Ellipsoidal Reflector Spotlight, ERS, or just Ellipsoidal is the correct generic name for a large class of lighting instruments more commonly called Leko, Source Four, or Profile Spot (in Europe and other non-US locations). LekoLight (the contraction of the surnames of inventors Joseph Levy and Edward Kook of Century Lighting) was a trademark in 1932 that improved the technology which made it the best ERS of its day. For decades (and continuing today, although the practice is slowly dying), all ERSs were erroneously called Lekos, whether or not they were made by Century or any of its iterations.
In 1992, Electronic Theater Controls (ETC) redefined the technology when they created the Source Four. The key developments are a glass dichroic reflector designed around the HPL lamp with four filament segments--hence the name Source Four. This fixture has been widely adopted as the industry standard today, with well over two-million units sold since its introduction in 1992.
ERS instruments get their name from the elliptically shaped mirror chamber which surrounds the lamp. An ellipse is a shape with two foci. In theory, by placing the lamp at one focus of an elliptically shaped mirror, all light leaving that lamp will be reflected through the other focus. In practice, the ellipse is cut in half before that second focus and instead light is directed through the gate into a lens tube. At this gate are four framing shutters which may be used to shape the beam. Optionally an iris may be placed here to create a smaller circular beam, or a huge variety of steel or glass gobos may project patterns, logos, text, or images.
Older ERS instruments were classified by their lens diameter and focal length: 6x9, 6x12, 6x16, 8x13, 10x23, and so on. Modern ERS instruments are classified by their beam angle: 36°, 26°, 19°, 10°, 5°, et cetera.
ERS instruments can be divided into two categories, based upon how the lamp enters the reflector: the obsolete, less-efficient Radial ERS, and the modern Axial ERS.
Another modern, advanced design is the Strand Selecon Pacific series. Other modern ERSs are the Altman Shakespeare (replaced by the Altman Phoenix October 2011), Strand SL Coolbeam (discontinued July 2009), the most recent incarnation of the Strand LekoLite, Selecon SPX and Pacific, and Leviton Leo. See also Ellipsoidal Reflector Spotlight-Ancient History.
While many manufacturers (most notably Coemar, Robert-Juliat, Prism Projection, Strong, Wybron) are developing LED-based or other non-incandescent ERS fixtures, none on the market currently can compete with the intensity of a conventional unit eqipped with a tungsten-halogen lamp.