became enhanced and evolved into high quality, high definition video recording. Michael C. Andersen generated the first known mathematical formula of the video feedback process, and he has also generated a Mendeleev's square to show the gradual progressive formulaic change of the video image as certain parameters are adjusted. In the 1990s the rave scene and a social return to art of a more psychedelic nature brought back displays of video feedback on large disco dance floor video screens around the world. There are filters for Adobe Photoshop and non-linear video editors that often have video feedback as the filter description, or as a setting on a filter. These filter types either mimic or directly utilize video feedback for its result effect and can be recognized by its vortex, phantasmagoric manipulation of the original recorded image. In entertainment Many artists have used optical feedback. An example is Queen's music video for "Bohemian Rhapsody" (1975). The effect (in this simple case) can be compared to looking at oneself between two mirrors. Other videos that use variations of video feedback include: Doctor Who – An Unearthly Child (series, 1963–1973, opening title sequence.) Earth, Wind & Fire — "September" (1978) The Jacksons – "Blame It On the Boogie" (1978) Kate Bush – "Hammer Horror" (1978) Amii Stewart – "Knock on Wood" (1979) Kool & the Gang – "Get Down On It" (1981) Todd Rundgren – "Something to Fall Back On" (1985) Smashing Pumpkins – "Ava Adore" (1998) Klaxons – "Gravity's Rainbow" (2008) Boxcutter – "TV Troubles" (2011) Django Django – "Life's A Beach" (2012) "Howl-around" and Doctor Who title sequence This technique- under the name "howl-around"- was employed for the opening titles sequence for the British science fiction series Doctor Who, which employed this technique from 1963 to 1973. Initially this was in black and white, and redone in 1967 to showcase the show's new 625-line broadcast resolution and feature the Doctor's face (Patrick Troughton at that time). It was redone again, in colour this time, in 1970. The next title sequence for the show, which debuted in 1973, abandoned this technique in favour of slit-scan photography. In science An example of optical feedback in science is the optical cavity found in almost every laser, which typically consists of two mirrors between which light is amplified. In the late 1990s it was found that so-called unstable-cavity lasers produce light beams whose cross-section present