The earliest opto-isolators, originally marketed as "light cells", emerged in the 1960s. They employed miniature incandescent light bulbs as sources of light, and cadmium sulfide (CdS) or cadmium selenide (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 neon lamp. These devices (or just their LDR component) were commonly named "Vactrols", after a trademark of Vactec, Inc. The trademark has since been genericized, but the original Vactrols are still being manufactured by PerkinElmer.