Physical characteristics of a commercially available electronic portal imaging device (EPID), relevant to dosimetric applications in high-energy photon beams, have been investigated. The EPID basically consists of a fluorescent screen, mirrors and a CCD camera. Image acquisition for portal dose measurement has been performed with a special procedure, written in the command language that comes with the system. The observed day-to-day variation in local EPID responses, i.e. measured grey scale value (EPID signal) per unit of delivered portal dose, is 0.4% (1 SD); day-to-day variation in relative EPID responses (e.g. normalized to the on-axis response) are within 0.2% (1 SD). Measured grey scale values are linearly proportional to transmitted portal doses with a proportionality constant which is independent of the thickness of a flat, water-equivalent absorber in the beam, but which does significantly depend on the size of the applied x-ray beam. It is shown that the observed increased in EPID response with increasing field size is mainly due to contributions to the EPID signals from scattered light: visible photons produced by the x-ray beam in a point of the fluorescent screen not only generate a grey scale value in the corresponding point of the EPID image, but also lead (due to scatter from components of the EPID structure onto the CCD chip) to an increased grey scale value at all other points of the image. A point spread function, derived from measured data and describing the increase in EPID response at the beam axis due to off-axis irradiation of the fluorescent screen, has been successfully applied to connect portal doses with grey scale values measured with the EPID.

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