An area dosimeter for measuring the ambient equivalent dose of photon radiation with a diffuser, and a detector card with at least one pair of detection elements, preferably LiF-chips. A first of the two detection elements is positioned between two filters in order to spectrally filter the photon radiation. A second of the two detection elements is not positioned between such filters in order that the photon radiation arriving at the second detection element will have a different spectral distribution from the spectrally filtered photon radiation arriving at the first detection element. The two measurement values are used to obtain a weighted sum in order to achieve an optimized response characteristic.