The collimator is, overall, the most efficient beam-restricting device. It is attached to the tube head, and its upper aperture, the first set of shutters, is placed as close as possible to the x-ray-tube port window. This is done to control the amount of image degrading off-focus radiation leaving the x-ray tube (i.e., radiation produced when electrons strike surfaces other than the focal track). The next set of lead shutters (blades or leaves) actually consists of two pairs of adjustable shutters—one pair for field length and another pair for field width. It is these shutters that the radiographer adjusts when changing the field size and shape. Another important part of the collimator assembly is the light-localization apparatus. It consists of a small light bulb (to illuminate the field) and a mirror. For the light field and x-ray field to correspond accurately, the x-ray-tube focal spot and the light bulb must be exactly at the same distance from the center of the mirror. If the light and x-ray fields do not correspond, IR alignment can be off enough to require a repeat examination. Collimator accuracy should be checked regularly as part of the QA program. NCRP guidelines state that collimators must be accurate to within 2% of the SID. The nine-penny test can also be used to evaluate collimator accuracy, but it has been replaced by the ruled measuring device shown in the figure