carry the shadow image to a photo diode, and the image is converted to electrical signals. Velocity, or the Doppler signal, is obtained from scattering light when particles pass through the measuring volume. The scattering light is collected by a collecting lens and converted to electrical signals by a photo-multiplier. Both shadow and velocity signals are digitized and transferred to a computer that processes them into velocity and shape information.
Exhibit A: SDPA Construction
Exhibit B: System Diagram
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