Projectors that use an optical system to project the image on a slide through the lens. The slide was originally a square frame that supports a piece of glass. The glass had imagery painted or printed on it. These type of projectors vary significantly not only in size and brightness, but also in the way slides are moved or transported into the focus plane. Small (domestic) Carousel projectors famously use the eponymous circular slide carousel to show many slides sequentially. The large Austrian made Pani projectors and German Reiche & Vogel projectors initially only allowed for a single large glass slide to be shown. French made ETC Audiovisuelle's PIGI projectors and Hardware Xenon's models later (late 1970s) used a scrolling film mechanism. They all need sophisticated cooling mechanisms to stop the slides from burning out and the colors from fading as a result. When used judiciously  color brilliants and resolution of slide projectors is still unmatched by video projectors, but that gap is closing.