• Manafacturer: Stratton Park Engineering
• Model Number: CPI-230

• Purchase: $100,000-$200,000
• Maintenance: N/A
• Calibration: N/A

Figure 1: Top side view of Cloud Particle Imager Optical Bench assembly.

• Airborne instrument
• Two beam Particle Detection System (PDS) strobes a high power laser to flash at the instant a particle is in the object plane of the imaging system.
• Processed image is cropped to region of interest by imagin processing software proprietary to SPEC, Inc.
• 1000 X 1000 pixel digital camera that has 8 bits of resolution and effective pixel size of 2.3 um.
• Image plane is oriented 45 degree to particle's trajectory, and this implies a max sample volume at 100 m/s of 529 centimeters cubed.
• Two interesting laser beams each 2.3 um wider placed at a 45 degree angle to the particle, are located in the center of the sample tubed. The object plane is located on the trailing edge of this intersection. Each PDS is focused to a spot where the collimated light is blocked and the light scattered forward by a particle is collected with two photo detectors.
• When the sensor electronics detects simultaneous pulses lasting longer than a minum duration, a high power pulse laser is armed.
• The pulse will fire on the trailing edge of the detection pulse, which is when the particle is in the object plane of the camera.

• Sensor Head: Hollow pylon and cover into which the optical bench/electronic assembly fits.
  • Elctrically Heated
  • 37.5 pounds
  • Dalsa CA-D4 CCD Camera
  • Two commercial avalanche photodiode detectors
  • Three lasers
  • Set of SPEC custom cards

Figure 2: Spec custom cards fixed to the Cloud Particle Imager Optical Bench assembly.

• Digital Signal Processor: Controlls and monitors housekeeping card, Analog Processor Card, and AC Switch Card.
• Housekeeping Card:
  • Monitors slowly carying signals from scattered locations around the probe.
  • Conditions signals from transducers, sensors, and voltage supplies; digitizes these signals, buffers these data for retrieval by the Digital Signal Processor.
  • Monitors 20 temperatures
• Analog Processing Card:
  • Provides the interface to the elctro-optics in the system.
  • Contains laser drivers for the continous lasers, bias supplies for the high-power laser driver, pulse processing for avalanche photodiode detectors and the sate machine which processes pulses and decides when a laser pulse can occur.
• AC Switch Card:
  • Distributes AC power to the various heat zones of the probe.

Figure 3: Schematic of Cloud Particle Imager sample volume and laser system.

• Particle Detection System:
  • 45 degree and 90 degree Particle Detection Systems consists of same primary components. The continuous wave diode lasers have an output power of 30 mW and a wavelength of 788 nm at 25 degree Celsius approximately. The lasers are collimated to form an elliptical beam, which is then compressed, and passed through an aperture to produce a beam with a rectangular cross section to illuminate the sample volume. After exiting the sample volume the beam is focused onto a dump spot. When a particle crosses the beam, light scatters around the dump spot onto an avalanche photodiode detector.

Figure 4: Simplified schematic of the Particle Detection System laser path for each laser (PDS 90 and 45). This diagram depicts the laser path in both non-scattered and scattered light scenarios.

• Avalanche Photodiode Detector:

Figure 5: Picture of Avalanche Photidiode Detector.

• Multiplies tiny current created by particle scattered energy.
• Main purpose is to measure the energy scattered by particle.

• Imaging System:
  • Imaging laser is a pulsed three segment stacked laser diode array with a maximum output power of 80 W, pulse width of 25 ms, and a wavelength of 850 nm approximately.
  • The imaging optics magnify the object plane to the size of the CCD array. Specifically a 2.3 um X 2.3 um pixel in the object plane is mapped to a 12 um X 12 um pixel on the CCD array.
  • Again, since the object plane is located at the rear of the sample volume, the PDS system guarantees that at least one particle per image will be in focus.

Figure 6: Schematic of Cloud Particle Imager Imaging Laser optical path.

• Charged Coupled Device (CCD):

Figure 7: Picture of Charged Coupled Device (CCD) used in commercially available digital camera.

• Image projected onto capacitor array (photo-active region).
• Capcitor accumulates electrical charge proportional to the light intensity at that location.
• Two-dimensional array captures picture.
• Charge converted to voltage and digitized.

Figure 8: Picture of data system components. (From right to left) User interface, Rack mount, and PC.

