Abstract: Method and apparatus for determining the location at which radiation is incident on the sensor system which includes an EMI screening mesh by comparing the actual image detected, which includes the higher order diffraction effects, with a search or sample image applied sequentially at different locations and determing the location of the sample image which gives the closest match. The sample image corresponds to an expected pattern or image from a point source incident at a particular location on a particular window/sensor system, and may be generated from a real source of known location and properties incident on the system, or may be a generated image, such as a standard image such as a cross pattern or determined mathematically, for example using Fourier transforms. Preferably, once the location of the point of incidence of the radiation has been determined, image processing techniques are applied to remove the diffraction spots.