Vactec Inc.
2423 Northline Industrial Blvd., Maryland Heights, MO

In the 1960s - exact date is unknown - Vactec Inc. figured among the first producers of Resistive Opto-Isolators (RO), also called Photoresistive Opto-Isolator, VACTROL (after a genericized trademark introduced by Vactec, Inc. in 1967), analog opto-isolator or lamp-coupled photocell.

An 'RO' is an optoelectronic device consisting of a source and detector of light, which are optically coupled and electrically isolated from each other. The light source is usually a light-emitting diode (LED), a miniature incandescent lamp, or sometimes a neon lamp, whereas the detector is a  photoresistor made of cadmium selenide (CdSe) or cadmium sulfide (CdS). The source and detector are coupled through a transparent glue or through the air.

Vactec Inc. also produced a wide range of LDRs (light dependent resistors, photoconductive cells) for varios applications:

• Ambient light sensor for day/night threshold controls,
• automatic brightness contriols for CRT terminals, TV sets, digital clocks,
• automotive, and aircraft instrument displays.

As of 2010, the Vactrol ROs were produced by PerkinElmer, the successor of Vactec Inc. until its illumination and detection solutions businesses were divested as an independent company Excelitas Technologies in November 2010. Excelitas ended the production of ROs in December 2015. As of 2022 Vactrol type ROs are still manufactured by successor of Silonex the Advanced Photonix.and at least two factories in Shenzhen, China.