• Components:
  • 19” Rack Mount Chassis
  • PC with 1024 MB ram, 80 GB, and 120 GB har drives, and Windows 2000 Professional
  • PC-DIG RS422 Digital Image Capture Board
  • VisionNow high performance image acquisition and analysis software application, SPEC Inc.
  • Licensed copy of Interactive Data Language software
  • CPIview licensed installation, SPEC Inc.

• Hardware Settings:
  • Set particle detection lasers respective operating power using the VisionNow software. Power settings determine range of particles sizes imaged. Higher power settings imply fewer smaller particles will be imaged by the Cloud Particle Imager.
  • Other settings accessible in the VisionNow software are too numerous to list on this page, but some ones worth mentioning are:
    • Heat, Minimum Transit Time, Background Mean, and Image Mean.

• CPIview Software
  • Automatically sizes image and trims white space around image based on settings made by user during data collection.
  • Calculates liquid water content and Ice water content.

Figure 9: Sample image of CPIview's mosaic view. Mosaic view implies that the position of each image is its respective location on the image plane at the time of its being imaged by the imaging laser.

The following is a list obtained from the Cloud Particle Imager User's Manual:

1. PDS detects a particle
2. State machine fires the laser
3. DSP sends a PDS packet to the data system
4. Data system searches the next image for particles
5. Data system sends an EXSYNC pulse to the camera
6. The camera shifts data from the collection array into the output array
7. The camera sends a FVAL risin edge tot he state machine
8. The state machine resumes the search for the next particle

The following figures use data collected by the CPI for the CRYSTAL-FACE mission. This mission, conducted in summer 2002 over the south florida peninsula, was undertaken to better understand tropical cirrus cloud physical properties and formation processes.

Figure 10: Flight path of University of North Dakota's Cessna Citation research aircraft on day depicted in figure.

Figure 11: Typical flight profile used during mission operations.

Figure 12: Processed image data obtained during 60 second time frame at 12 km altitude.

Figure 14: Processed image data obtained during 60 second time frame at 9 km altitude.

Figure 15: Particle spectrum that was derived from data obtained during 60 second time frame at 12 km altitude.

Figure 16: Particle spectrum that was derived from data obtained during 60 second time frame at 9 km altitude.

• Complex and completed at manufacturer
  • Beads of a known size
  • Opaque disks of a known size on a glass plate placed perpendicular to imaging laser.
  • Optical and laser alignment process too detailed to list on this page and too complex for the normal user, and it is recommended highly that scientists with thorough experience operate this instrument during field campaigns.
  • SPEC developed correction algorithm to correct for sizing error and counting error.

Figure 17: Example image of VisionNow software window viewable during probe operation. More importantly, the images in this figure are from a calibration. Notice the dark circles that are disks of a known size on a glass slide.

• Depth of Field
  • Leads to out of focus image and bias towards larger particle diamater due to low contrast edges of particle in image.
• Dead Time
  • Not mentioned in literature, but could be a possibility due to time between imaging laser being fired and particle detection strobe being reset.

The Cloud Particle Imager is a highly automated stand alone instrument with numerous user defined parameters. This instrument can be turned on with relative ease and collect data continuously. The only forseeable issues would be hard disk memory capacity, electronic component usable life spans, and dirty optics. The following listed items are settings and techniques used by University of North Dakota personnel during field campaigns:

• Background mean: 100 to 120 for optimum lighting
• Image mean: 100 to 120
• Image minimum: 70
• Image maximum: 170
• Imaging laser current: ~30 V
• Imaging laser pulse width: ~30 V (corresponds to 30 ns)
• PDS 45 and PDS 90 Threshold Levels: 300 (0.1831 volt for physical unit scale) This setting determines the pulse level needed by the imaging laser for its being allowed to fire. In short: higher the threshold value the larger the minimum particle size.
• PDS 45 and PDS 90 Laser Power: 30 mW
• PDS 45 and PDS 90 Detector DC levels: 6 to 7 V
• Minimum transit time: 8.33 E-7 ms

• Artifact rejection: Will eventually be an automated algorithm to reject artifacts, but for the current time it is tool that can only be utilized manually by a user for each frame of interest. Those rejected artifacts will not be used in calculating particle spectrum plots.

The cloud particle imager has vertain features as listed in the calibration section that must be set by the respective user. The background mean or minimum is a way of setting a light level required for an image to be captured. This helps ensure that dark frames are not stored into memory unnecessarily. Otherwise, any particle that crosses through the sample volume for a minimum amount of time will be captured and stored into memory regardless of focus quality. It can be inferred that quality assurance belongs to the user. The level of quality is a function of user experience largely.