Abstract: A dosimeter for ionizing radiation is of oblong shape. It comprises a gas-filled measuring chamber surrounded by a casing. Two opposite side walls are manufactured of material transparent to the ionizing radiation. One of the transparent side walls is provided with a transparent plate-like electrode. The other transparent side wall is provided with a number of strip-like electrodes extending transversely to the longitudinal direction of the measuring chamber. A guard electrode surrounds the plate-like electrode. The dosimeter being transparent to X-rays can be used particularly in slit radiography equipment in which the slitwidth can be controlled locally and independently along the length of the slit.