Back-scatter Cloud Probe with Polarization Detection

The BCPD measures cloud droplet size distributions, which are then used to derive the total number concentrations, liquid water content (LWC), median volume diameter (MVD), and effective diameter (ED). The BCPD’s non-intrusive optical housing allows use in a range of ground-based or airborne applications with no contamination from ice crystal shattering and no airflow distortion.


Discriminate between ice and water particles with the BCPD’s polarization feature. For example, use the BCPD to detect when an aircraft encounters abnormally high concentrations of ice crystals.

Minimize airflow disturbance and enable undisturbed particle measurement with the BCPD’s aerodynamic design. Mount the BCPD easily on aircraft, even when space is at a premium.

Monitor multiple output variables like total particles, average transit time, over-range particles, and various probe health indicators. Get particle classification by size in histogram form.

Depend on automatic correction for drifts in polarization signals through the dynamic thresholding feature.

Program the instrument's sample rate and size bin thresholds. Plus, get size distributions accumulated in the probe, with serial transmission to a computer via any standard RS-422 communications port.

Particle light detection and polarization detection

  • Particles passing through the laser beam scatter light in all directions.
  • Some of this light is transmitted within a cone with subtending angles between 143° and 169° (156° ± 13°). These photons are directed onto a photodetector that converts their photon pulses into electrical pulses, which are then transmitted to a signal processor that amplifies and digitizes them.
  • Mie theory is used to determine the particle’s size from the peak amplitude of the scattering signal. At programmable intervals, the BCPD sends out a particle size distribution. This information is then used to determine particle number concentration.
  • A beam splitter separates P-polarized from S-polarized light, and a comparison of the components allows separation of water droplets from ice crystals.
Parameter Specification
Measured Parameters

Single-particle light scattering:

  • S polarized back-scattered light
  • P polarized back-scattered light
Auxiliary Parameters
  • Temperature
  • Pressure
Derived Parameters
  • Particle diameter
  • Particle number concentration
  • Liquid water content (LWC)
  • Effective Diameter (ED)
  • Median volume diameter (MVD)
Particle Size Range

2 – 50 µm

Number of Size Bins
  • 10, 20, 30, or 40 (configured at time of purchase)
  • User-selected boundaries
Number of Concentration Range

0 – 1,000 particles/cm3

Air Speed Range

10 – 250 m/sec

Sampling Frequency

Selectable, .04 sec to 20 sec

Light Collection Angles

Center-line: 155°, +/- 18.5°


658 nm

Laser Power

85 mW or less

Data System Interface

RS-422 serial interface

Environmental Operating Conditions
  • Temperature: -40° to 40°C
  • Relative Humidity: 0 – 100%, non-condensing
  • Altitude: 0 – 50,000 feet (0 – 15,000 meters)
  • 1.4 kg for head and window interface
  • 0.9 kg for electronics box
Probe Dimensions
  • Probe alone: 14.7 cm L x 13.3 cm W x 6.4 cm H
  • With window interface: 16.7 cm L x 13.3 W x 8.3 cm H
Electronic Box Dimensions

21.6 L cm x 12 cm W x 8.2 cm H

Power Requirements

28 VDC, 5 A for system and heaters

Droplet offers a three-year service plan to ensure that your BCPD continues to deliver high-quality data. This plan locks costs upfront at the time of funding application and covers annual calibration, alignment, cleaning, wear-item replacement, calibration documentation, and shipping. Specific services provided for the BCPD include:

  • Freight to and from Droplet facility for service
  • Incoming instrument evaluation and health check
  • Optics cleaning and alignment
  • Clean and replace heater heatsink compound
  • Laser evaluation and adjustments
  • Laser splitter block alignment
  • Offset voltage verification
  • Baseline amplitude verification
  • Baseline voltage evaluation on DSP board and PADS
  • O-ring replacement
  • Hardware replacement
  • Reseal windows
  • Extreme cold testing of probe
  • Software, firmware, and antivirus update
  • 100% unit functionality tests
  • Calibration data, Calibration Report, Pitot Calibration Report, and Certificate of Calibration provided

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