High Speed Imaging Flight Probe
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.
The HSI optics consists of a transmitter and a receiver optics. Multiple, pulsed, laser beams cross at the same point to form a measurement volume where a particle is probed when its presence is detected. Each beam is produced by its own laser source in the transmitter and is therefore optically incoherent with respect to other beams. The intersection of multiple laser beams provides an intense uniform illumination in the sample volume. Since each laser beam takes a different path from the transmitter optics to the sample volume, particles that may momentarily block parts of any one beam will not, at the same time, block any of the other beams. As a result, unlike conventional imaging systems that use only a single beam path to illuminate the particle field, the illumination profile at the sample volume of the HSI system does not lose uniformity at any time. The transmitted laser beams are re-focused onto a fast frame rate imaging sensor where a shadow of the particle is cast. The images are processed in real-time by the AIMS software to extract various size and shape parameters.
Particle imaging with multi-laser beam illumination
- 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 system 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.