This invention relates to a method of measuring a property of an ion beam, for example an ion beam current profile or the emittance of an ion beam. A Faraday array comprising an array of ion beam current sensors is employed. The array can provide an ion beam current profile at the plane of the array. The Faraday array is also used in conjunction with an occluding element that may be moved through the ion beam upstream of the Faraday array, there obscuring varying portions of the ion beam from the Faraday array. Suitable manipulation of the signals from the Faraday allows the ion beam current profile to be determined for the plane of the occluding element, and also for the emittance of the ion beam at the plane of the occluding element to be determined.