They are used as collimators or lenses for filter components. Fresnel lenses are also used as collimators in overhead projectors. Collimators (beam limiting devices) are used in linear accelerators used for radiotherapy treatments. Most lead collimators let less than 1% of incident photons through. The un-deflected cluster ions are selected with appropriately positioned collimators. Its purpose was to hold the collimators above the Ge crystals at known, fixed positions. Attempts have been made to replace collimators with electronic analysis. Most collimators have grid patterns for rechecking the zero after the barrel is sighted. Typically collimators are used on either side of the isolator. Collimators may be used with laser diodes and CO cutting lasers. In lighting, collimators are typically designed using the principles of nonimaging optics. Collimators are also used in radiation detectors in nuclear power stations to make directional sensitivity.