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