An optical isolator, or optical diode, is an optical component which allows the transmission of light in only one direction. It is typically used to prevent unwanted into an, such as a. The operation of conventional optical isolators relies on the (which in turn is produced by ), which is used in the main component, the The earliest opto-isolators, originally marketed as light cells, emerged in the 1960s. They employed miniature as sources of light, and (CdS) or (CdSe) photoresistors (also called light-dependent resistors, LDRs) as receivers. In applications where control linearity was not important, or where available current was too low for driving an incandescent bulb (as was the case in vacuum tube amplifiers), it was replaced with a. These devices (or.