Kurt Kramer introduced a selenium photocell to medical research. In 1940, Glenn Millikan built the first practical selenium-based oximeter to monitor the physical condition of the Royal Air Force pilots. It was a RO where the light source and detector were separated by the ear lobe of the pilot. In the early 1950s, Teletronix used the "T4" optical attenuator in the LA-2 compressor, furthermore, for their unique sound, they are still used today by Universal Audio in their reproductions of the LA-2. After the 1950s selenium in photocells was gradually replaced by CdS and CdSe. By 1960, ROs based on incandescent lamps and CdS/CdSe photoresistors were used in feedback circuits in the industry, for example, for controlling rotation speed and voltage. In the early 1960s, the introduction of sensitive and compact CdS/CdSe photoresistors resulted in the mass production of cameras with automatic exposure. However, these photoresistors were not adopted in medicine because of their memory effect and rapid aging – they required regular recalibration that was not acceptable for medical practice. In the early 1960s, Gibson and Fender started using ROs to modulate the tremolo effect in guitar amplifiers. Both companies were assembling their ROs from discrete lamps, photoresistors and coupling tubes. While Gibson used cheap but slow incandescent lamps as light sources, Fender replaced them with neon lamps, which increased the maximum frequency to tens of Hz and reduced controlling currents, but resulted in a nonlinear modulation. Therefore, other producers preferred incandescent lamps for their linearity. In 1967 Vactec introduced a compact RO branded as Vactrol. Unlike the tube-coupled ROs of Fender and Gibson, Vactrols were sealed and sturdy devices. In the early 1970s, Vactec replaced incandescent bulbs with LEDs. This increased the switching speed, but not to the level required for digital devices. Therefore, the introduction of the faster photodiodes and phototransistors in the 1970s pushed out ROs from the market. ROs retained narrow application niches in sound equipment and some industrial automation devices that did not require high speeds. Vactec did not extend their rights to the trademark Vactrol, and it has become a household word in the English language for any RO used in audio equipment, including the ROs of Fender and Gibson.