Tube projector

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A Zenith 1200 tube projector in a home theater

A tube projector is a projector for reproducing video and computer signals.


In contrast to digital projectors such as LCD projectors or DLP projectors , a tube projector does not use an incandescent or metal halide lamp to generate light, but uses very bright optics to project the light generated by three cathode ray tubes directly onto a screen or, in the case of a rear projection screen, via a mirror and a Fresnel lens on a screen. In principle, a tube projector consists of three single-color televisions connected together, whose images are projected congruently onto a shared screen.

The advantages of tube projectors are natural color reproduction , ideal black level , long service life in video mode and extremely high contrast . Due to the elimination of a screen mask , as found on normal tube color televisions, the rasterization that can be determined there and on LCD and plasma televisions is also omitted , which means that all television standards including PAL, NTSC and HDTV can be easily scaled (the latter only multiscan devices, horizontal frequency higher 15.75 kHz) and results in an image without a fly screen structure. However, the devices should be operated in the so-called sweet spot , an area in which they deliver very bright images and at the same time can still depict the required resolution sharply. Resolutions of up to 1920 × 1080 pixels are not uncommon, but this is only an addressable resolution, not the optical one. The optical resolution depends on many factors, provided that the device is correctly set, the most important here: technical implementation of the electrical focus of the electron beam electrostatically or, better, electromagnetic, the active tube size, i.e. the image area used on the tube, the phosphor mixture and the quality of the lenses.

However, direct control of high-resolution multiscan devices is not easily possible because the line structure of the respective video standard is more or less visible. Scalers , formerly line doublers or quadruplers , are required that interpolate an integral multiple of the number of lines if possible. With up to 30,000 hours in video mode, the service life of the tubes is much longer than that of the lamps in other types of projectors.

The very short, fixed ratio of the distance to the projection surface to the image width is to be rated as positive as well as negative; Depending on the lens, an image width of 2 meters can be achieved at a distance of 2.2 meters without perspective distortion. As mentioned, there is no zoom functionality. Otherwise the convergence (of the three cathode ray tubes), which is laborious to set, would have to be readjusted again and again. A short distance from the seat to the screen can only be achieved by mounting the device on the ceiling.

Disadvantages are high weight (from 35 kg), high price (NP from 10,000 €), complex installation (permanent installation is necessary and to get the three images congruent by means of test images through various analog or digital controllers, see convergence ), as well as the Low absolute brightness, which means that the devices can only be used in a relatively dark environment for desktop use (presentation). The latter, however, will not come into play in video mode, due to the high peak brightness (contrast ratio!) And the low proportion of white in video mode (film). To understand the effect of peak brightness on tube projectors: A 10 cm² white surface on the screen is illuminated much brighter than a white surface distributed over the entire screen surface, depending on the device, the image will usually appear brighter in video mode than with an equivalent digital projection . The tubes, like other picture tubes, are also sensitive to burn-in , which is why, if possible, no still images should be displayed over long periods of time.

Tube projectors are only produced by a few manufacturers ( Barco , Runco , VDC ) today . Because of the high price and the properties that are almost ideal for video operation, there is a very active market for used devices.

See also

Web links

Commons : CRT projectors  - collection of images, videos and audio files