Cloud, Aerosol, and Precipitation Spectrometer (CAPS)



A multipurpose, research-grade cloud spectrometer that includes three DMT instruments plus temperature and relative humidity sensors.
Special Features: Polarization data (included); grayscale imaging, finer resolution, anti-shatter tips (all optional)




  • Cloud particle research
  • Climate studies
  • Huricane and storm research
  • Weather modification
  • Contrails and contrail-induced cirrus
  • Cloud chamber studies
  • Aircraft icing
  • Spray characterization

Photo: the CAPS mounted on a Twin Otter research aircraft. Photo courtesy of British Antarctic Survey.



The CAPS combines multiple instruments in one flight canister, resulting in an exceptionally broad array of data:

  • Aerosol particle and cloud hydrometeor size distributions from 0.51 to 50 µm
  • Precipitation size distributions from 25 µm to 1550 µm, or 15-930 um with optional 15-micron resolution
  • Particle optical properties (refractive index)
  • Particle shape assessments (discrimination between water and ice for probes with depolarization feature)
  • Liquid water content from 0.01 to 3 g/m3
  • Aircraft velocity
  • Atmospheric temperature and pressure


Special Features

The CAPS's polarization feature allows it to differentiate between water and ice particles for particles in the 0.5 - 50 µm range. The CAPS measures forward-scattered light and the S-state and P-state polarizations with two backscatter detectors. For spherical particles, typically droplets, the polarization of the incident light will be retained and the crossed polarization in the back-scatter will not generate any signal. Depending on the asphericity of the particles, there will be increased signal in the backscatter detector with the crossed polarizer.

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 a CAS, a component instrument of the CAPS. 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.

Grayscale imaging provides detail about particle composition that is missed with monoscale imaging. In particular, grayscale imaging gives three levels of shadow recording on each photodetector as opposed to one. The pictures below show the same particles recorded with grayscale imaging (left) and monoscale imaging (right).


The CIP is offered in a 15 µm resolution as well as the standard 25 µm resolution.
The CIP can be purchased with special Korolev anti-shatter tips, shown below. Korolev tips significantly reduce the incidence of particle artifacts in the sample area.




Two optional software packages are available  for CAPS users. The Particle Analysis and Display System (PADS) allows for instrument control and real-time data display.

The second software package, the Optical Array Shadow Imaging Software (OASIS), facilitates post-processing particle analysis. More information about these two packages is given below.


Particle Analysis and Display System (PADS)

PADS displays a user-friendly virtual instrument panel that enables the user to do the following tasks:

  • Start data recording and sampling
  • View real-time particle image data acquired by the CIP
  • View particle volume and number concentrations, as well as Median Volume Diameter (MVD) and Effective Diameter (ED)
  • View LWC as measured or calculated by the hotwire LWC, CIP, and CAS
  • Monitor instrument parameters like CIP laser current and various electronics voltages
  • Play back data for post-flight viewing

Online PADS Software Manual



Optical Array Shadow Imaging Software (OASIS)

OASIS, shown at right, is designed for rigorous post-processing analysis. It provides additional particle statistics beyond those generated by PADS. In addition, OASIS allows users to filter particles by various criteria, including particle area, particle shape, area fraction, and inter-arrival time. Written in IGOR Pro, OASIS leverages IGOR’s robust statistical analysis features for enhanced data analysis and graphing capability. 


How it Works


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

The CIP, which measures larger particles, operates as follows. Shadow images of particles passing through a collimated laser beam are projected onto a linear array of 64 photodetectors. The presence of a particle is registered by a change in the light level on each diode. The registered changes in the photodetectors are stored at a rate consistent with probe velocity and the instrument’s size resolution. 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. A slice must be stored each time interval that the particle advances through the beam a distance equal to the resolution of the probe.

The CAS, which measures smaller particles, relies on light-scattering rather than imaging techniques. Particles scatter light from an incident laser, and collecting optics guide the light scattered in the 4° to 12° range into a forward-sizing photodetector. This light is measured and used to infer particle size. Backscatter optics also measure light in the 168° to 176° range, which allows determination of the real component of a particle’s refractive index for spherical particles.

The Hotwire LWC instrument estimates liquid water content using a heated sensing coil. The system maintains the coil at a constant temperature, usually 125 °C, and measures the power necessary to maintain this temperature. More power is needed to maintain the temperature as droplets evaporate on the coil surface and cool the surface and surrounding air. Hence, this power reading can be used to estimate LWC. Both the LWC design and the optional PADS software contain features to ensure the LWC reading is not affected by conductive heat loss.


