A “vactrol” is a combination of a light dependent resistor (LDR) and light source both enclosed in into a small, light-proof case. In early designs, incandescent lamps were used as light sources. Today LEDs are used due to their faster response, longer lifespan, and lower current consumption. The illumination changes are much faster for LEDs compared to incandescent lamps.
Increasing the LED brightness decreases the LDR resistance. This effect can be used in circuits that require variable resistors to obtain the desired function (e.g., VCFs, VCAs, and others).