April 20198 MANUFACTURINGTECHNOLOGYINSIGHTSIN MY OPINIONBy Arun Dutta, Director Strategic Innovation, LEDVANCE LLCSolid sstate lighting involving light emitting diodes (LEDs) continues to play a dramatic role in reduction of electrical energy usage in the USA. Use of LEDs in residential lighting, for example, results in a reduction of electrical power consumption by about 80-88% at equivalent light output. Several of these light sources consist of LEDs placed on suitable substrates, housed in glass envelopes and driven by sophisticated electronics. The glass envelopes often employ specialized coatings on the surface to achieve one or more objectives: glare reduction, optimal angular distribution of the light, spectral tuning, lumen maintenance etc. The coatings are composed of one or more inorganic materials like oxides, silicates, aluminates and phosphates. The 50% particle size of the powder ranges from 20 µm to 40 µm.In some applications, the powder layer is deposited via an electrostatic coating process. In this process, the glass envelope is rotated around a vertical axis with a flame impinging on the external surface of the glass. Powder is supplied via a gravimetric feeder into a venturi powered by a carrier gas with controlled psychrometric properties. The gas carries the powder particles into the angled nozzles of a coating gun which has a central high voltage probe running at around 60-75kV that creates a corona discharge to charge the particles. The flame serves to both heat the glass envelope to a temperature for optimal electrical conductivity and to create a ground potential for the envelope. The former is required for creating a mirror charge on the inside glass surface for initial adherence of the charged particles. The latter helps to create the electric field for migration of the particles from the probe to the inside glass surface. A subsequent heat treatment enhances the adherence of the powder layer to the glass.The charge to mass ratio (q/m) of the particles is a key parameter that determines the quality of the electrostatic coating. A low value of q/m means poor particle charging and inadequate adhesion of the powder layer. The ratio is directly proportional to the electric field strength and inversely proportional to the particle size and density. The ratio is also influenced by the dielectric constant which varies from 4 to 6 for the materials involved.Typical q/m for the coating process varies from 1-3 µC/g.The adjoining chart shows the angular light distribution that a LED lamp must comply with in order to be classified as an Omni-directional lamp per the US Energy Star voluntary Program.Not only must a certain amount of light be emitted at low angles near the base but the overall light distribution from 0 to 135° needs to have a minimum threshold of uniformity. Electrostatic powder coating of the inside of the glass bulb, via the process discussed here, helps to achieve this Omni-directionality. The coating powder is silica and/or alumina. In some applications, the light emitted by the LED lamp is tailored to be richer or deficient in certain regions of the visible spectrum. This helps to render objects of color more vividly. COATINGS TECHNOLOGY FOR LIGHT SOURCESArun DuttaCoatings play an important role in lighting and will continue to do so because of the value they add
< Page 7 | Page 9 >