Cloud, Aerosol, and Precipitation Spectrometer with Depolarization

The CAPS-DPOL is a multipurpose, research-grade cloud spectrometer that includes three Droplet instruments (CIP, LWC, and CAS); temperature and relative humidity sensors; and polarization measurement to discriminate between water and ice.


Combine multiple probes with ranges optimized to measure cloud particles prior to and during transition to precipitation.

CAS measures aerosol particle and cloud hydrometeor size distributions from 2 µm to 50 µm; CIP measures precipitation size distributions from 25 µm to 1550 µm, and LWC measures liquid water content from 0.01 g/m3 to 3 g/m3, along with aircraft velocity, and atmospheric temperature and pressure.

Gather data on particle optical properties (refractive index), as well as particle shape assessments (discrimination between water and ice for probes with depolarization feature).

Particle imaging, polarization detection, and hotwire sensing

The three Droplet instruments included in the CAPS-DPOL are the Cloud Imaging Probe (CIP), the Cloud and Aerosol Spectrometer (CAS), and the Hotwire Liquid Water Content Sensor (Hotwire LWC).


  • Shadow images of particles passing through a collimated laser beam are projected onto a linear array of 64 photodetectors to determine presence of particles.
  • Particle images are reconstructed from individual “slices,” where a slice is the state of the 64-element linear array at a given moment in time.


  • The CAPS-DPOL collects forward-scattered light to determine particle size, and polarized back-scatter light to differentiate between spherical and aspherical particles.


  • The LWC features a coiled wire that is maintained at 150 °C, which acts as variable resistance in a Wheatstone Bridge circuit.
  • The resistance of the sensing coil is directly proportional to its temperature.
  • Power dissipated by the sensor to maintain temperature is used to calculate vaporization, and in turn, liquid water content.
Parameter SpecificationCAS SpecificationLWC Specification

Optical Array Probe with 64 elements: 62 sizing elements, end diodes reject

Forward- and Back-Scatter Light SensorsTemperature-Controlled Hotwire Sensor
Measured Particle Size Range

12.5 µm – 1.55 mm (standard)

0.51 µm to 50 µmN/A; measured LWC range is 0 - 3 g/m3
Sample Area

10 cm x 1.55 mm

11.1 mm x 120 µmN/A
Upper Concentration Range

Depends on particle size, but up to 500 particles/ cm3 for a CIP with standard tips and arm width

Greater than 1,000 particles/cm3 after corrections for coincidence that are about 25% at 800 and 30% at 1,000 particles/cm33 g/m3
Air Speed Range

10 – 300 m/s

10 - 200 m/s10 - 200 m/s
Number of Size Bins


Selectable; 10, 20, 30, or 40N/A
Sampling Frequency

1D histogram data: 0.05 to 40 Hz
2D image data: variable interval, when buffer fills

Selectable, 0.05 to 40 HzN/A

658 nm, 30 mW

658 nm, ~50 mWN/A
Light-scattering Parameters


Non-absorbing refractive index: 1.3 – 1.7
Light collection angles: 4° - 12°, 168° - 176°
Auxiliary Parameters

Ambient Temperature, Relative Humidity, Static Pressure, Dynamic Pressure (CIP)

Data System Interface:

2D CIP data: RS-422, High Speed, 4 Mb/sec Baud Rate
System data: RS-422, 56.6 kb/sec Baud Rate


45 lbs./20.4 kg

Power Requirements

28VDC: 10A for probe system, and 45A for anti-ice heaters, optional AC voltages for anti-ice heaters

Environmental Operating Conditions

Temperature: 0 – 40°C (32 – 104 °F)
RH: 0 – 100%, non-condensing

  • PADS software and laptop
  • Data Acquisition System

Droplet offers a three-year service plan to ensure that your CAPS-DPOL 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 CAPS-DPOL include:

  • Freight to and from Droplet facility for service
  • Incoming instrument evaluation and health check
  • Optical component inspection and cleaning
  • 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 all windows
  • Extreme cold testing of probe
  • Calibration of instrument using multiple sizes of glass beads
  • Beam-mapping 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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