The peculiarities of 3D objects image formation with clear shadow projection based on the constructive theory of 3D objects formation under illumination by partially coherent and perfectly incoherent light are investigated. Threshold algorithms for determining the position of boundaries of geometric 3D objects are developed, algorithms taking into account object thickness, light source angular sizes, and projection system angular apertures. These algorithms are based on the application of a true (calculated) threshold or a standard one using the corrective component for thresholds. Cases of weak and strong 3D object volumetricity for partially coherent and incoherent illumination are studied. The analytical equations for these algorithms are given. It is shown that the use of algorithms can significantly improve the measurement accuracy of the extended objects.