Monoframe droplet shadow velocimetry (mf-DSV) is a cost-effective method in the quality control process of the injectors. In this method, the planar (2D) velocity of the droplets is measured by tracking the droplets in a volumetric illumination, known as shadowgraphy technique. An image with two footprints of each droplet in time lapse of two light pulses is used to determine the displacement/velocity. The mf-DSV method could be established by a much less expensive setup than the traditional velocimetry method. A high-power LED-based light source and ordinary camera without capability of double shutter mode are the requirements of this method. In this paper, different stages of image processing including binarization, clustering and diagnostics of droplets pair, and final screening are explained in detail. The challenge of out-of-focus droplets in the shadowgraphy technique is solved by using an accurate method to predict the real size of the droplets. After analyzing the errors raised in the mf-DSV method, a number of dummy and binary images are used to verify the search algorithm for droplet pair detection. Comparing the exact velocity with, that obtained by mf-DSV indicates that the relative error originating from overlapped droplets and their elimination for velocimetry is about 0.2%. The simultaneous measurement of velocity and diameter distributions of a pressure swirl atomizer confirms the performance of mf-DSV in droplet velocimetry and sizing, especially in the secondary atomization with dilute conditions.