There are three general types of two-dimensional imaging probes:
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. The 2D Probes section of RAF Bulletin 24 provides some additional information. The optical configuration is shown in the following diagram:
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.
Overall estimate of uncertainty: For sizes above 125 micrometers, the uncertainty in concentration is approximately as determined by counting statistics, but for sizes below 125 micrometers the concentrations can be in error by as much as a factor of five (arising from mis-application of the depth of field to mis-sized images). Often in processed files there is no correction for reduced depth-of-field at small sizes, so users must apply corrections or else use only the size range where the depth-of-field is limited by the sample arms (i.e., >= 125 micrometers).
A spinning disk with holes of known size or images of known size is used to check the size resolution.
In about 2005, the electronics and data-transmission rates were increased to accommodate the higher airspeed of the GV. In 2011, a development was made to provide a unit with increased resolution.
An example of images from an all-ice region containing large crystals and aggregates is shown below:
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