The idea of optically coupling a solid-state optical emitter with a semiconductor sensor was proposed in 1963 by Akmenkalns et al. (US patent 3,417,249). Photoresistor-based optocouplers were introduced in 1968 and are the slower, but most linear, isolating components that still have a niche in the music and audio industry. The commercialization of LED technology in 1968–1970 led to a significant growth in optoelectronic engineering, and by the end of the 1970s various major optoelectronic coupling elements had been developed. The workhorse of the photocoupling element is the bipolar silicon photocrystal sensor, which can achieve enough transmission speed to be used in applications such as electroencephalography, and the fastest photocoupling element is a PIN type diode that utilizes the light guide mode.