| 摘要 |
A collimating arrangement for use in radiotherapy apparatus has a set of mutually contiguous members (8) for substantially attenuating radiation. The positions of the members (8) are adjustable so that edges (8a) thereof form an adjustable aperture (9) for defining the cross-sectional shape of the beam. A lamp (12) illuminates an area including the beam-defining edges (8a). Each of the members (8) is provided with a respective retroreflector (19) for reflecting illumination from the lamp (12) into the camera (13) substantially independently of the position of the beam-defining edges (8a). A detecting arrangement, including a video camera (13), more responsive to the higher intensity of illumination reflected from the retroreflectors (19) than from respectively adjacent regions in the illuminated area, is used to detect the positions of the retroreflectors (19) which are respectively representative of the positions of the edges of the members. |
