There are three general types of two-dimensional imaging probes:

• 2D-C, cloud probe, with elements representing 25 um pixels in the image. Older probes had 32 such elements for a sample aperture 0.8 mm wide; recently modified probes have 64 elements and a sample aperture of 1.6 mm. This probe is the focus of the corrent instrument page.
• 2D-P, precipitation probe, with pixels covering either 100 or 200 um on the particle image. Older 2D-P probes had 32 elements each representing 200 um pixels, for an aperture of 6.4 mm; recently modified probes have 64 100 um pixels and provide the same sample aperture. This instrument has characteristics very similar to the 2D-C probe, but there is a separate page that describes the differences.
• 2D-gray, which produces gray-scale images by recording shadowing at three different thresholds, with 25 um pixels covering an 0.8-mm aperture. This probe has not been modified for use on current high-speed aircraft and is now obsolete, but there are some projects that still archive data from this probe.

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• The 2DC records the two-dimensional shadow cast by a hydrometeor as it passes through a laser beam. The image of the shadow is focused onto a plane containing a linear photodiode array, and the status of the elements of that array (illuminated or not) is recorded at a rate that permits reconstruction of the image. Fpr the 2DC, the array has resolution for the image of 25 micrometer along the linear array, so to provide rectangular resolution at 100 m/s the array must be sampled at 4 MHz. This section of RAF Bulletin 24 provides some additional information. The optical configuration is shown in the following diagram:

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  • Optical Diagram of a 2DC probe. Additional mirrors and lenses are not shown here. The shadow of the hydrometeor passes over the diode array as the aircraft moves in the flight direction, so rapid sampling of that array records the image of the hydrometeor.

  • From the recorded images, it is possible to determine size distributions and associated derived quantities, and with additional non-standard processing to classify images and recognize habits of hydrometeors. Although the minimum size detected is the size of an equivalent pixel in the array, which is 25 micrometers for a standard 2DC probe, the optical depth-of-field for the 2DC is smaller than the distance between the arms of the probe (cf. the above photograph) for hydrometeor sizes smaller than about 150 micrometers. For the original 2DC, the correction needed was also a strong function of the airspeed; see Baumgardner and Korolev (1997), referenced in the "Documentation" section below. Applying corrections for the change in sample volume is difficult, however, because the image can be mis-sized as well and if the wrong size is used to determine the depth-of-field large errors in concentration can result. Modern versions of the 2DC (used at the RAF since about 2009??) have much faster electronic circuitry and do not miss the first scan of an image as did the original version, so the corrections for small size are different from those published by Baumgardner and Korolev (1997) -- although their zero-airspeed result is a good approximation to the behavior of modern probes.