Abstract: A collimator which is adapted to be mounted on the tube head cone of any existing panoramic X-ray machine used in dental examinations. The device personalizes the radiation from an X-ray machine to the size and shape of a patient's head by accurately allowing the X-rays to be directed only to that part of the head which requires the examination. The collimator of our invention comprises a housing which is mounted on the tube head cone of an X-ray machine, the housing having a vertical slot adapted to be aligned with the vertical slot in the X-ray tube head. A vertically-adjustable slide member regulates the open length of the slot in the housing, the slide member being regulated in its position by a pointer which extends from the slide and has an end disposed at a selected part of the patient's head. Thus, the length of the slot in the housing may be regulated to the shape of the patient's head to eliminate radiation to the eyes and retinal tissue, in the upper limit, and under the chin in the lower limit.