Abstract: The invention is an instrument comprising a unit having a frame defining the perimeter about an aperture, and a manually adjustable rack connected to the frame for supporting the unit with its aperture disposed between a source of x-rays and the body of a patient. Radio-opaque flexible panels depending from the frame are pivotable relative to the aperture to collimate and limit x-rays issuing from the source through the aperture to a particular area of the body of the patient. Translucent ports along lower edges of the panels serve to provide upon a photographic film below the patient a scale indication of the area of the body of the patient subjected to the x-rays.