HYahoo624APP30. proaudio ws880ims 03 10 2017.Umfangreiche Linkliste zu Selbstbau, Reparatur und Modifikation von Fotoequipment Kameras, Objektive, Beleuchtung, Labor, Studio.Casio Bm 200 W Manual' title='Casio Bm 200 W Manual' />Phosphor Wikipedia.Example of phosphorescence.A phosphor, most generally, is a substance that exhibits the phenomenon of luminescence.Somewhat confusingly, this includes both phosphorescent materials, which show a slow decay in brightness 1 ms, and fluorescent materials, where the emission decay takes place over tens of nanoseconds.Phosphorescent materials are known for their use in radar screens and glow in the dark materials, whereas fluorescent materials are common in cathode ray tube CRT and plasma video display screens, fluorescent lights, sensors, and white LEDs.Phosphors are often transition metal compounds or rare earth compounds of various types.The most common uses of phosphors are in CRT displays and fluorescent lights.CRT phosphors were standardized beginning around World War II and designated by the letter P followed by a number.Phosphorus, the chemical element named for its light emitting behavior, emits light due to chemiluminescence, not phosphorescence.PrincipleseditA material can emit light either through incandescence, where all atoms radiate, or by luminescence, where only a small fraction of atoms, called emission centers or luminescence centers, emit light.In inorganic phosphors, these inhomogeneities in the crystal structure are created usually by addition of a trace amount of dopants, impurities called activators.In rare cases dislocations or other crystal defects can play the role of the impurity.The wavelength emitted by the emission center is dependent on the atom itself and on the surrounding crystal structure.The scintillation process in inorganic materials is due to the electronic band structure found in the crystals.An incoming particle can excite an electron from the valence band to either the conduction band or the exciton band located just below the conduction band and separated from the valence band by an energy gap.This leaves an associated hole behind, in the valence band.Impurities create electronic levels in the forbidden gap.The excitons are loosely bound electronhole pairs that wander through the crystal lattice until they are captured as a whole by impurity centers.The latter then rapidly de excite by emitting scintillation light fast component.In case of inorganic scintillators, the activator impurities are typically chosen so that the emitted light is in the visible range or near UV, where photomultipliers are effective.The holes associated with electrons in the conduction band are independent from the latter.Those holes and electrons are captured successively by impurity centers exciting certain metastable states not accessible to the excitons.The delayed de excitation of those metastable impurity states, slowed down by reliance on the low probability forbidden mechanism, again results in light emission slow component.Phosphor degradationeditMany phosphors tend to lose efficiency gradually by several mechanisms.The activators can undergo change of valence usually oxidation, the crystal lattice degrades, atoms often the activators diffuse through the material, the surface undergoes chemical reactions with the environment with consequent loss of efficiency or buildup of a layer absorbing either the exciting or the radiated energy, etc.The degradation of electroluminescent devices depends on frequency of driving current, the luminance level, and temperature moisture impairs phosphor lifetime very noticeably as well.Harder, high melting, water insoluble materials display lower tendency to lose luminescence under operation.Examples Ba. Mg. Al.O1. 7 Eu. BAM, a plasma display phosphor, undergoes oxidation of the dopant during baking.Three mechanisms are involved absorption of oxygen atoms into oxygen vacancies on the crystal surface, diffusion of EuII along the conductive layer, and electron transfer from EuII to adsorbed oxygen atoms, leading to formation of EuIII with corresponding loss of emissivity.Thin coating of aluminium phosphate or lanthanumIII phosphate is effective in creation a barrier layer blocking access of oxygen to the BAM phosphor, for the cost of reduction of phosphor efficiency.Addition of hydrogen, acting as a reducing agent, to argon in the plasma displays significantly extends the lifetime of BAM Eu.EuIII atoms back to EuII.Y2. O3 Eu phosphors under electron bombardment in presence of oxygen form a non phosphorescent layer on the surface, where electronhole pairsrecombine nonradiatively via surface states.Zn. S Mn, used in AC thin film electroluminescent ACTFEL devices degrades mainly due to formation of deep level traps, by reaction of water molecules with the dopant the traps act as centers for nonradiative recombination.The traps also damage the crystal lattice.Phosphor aging leads to decreased brightness and elevated threshold voltage.Zn. S based phosphors in CRTs and FEDs degrade by surface excitation, coulombic damage, build up of electric charge, and thermal quenching.Electron stimulated reactions of the surface are directly correlated to loss of brightness.The electrons dissociate impurities in the environment, the reactive oxygen species then attack the surface and form carbon monoxide and carbon dioxide with traces of carbon, and nonradiative zinc oxide and zinc sulfate on the surface the reactive hydrogen removes sulfur from the surface as hydrogen sulfide, forming nonradiative layer of metallic zinc.Sulfur can be also removed as sulfur oxides.Zn. S and Cd. S phosphors degrade by reduction of the metal ions by captured electrons.The M2 ions are reduced to M two M then exchange an electron and become one M2 and one neutral M atom.The reduced metal can be observed as a visible darkening of the phosphor layer.The darkening and the brightness loss is proportional to the phosphors exposure to electrons and can be observed on some CRT screens that displayed the same image e.EuropiumII doped alkaline earth aluminates degrade by formation of color centers. Call Of Duty 4 Mw Wallhack Download For Cs . Y2. Si. O5 Ce. 3 degrades by loss of luminescent Ce.Zn. 2Si. O4 Mn P1 degrades by desorption of oxygen under electron bombardment.Oxide phosphors can degrade rapidly in presence of fluoride ions, remaining from incomplete removal of flux from phosphor synthesis.Loosely packed phosphors, e.E. g. In. BO3 Tb.MaterialseditPhosphors are usually made from a suitable host material with an added activator.The best known type is a copper activated zinc sulfide and the silver activated zinc sulfide zinc sulfide silver.The host materials are typically oxides, nitrides and oxynitrides,1.The activators prolong the emission time afterglow.In turn, other materials such as nickel can be used to quench the afterglow and shorten the decay part of the phosphor emission characteristics.Many phosphor powders are produced in low temperature processes, such as sol gel and usually require post annealing at temperatures of 1.C, which is undesirable for many applications.However, proper optimization of the growth process allows to avoid the annealing.Phosphors used for fluorescent lamps require a multi step production process, with details that vary depending on the particular phosphor.Bulk material must be milled to obtain a desired particle size range, since large particles produce a poor quality lamp coating, and small particles produce less light and degrade more quickly.During the firing of the phosphor, process conditions must be controlled to prevent oxidation of the phosphor activators or contamination from the process vessels.After milling the phosphor may be washed to remove minor excess of activator elements.Volatile elements must not be allowed to escape during processing.Lamp manufacturers have changed composition of phosphors to eliminate some toxic elements, such as beryllium, cadmium, or thallium, formerly used.The commonly quoted parameters for phosphors are the wavelength of emission maximum in nanometers, or alternatively color temperature in kelvins for white blends, the peak width in nanometers at 5.ApplicationseditLightingeditPhosphor layers provide most of the light produced by fluorescent lamps, and are also used to improve the balance of light produced by metal halide lamps.Various neon signs use phosphor layers to produce different colors of light.
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