Calculation of the effect of slit size on emittance measurements made by a two-slit scanner

Parallel slit-slit devices are commonly used to measure the transverse emittance of a particle beam, selecting a portion of the beam with the front slit and measuring the angular distribution with the rear. This paper calculates the effect of finite slit sizes on measured emittance and Twiss functions in the case of Gaussian spatial and angular distributions of the oncoming beam. A formula for recovering the true emittance from the measured values is derived.