AbstractA new method is introduced to analyze the accuracy of phase‐Doppler anemometry (PDA) for sizing large particles in two‐phase flows. The method is based on Fourier optics theory (FOT) and geometrical optics theory (GOT) to calculate the intensity ratio of refractive and reflective light scattered by a sphere under a slit constraint. To examine the accuracy in calculating the light intensity, the results from the GOT method were compared with those of the direct simulation based on the generalized Lorenz‐Mie theory (GLMT). This newly developed FOT/GOT method also predicts the results of phase jump due to the slit constraint. The performance of dual‐mode phase‐Doppler anemometry can also be simulated by th