The term beamer, which is now used for all projectors, originally comes from the tube projectors. This technology was already used for rear projection in the early years of television technology.
The tube projection works with three cathode ray tubes for the three primary colorsred, green and blue. Each cathode ray tube generates the corresponding color separation and projects it onto the projection surface. Since the three cathode ray tubes are arranged at a certain distance from each other, convergence distortions occur between the individual color images.
Since the light energy of the tubes is distributed over the entire projection surface, the tubes must be operated at maximum brightness. This is reflected not least in the life expectancy of the tubes. Despite all the technical refinements, such tube projectors only achieve projected brightnesses of less than 500 ANSI lumens.
The display on the cathode ray tubes is line and image based, as in television and video. Due to improved technologies and the relatively low brightness, which allows projections only in darkened rooms, the tube projection is replaced by other projection technologies.