Back-scatter Cloud Probe (BCP) and BCP with Polarization Detection (BCPD)


A compact probe that measures 5µm to 75µm particles with no contamination from ice crystal shattering and no airflow distortion.
Special Features: Polarization Detection (optional) for determining particle phase





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

Photo at near right: The BCP, circled, on a IAGOS research aircraft. The IAGOS project involves mounting climate-research instruments on commercial airline jets from Lufthansa, China Airlines, Air France, and other airlines.

Photo at far right: A BCPD with non-standard housing measuring particle distributions from the Sakurajima volcano in Japan. Photo courtesy of Martin Lange.



The BCP and BCPD offer several advantages:

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


The BCP monitors multiple output variables like total particles, average transit time, over-range particles, and various probe health indicators. The user can program the instrument's sample rate and size bin thresholds. Size distributions are accumulated in the probe, with serial transmission to computer via any standard RS-232 or RS-422 communications port.
The instrument automatically adjusts both sizer and qualifier signals to adjust for temperature drifts and ensure accurate particle sizing.


Polarization Detection (Optional 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.




The Particle Analysis and Display System (PADS, right) is optional software that provides a user-friendly virtual instrument panel.

PADS allows the user to control the BCP and display real-time data and logs. 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 BCP
  • 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

Online BCP Software Manual 

Online BCPD Software Manual


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 BCP sends out a particle size distribution. This information is then used to determine particle number concentration. 


Parameter Specification
Measured Parameters Single-particle light scattering
Auxiliary Parameters Temperature
Derived Parameters Particle diameter
Particle number concentration
Liquid water content (LWC)
Effective Diameter (ED)
Median volume diameter (MVD)
Particle Size Range 5 - 75 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: 156°, +/- 13° (see "How it Works" diagram)
Laser 658 nm
Laser Power 50 mW or less
Data System Interface RS-232 or 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 1.5 kg
Probe Dimensions 11.7 cm x 10.7 cm x 4.5 cm, with 5.9 cm diameter mounting flange
Electronic Box Dimensions 21.6 cm x 12 cm x 5.7 cm
Power Requirements 28 VDC, 5 A for system and heaters

Specifications are subject to change without notice.

LASER WARNING: The requirement for the BCP 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

  • Instrument
  • Electronics box and cable
  • Shipping case
  • One day of training at DMT facility
  • Operator manual

Accessories (Purchased separately)

  • PADS software and laptop


How to Order

Contact DMT for pricing or more information.


Phone: +001 303 440 5576
Fax: +001 303 440 1965

Selected Bibliography

Beswick, K., Baumgardner, D., Gallagher, M., and Newton, R.: The Backscatter Cloud Probe – a compact low-profile autonomous optical spectrometer, Atmos. Meas. Tech. Discuss., 6, 7379-7424, doi:10.5194/amtd-6-7379-2013, 2013. Link