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PDI: Phase Doppler Interferometer

Droplet size and velocity measurements

Overview

Phase Doppler Interferometry (PDI) is an extension of Laser Doppler Velocimetry (LDV) for measuring the size of spherical droplets in addition to its velocity. Like the LDV, the PDI uses two coherent laser beams to intersect and form a measurement probe volume. Particles passing through the beam intersection region will scatter light that is collected by a receiver placed at a suitable angle, typically in the forward scatter direction. However, unlike an LDV system, the receiver lens of a phase Doppler system is partitioned into four segments and the scattered light collected by these segments are directed to separate photodetectors. 

The Doppler difference frequency observed by each of the photodetectors will be identical and any one of them can be used to infer the particle velocity as in a traditional LDV. In addition, since the phase difference between any two Doppler burst signals can be shown to bear a nearly monotonic, linear relationship with the particle or droplet diameter, in PDI, the phase is measured and used to infer the diameter of a spherical particle. Non-spherical particles will also produce a Doppler signal but will be rejected based on proprietary phase validation logic that is incorporated into the system software.

Read more about the applications and

uses of PDI technology here: 

NASA Spinoff: Technology Transfer Program

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Theory of Operation
Advantages of the PDI Method for Particle Sizing
System Components
Competitive Advantages
Comparison of the PDI and intensity-based light scattering particle sizing methods
Product Brochure
Software
Signal Processing