A recently developed digital image analysis technique potentially capable of sizing particles of arbitrary shape and size and with wide dynamic range is examined. Shadow images of droplets or particles are produced in this case by back-illumination using an infra-red diode laser. The measurement performance of the particle/droplet image analysis (PDIA) system has been assessed initially in terms of individual object diameters and the aim of this experimental investigation was to assess the robustness and accuracy of the technique. The first part of this paper provides a description of the fundamental principles of the technique followed by a thorough description of the calibration procedure in which the image processing routine was verified with calibration data of known particle sizes. The lens-camera optical behavior has been characterized whilst the relative uncertainties of various parameters such as the depth-of-field dependence on particle diameter, threshold level sensitivity and image signal-to-noise issues have been quantified and their effects on measurement accuracy discussed. Calibration data revealed that the depth-of-field varied with object diameter approximately linearly in the measured range 18 to 145 mum. The calibration data is subsequently incorporated into the PDIA processing algorithm and for defocused droplets or particles, enables their true diameter to be estimated by analyzing the droplet image properties. Greater emphasis on the application of the PDIA technique is deferred to the second part of this two-part paper where droplet size data obtained from a hollow-cone spray is compared with similar data measured via phase Doppler anemometry (PDA).