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A new dosemeter was developed for environmental dosimetry to measure the ambient dose equivalent. The dosemeter consists of a slide containing four CaSO4:Dy pellets and a cilindrical metal filter filled with polymethylmethacrylate. Metal filters of aluminium, stainless steel and copper were studied for compensation of the photon energy dependence of the response of CaSO4:Dy. Three prototype dosemeters were manufactured based on calculations using mass attenuation coefficients. However, a disagreement was found between the calculated and measured response, especially for the aluminium prototype. Therefore additional calculations of the photon energy dependence were performed using a Monte-Carlo code. With these calculations it was possible to reproduce the measured values. On basis of the measurements and the Monte-Carlo calculations a 2 mm copper filter was recommended.