A PC-based spray imaging system has been constructed to count and to size small particle images (calibrated down to 3.9 mum in diameter). The main objective was to develop a reliable calibration procedure in the spray imaging system for drop sizing, particularly in the range of small particles, and eventually to construct an image processing algorithm for automatic sizing and counting of the particle images. The image characteristics of the calibration reticle were investigated for this purpose, and it was found that there were, in fact, useful parameters appropriate in determining the optimum threshold gray level for drop sizing and selecting the in-focus droplets. Using these parameters, it was found that a rather simple linear relationship existed between D50 (diameter measured with 50% threshold gray level) and real size. Very small errors were detected in the in-focus droplet selection and drop sizing, which were confirmed by the tests employing the calibration reticle and applying the above-mentioned calibration procedure and image processing algorithm.