Estimators for size distributions and flux measurements using the phase Doppler technique are examined. One important issue identified is the difference between detection and illuminated volumes, the detection volume being a function of both particle size and particle trajectory. Previous estimate of the detection volume dependence on these parameters are reviewed, and an improvement is proposed. A further consideration is the count error due to multiple particles in the volume. A statistical correction for this effect is introduced and shown to be already significant in relatively sparse sprays. Comparisons between various estimators and a simple patternator confirm the improvements of the new estimators.