Two measurement techniques capable of monitoring droplet sizes and number density in optically thick sprays are presented: both techniques use infrared probe beams in order to minimize the attenuation from the high number density of droplets in the spray. The first technique relies on multiple wavelength extinction from coaxial beams (wavelengths 1.06 and 9.27 μm). This method provides a line-of-sight measurement of the Sauter mean diameter for the spray. The second technique uses forward scattering from a 9.27-μm beam and 90° scattering from a 1.06-μm beam to produce size, again Sauter mean diameter, at specific locations within the spray. Simultaneous application of the two techniques to the same region of the spray has been used to cross-validate the measurements: agreement on droplet size is excellent and well within the predicted error levels. In addition to providing details of the diagnostic technique, this paper discusses potential sources of error for the measurement, namely, detector noise and calibration, size distribution effects, multiple-scattering and beam-steering considerations, droplet sphericity, optical thickness effects and a correction for optical thickness, and the effect of size distribution widths. Results for the example spray used in this work, a pressure-atomized single-hole diesel injector, indicate droplet diameters of 3 μm at 25 mm from the injector tip along the spray axis on the spray centerline, compared with 4 and 7 μm at radii 2 and 3 mm from the centerline, respectively. The diagnostic shows great promise for providing detailed information on the structure and temporal character of diesel-type sprays in a region that is relatively unexplored: the optically thick zone near the injector orifice.