A @W06659@ converter converts @R05048@ at one @W06659@ to radiation at another detectable @W06659@ or at a @W06659@ of improved @R05347@ of the detector. The classical @W06659@ converter consists of a screen of luminescent material that absorbs radiation and radiates at a longer @W06659@. Such materials are often used to convert @UT07492@ to @VT07496@ radiation for detection by conventional phototubes. In @X06718@ a converter that emits optical radiation is called a @S05507@. In most cases @W06659@ conversion is from short to long @W06659@, but in the case of conversion of long to short @W06659@ the process is sometimes called @U06574@. Wavelengths of coherent sources can be converted using nonlinear optical techniques. A typical example is @F02527@.