Projection television

Optoelectronics covering of displays, from small LCDs in cellular phones to large rear-projection television sets or screens designed for audio-visual presentations... Transparent technical parts indicators, dials, inspection holes, caps, casings, hoods and other electrical parts... 

Oxyhalides. The oxyhalides of yttrium, lanthanum, and gadolinium are good host lattices for activation with other rare-earth ions such as terbium, cerium, and thulium. The use of LaOCl Tb3+ as the green component in projection-television tubes has been discussed [5.419]. LaOBr Tb3+ and LaOBr Tm3+ exhibit high X-ray absorption, and they are used in X-ray intensifying screens [5.420]. 

In contrast to organic chromophores, luminescent lanthanide complexes are believed to be promising candidates to solve this problem. The spectroscopic properties of some lanthanide ions are ideal for use in full color displays, as is known from inorganic luminescent materials in cathode-ray and projection television tubes. Luminescent lanthanide complexes belong to a special class of emitters, exhibiting the following important advantages. 

L.J. Hornbeck, Current status of the digital micromirror device (DMD) for projection television applications, IEEE International Electron Devices Meeting, Tech. Digest 1993, 381-384. 

P-55 ZnSrAg + Y202S Tb -1-(Zn,Cd)S Cu Al blue + green + red medium projection television... 

One fast electron creates in the luminescent material many electron-hole pairs which recombine on the luminescent center. This multiplication is one of the factors which have determined the success of the cathode-ray tube as a display. It will be clear that the luminescent materials applied belong to the class of materials where excitation occurs in the host lattice. They will be discussed in Chapter 7 where we will also deal with materials for projection television. In this way the display screen can have a diameter of 2 m. This application puts requirements on luminescent materials which are hard to satisfy. 

Since we are dealing with a vacuum tube, the upper limit of the screen which can be reached by present-day technology is about 73 cm. Piaures with a 2 m diameter can be obtained in projection television (PTV). For each of the three colors a small (monochrome) cathode-ray tube is used. Their images are optically projected and superimposed on a projection screen using a lens system. In such a way a composite picture in full color is shown on the screen (Fig. 7.3). In order to obtain high illumination levels on the large screen, much higher current densities have to be used in PTV than in direct-view cathode-ray tubes. 

As argued in Sect. 7.1 one of the problems with cathode-ray phosphors in projection television is the saturation of their light output under high excitation density. Related to this is the temperature increase of the phosphor under these circumstances the screen temperature can rise to 100 C [8]. 

In the fields of projection television phosphors the situation is less. satisfactory. It is clear that luminescent materials have difficulties in meeting the high requirements. At the moment the largest need is for a blue-emitting phosphor with an acceptable saturation. The bad situation is well illustrated by the blue phosphor used, viz. ZnS Ag. Its high radiant efficiency in direct-view television tubes ( 20%) decreases to less than 5% under the conditions of the projection-television tube. Nevertheless it has not been possible, up till now, to find an acceptable alternative. 

The reader will recognize these from earlier paragraphs Y203 Eu + as a lamp phosphor (Sect. 6.4.1.4) and a projection-television phosfi or (Sect. 7.3.4),... 

Part of the saturation effects in projection-television phosphors (see Sect. 7.3.4) can be ascribed to an upconversion process of the type shown in Fig. 10.7. This is very similar to the Auger processes mentioned for semiconductors (see Sect. 4.6). Often this type of upconversion prevents a material from becoming a good laser matmal. If the stimulated emission radiation is reabsorbed by ions which are still in the excited state, the laser efficiency drops. From Fig. 10.7, it becomes clear that the upconversion process, which is, in this case, usually called excited state absorption, influences the population inversion in a negative way considering the two ions in Fig. 10.7, the population inversion is complete before the upconversion occurs, but after upconversion and nonradiative decay to the emitting state, the population... 

In color television, where the image is reproduced by selective cathode excitation of three phosphors (blue, green and red) deposited on the internal face of the screen, yttrium oxysulfides activated with trivalent europium (Y202S Eu ) facilitate such a gain in the brilliance of red over ZnS Ag (more than double it) that they have totally replaced it at a cost about five times less. The exceptional performance of the rare-earth phosphors has also been used gainfully in a vast number of cathode tubes for professional applications color computer monitors, tubes for aviation use, projection television, etc. 

There are plenty of advanced products for phosphors used in tri-chromatic fluorescent lighting, phosphor converted light emission diodes (pcLED), X-ray imaging, specialty lamps, back lighting for liquid crystal displays (LCD), cathode ray tubes (CRT) for televisions and monitors, plasma display panels (PDF), projection televisions (PRT), and field emission displays (FED). 

Raue R., Vink A. T. and Welker T. 1989. Phosphors screens in cathode ray tubes for projection television Phillips, Technol. Rev. 44 335-347. 

The refractive index varies with wavelength of light and is measured by the optical dispersion—that is, the difference in the refractive indexes for different wavelengths. It is responsible for the spectrum-separating ability of a prism in a spectroscope. Most plastics have relatively low optical dispersion. This property makes them more suitable for eyeglasses and large lenses for projection television. 

Polymers can also be used to manufacture lenses and screens for projection television systems. These are most conveniently made from PMMA, or combinations of glass and PMMA, to counteract the high thermal expansion of the polymer. The use of ultraviolet curable coatings for lens replication and protective layers is widespread, and these systems are based on diacrylate or dimethacrylate monomers mixed with photoinitiators such as... 

Good, W. 1975. Projection television. IEEE Trans. Consumer Elec. CE-21(3) 206-212. 

Poorter, T. and deVrijer, F.W 1959. The projection of color television pictures. /. SMPTE 68 141. Robertson, A. 1979. Projection television—1 review of practice. Wireless World 82(Sept) 47-52. 

Schiecke, K. 1981. Projection television Correcting distortions. IEEE Spectrum 18(11) 4(LA5. 

Whitaker, J.C. 1994. Electronic Displays Technology, Design and Applications. McGraw-Hill, New York. Yamamoto, Y, Nagaoka, Y, Nakajima, Y, and Murao, T. 1986. Super-compact projection lenses for projection television. IEEE Trans. Consumer Elec. (Aug.). 

High-paformance color laptop computer screens and televisions typically use this display they are also used in projection televisions, but the problem of low light throughput requires subdued ambient lighting conditions to be used. Developments in reactive liquid crystals (Section 2.6) to provide polarizing beam splitters may be helpful. 

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