How It Works
Polarization Features
Included Items
How to Order
Print Version
BCPD Manual

The Backscatter Cloud Probe with Polarization Detection (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.



  • Classification of particles by size in histogram form
  • User-programmable sample rates and bin thresholds 
  • Size distributions accumulated in the probe, with serial transmission to computer via any standard RS-422 communications port
  • Monitoring of multiple variables, e.g. total particles, average transit time, over-range particles, and various probe health indicators
  • Automatic correction for drifts in polarization signals through dynamic thresholding feature


Sleek, Non-Intrusive Design:

The BCPD’s aerodynamic design minimizes airflow disturbance and allows for undisturbed particle measurement See photo at right of the BCP embedded in simulated aircraft skin. In addition, the BCPD’s minimal size allows it to be mounted easily on aircraft, even when space is at a premium. 



  • Cloud particle research
  • Climate studies 
  • Fluid contamination detection (e.g., gas in a pipe)

The BCPD is suitable for both airborne and ground-based applications. It is currently being flown on Lufthansa commercial aircraft as part of the IAGOS program. 


How it Works

Particles passing through the laser beam scatter light in all directions. Some of this light transmits within a cone with subtending angles between 143° and 169° (156° ± 13°). These photons are directed onto a photodetector that converts their 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. 




The BCPD is calibrated at DMT with precision glass beads. 


Polarization Feature:

The BCPD’s polarization feature allows the instrument to discriminate between ice and water particles. This capability allows the BCPD to detect when an aircraft encounters abnormally high concentrations of ice crystals, for example.

The figure below illustrates how the change in polarization state of scattered light can be used to differentiate water droplets, ice crystals, and volcanic ash. This graph shows data acquired with another DMT instrument that measures polarized light, the CAS-POL. The polarization ratio measured for single particles (y-axis) is graphed against the ratio of the polarization signal to the sizing signal (x-axis). Three regimes are clearly seen that are related to the morphology of the particles. Polarization data from the BCPD allows users to similarly identify particle types.

Above figure courtesy of James Dorsey, University of Manchester.


The Particle Analysis and Display System (PADS) is optional software that provides a user-friendly virtual instrument panel for the control, data display, and data logging of the BCPD instrument. For instance, the program enables the user to do the following tasks: 

  • Start data recording and sampling
  • View a size histogram of particles measured by the BCPD
  • View particle volume and number concentrations, as well as Liquid Water Content (LWC), Median Volume Diameter (MVD) and Effective Diameter (ED)
  • Monitor instrument operational parameters like optic block temperature, electronic box temperature, and the baseline monitor voltage



Measured Parameters Single-particle light scattering:
- S polarized back-scattered light
- P polarized back-scattered light
Auxiliary Parameters Temperature
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° (see "How it Works" diagram)
Laser 658 nm
Laser Power 85 mW or less
Data System Interface RS-422 serial interface
Additional Components Electronics box
1 m connecting cable
Calibration Performed with glass beads at DMT facility
Routine Maintenance Optics cleaning before every field campaign
Recommended Service Annual cleaning and calibration at DMT service facility
Software Optional Particle Analysis and Display System (PADS) software
Environmental Operating Conditions Temperature: -40° to 40°C
Relative Humidity: 0 - 100%, non-condensing
Altitude: 0 - 50,000 feet (0 - 15,000 meters)
Weight 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
LASER WARNING: The requirement for the BCPD to be non-intrusive to aircraft operations (i.e., no external components) dictates that there be no laser beam-stop mechanism. The laser beam will project unimpeded from the optical window. The laser is not eye-safe, so precautions must be enforced for operation on the bench or ground. 


Included Items:

The following items are included in the purchase price:

  • Instrument
  • Shipping case (far right)
  • Operator manual
  • Electronics box and cable (near right)



  • PADS software, purchased separately


How to Order:

Contact DMT for pricing:

Phone: +1.303.440.5576,