April 20199MANUFACTURINGTECHNOLOGYINSIGHTSThis kind of spectral tuning of the light emitted by LED lamps can also be achieved by using powder coatings. In this case, a coating is applied on the inside of the glass bulb that absorbs yellow light in the 570-590 nm wavelength region. The resulting light is crisp white that renders objects more vividly. The coating material used for this application is a combination of certain silicates, phosphates, and aluminates with optimized spectral characteristics.In yet otherlighting applications, protective coatings are applied on photoluminescent powderscalled phosphors. Phosphors are materials that convert incident light of one wavelength to an emitted light of a longer wavelength. They are generally inorganic compounds doped with rare earth ions like Ce, Tb, Eu etc.Protective coatings may be needed to alter the surface chemistry of the phosphor particles. The coating could improve colloidal stability in aqueous media, increase shelf life by reducing solubility and improve performance by lowering affinity for adsorption of certain species. The phosphor particles are usually very fine sized, less than 20 µm in size, and cohesive. Van der Waals type attractive forces become significant compared to gravity forces, which makes the handling of these materials very challenging. An organo-metallic precursor is adsorbed on the surface of the phosphor particles in a low temperature multi-stage fluidized bed. These precursors are usually pyrophoric,so the process must be designed with layers of redundancy for safety. The adsorbed precursor is subsequently oxidized to the protective metal oxide coating in a second fluidized bed that operates at a temperature higher than the first bed. Electrostatic and CVD coating technologies have been discussed in this article. In some other lighting applications, water base coatings and RF sputtered coatings are also used. Coatings play an important role in lighting and will continue to do so because of the value they add.
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