The design, laboratory calibrations, and flight tests of a new optical imaging instrument, the twodimensional stereo (2D-S) probe, are presented. Two orthogonal laser beams cross in the middle of the sample volume. Custom, high-speed, 128-photodiode linear arrays and electronics produce shadowgraph images with true 10-mu m pixel resolution at aircraft speeds up to 250 m s(-1). An overlap region is defined by the two laser beams, improving the sample volume boundaries and sizing of small (< similar to 100 mu m) particles, compared to conventional optical array probes. The stereo views of particles in the overlap region can also improve determination of three-dimensional properties of some particles. Data collected by three research aircraft are examined and discussed. The 2D-S sees fine details of ice crystals and small water drops coexisting in mixed-phase cloud. Measurements in warm cumuli collected by the NCAR C-130 during the Rain in Cumulus over the Ocean (RICO) project provide a test bed to compare the 2D-S with 2D cloud (2D-C) and 260X probes. The 2D-S sees thousands of cloud drops <similar to 150 mu m when the 2D-C and 260X probes see few or none. The data suggest that particle images and size distributions ranging from 25 to -150 mu m and collected at airspeeds > 100 m s(-1) by the 2D-C and 260X probes are probably (erroneously) generated from out-of-focus particles. Development of the 2D-S is in its infancy, and much work needs to be done to quantify its performance and generate software to analyze data.