Parameter CIP Specification CAS Specification LWC Specification
Technique Optical Array Probe with 64 elements: 62 sizing elements, end diodes reject Forward and Back Scatter Light Sensors Temperature-Controlled Hotwire Sensor
Measured Particle Size Range 12.5 µm – 1.55 mm (standard) 0.51 µm to 50 µm N/A; measured LWC range is 0 - 3 g/m3
Sample Area 10 cm x 1.55 mm 11.1 mm x 120 µm N/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/cm3 3 g/m3
Air Speed Range 10 - 300 m/s 10 - 200 m/s 10 - 200 m/s
Number of Size Bins 62 Selectable; 10, 20, 30, or 40 N/A
Sampling Frequency 1D histogram data: 0.05 to 40 Hz 
2D image data: variable interval, when buffer fills
Selectable, 0.05 to 40 Hz N/A
Laser 658 nm, 30 mW 658 nm, ~50 mW N/A
Calibration Verification Spinning glass disk with opaque dots of known size Precision glass beads and latex spheres for sub-micron range Not Required
Light-scattering Parameters N/A 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) N/A N/A
Data System Interface: 2D CIP data: RS-422, High Speed, 4 Mb/sec Baud Rate
System data: RS-232 or RS-422, 56.6 kb/sec Baud Rate
Optional Software Particle Analysis and Display System (PADS) for all instruments
Optical Array Shadow Imaging System (OASIS) for CIP
Weight 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
Routine Maintenance DMT recommends conducting basic instrument performance checks and inspecting the CIP optical windows before a flight. A weekly calibration check of the CAS and CIP is also recommended.

Specifications are subject to change without notice. CAS specifications apply to both standard CAS and CAS with polarization.



Included Items

  • Instrument
  • Shipping case
  • Operator manual
  • Zero-count filter
  • Glass beads and dispenser for CAS calibration check
  • Spinning disk (pictured at right) for CIP calibration check
  • One day of training at DMT’s facility
  • One-year warranty
  • Email and telephone technical support

Accessories (Purchased separately)

  • Laptop and Particle Analysis and Display System (PADS) software
  • OASIS Software
  • Data Acquisition System


How to Order

Contact DMT for pricing or more information.


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

Selected Bibliography

Baumgardner, D., H. Jonsson, W. Dawson, D. O’Connor and R. Newton. “The cloud, aerosol and precipitation spectrometer (CAPS): A new instrument for cloud investigations,” Atmos. Res. 2001: 59-60, 251-264. Link

Baumgardner, D., H. Chepfer, G.B. Raga, G.L. Kok. “The Shapes of Very Small Cirrus Particles Derived from In Situ Measurements,” Geophys. Res. Lett., 2005: 32, L01806, doi:10.1029/2004GL021300, 2005. PDF Link

Chiriaco, M., H. Chepfer, P. Minnis, M. Haeffelin, S. Platnick, D.Baumgardner, P. Dubuisson, M. McGill, V. Noël, J. Pelon, D. Spangenberg, S. Sun-Mack, G. Wind. “Comparison of CALIPSO-like, LARC and MODIS Retrievals of Ice Cloud Properties over SIRTA in France and Florida during CRYSTAL-FACE,” J. Appl. Meteor., 2007: 46, 249-272. PDF Link

Gao, R.S., D. W. Fahey, P. J. Popp, T. P. Marcy, R. L. Herman, E. M. Weinstock, J. B. Smith, D. S. Sayres, J. V. Pittman, K. H. Rosenlof, T. L. Thompson, P. T. Bui, D. G. Baumgardner, B. E. Anderson, G. Kok, A. J. Weinheimer. “Measurements of relative humidity in a persistent contrail,” Atmos. Env. 2006: 40, 1590-1600. Link

Garrett, T.J., H. Gerber, D. Baumgardner, C. H. Twohy, E.H. Weinstock. “Small, Highly Reflective Ice Crystals in CRYSTAL-FACE Anvil Cirrus,” Geophys. Res. Lettrs. 2003: 30, NO. 21, 2132, doi:10.1029/2003GL018153 Link

Grosvenor, D.P., T. W. Choularton, T. Lachlan-Cope, M. W. Gallagher, J. Crosier, K. N. Bower, R. S. Ladkin, and J. R. Dorsey. “In-situ aircraft observations of ice concentrations within clouds over the Antarctic Peninsula and Larsen Ice Shelf.” Atmos. Chem. Phys. Discuss., 12, 17295–17345, 2012 Link

Heymsfield, A.J., C.G. Schmitt, A. Bansemer, D. Baumgardner, E.M. Weinstock, J.T. Smith and D. Sayres, “Effective Ice Particle Densities for Cold Anvil Cirrus.” Geophys. Res. Lettrs. 2004: 31, L02101, doi:10.1029/2003GL018311. Link

Heymsfield, A.J., C. Schmitt, A. Bansemer, G-J van Zadelho, M. J. McGill, C. Twohy, D. Baumgardner. “Effective Radius of Ice Cloud Particle Populations Derived from Aircraft Probes,” J. Atmos. Oceanic Tech. 2006: 23, No. 3, pp. 361–380. PDF Link

Lu, C., Y. Liu, S. Niu, and A. M. Vogelmann (2012), "Observed impacts of vertical velocity on cloud microphysics and implications for aerosol indirect effects," Geophys. Res. Lett., 39, L21808, doi:10.1029/2012GL053599. Link

Padmakumari B, Maheskumar RS, Morwal SB, Harikishan G. Konwar M, Kulkarni JR, Goswami BN. 2012. “Aircraft observations of elevated pollution layers near the foothills of the Himalayas during CAIPEEX-2009.” Q. J. R. Meteorol. Soc. DOI:10.1002/qj.1989 Link