The diffraction patterns of static particles deviate from those of dynamic particles of the same distribution characteristics. Use of diffraction solutions based on independent scattering to analyse such diffraction patterns leads to a narrower size distribution biased towards the smaller sizes. A 2-D coherent diffraction model using an inter-particle relation PDF function is developed. The model clearly demonstrates the terms due to the individual particle diffraction and the terms contributed by the inter-particle correlation, i. e. the coherent effects. The deviation of the static particle diffraction pattern from that of the dynamic particles can be well predicted. The results of this study provide a general solution for utilizing the forward diffraction technique in static particle measurement. One of the important applications of the new model is in the development of a diffraction reference reticle used for diffraction-based particle sizing instruments verification.