HSI: High Speed Imaging Flight Probe
Aircraft-based droplet and ice crystal measurements
While the PDI is an ideal instrument for measuring the size of spherical particles, there are many applications that involve the measurement of non-spherical particles. Particle imaging or shadowing methods are ideally suited for such applications. The HSI-FPDR, an innovative imaging-based instrument, has been developed specifically for aircraft-based icing cloud studies that require precise measurement of liquid droplets and ice crystals in mixed-phase environments. The HSI can differentiate between spherical and non-spherical particles and compute their respective size and velocity distributions, particle number density, liquid water content (LWC), and ice water content (IWC).
The HSI is a high-speed imaging system that takes advantage of the latest advances in CMOS sensing technology and combines it with an innovative particle illumination method to deliver precise measurements of particulate size and shape. The flight probe incorporates multiple diode lasers that are used to simultaneously illuminate the particulate field from multiple directions. A dedicated laser and photodetector is used to detect the presence of particles in the measurement probe volume. This information is used to pulse the multiple illumination beams. The laser beams are combined by a receiver lens which creates a frozen shadow (or bright-field image) of the particles on the image sensor. The use of multi-angle illumination significantly reduces measurement errors due to depth-of-field variations that are a problem for legacy imaging instruments. The optics and electronics are packaged in a rugged canister design that is proven to be air-worthy. The probe heads are well-heated to prevent ice accretion while flying in extreme icing environments. The pulsed diode lasers used in the probe provides stability, compactness, ruggedness, and high reliability.
Advantages of the HSI Method
Multi-beam, multi-directional illumination limits depth-of-field and improves measurement accuracy by removing background noise created by other particles that may be in the laser beam path but outside the measurement volume
Designed to eliminate shadows from other structures within the beam path but outside the measurement volume; shadows are formed only by particles at the overlap region
Not sensitive to particle speed because of the use of short laser pulses that freeze the shadow without causing blurs
Scalable platform using dual imagers to increase measurement size dynamic range
Can operate in high particle number densities
De-icing of the probes allows for use in icing conditions
Higher measurement resolution
The HSI Flight Probe offers turnkey operation with a fully automated setup feature. The complete instrument includes the flight probe, data acquisition computer, image acquisition card, and the AIMS software. The software analysis package includes sophisticated algorithms for identifying particles that are in-focus, calculating various shape parameters, and classifying ice crystals into its various habits. Methods for differentiating between liquid drops and ice crystals are also included.