Patent GB-827013-A (GB)
Title: Improvements in or relating to incandescent electric lamps
IPC: H01K1/32
Filed: 1957-01-15
Abstract: 827,013. Coating electric lamp bulbs internally. WESTINGHOUSE ELECTRIC CORPORATION. Jan. 15, 1957 [Jan. 20. 1956], No. 1483/57. Class 39(2) [Also in Group XV] An electric incandescent lamp with a light transmitting bulb or envelope has the internal surface of the envelope provided with a light diffusing coating comprising a mixture of finely divided silica and a finely divided substantially white material having a true density of at least about 4 grams per cubic centimetre, such as titania, the average thickness of the coating being at least 35 microns and the ratio of the coating bulk density to the true density of the coating material being from 0.018 to 0.43, with the product of coating thickness in microns and the coating density ratio being at least 0.9. The purpose of the added white material is to obscure pinholes, neck blows and other imperfections which are not revealed when the lamp is in operation but are apparent at other times. Alternatively the light diffusing coating may comprise a first layer of finely divided substantially white material having a coating bulk density to true material density ratio of at least 0.051, and of such thickness that it has from 0.3% to 3.5% light absorption and a second layer including finely divided silica and having an average coating thickness of at least 35 microns and a coating bulk density to true material density ratio of from 0.018 to 0.43 with the product of coating thickness in microns and coating density ratio being at least 0.9. The coating adjacent the envelope is densified to promote its adherence to the internal surface of the envelope whilst maintaining its light scattering property. The amount of white material admixed with finely ground silica may comprise 0.3% to 10% by weight of the silica coating. The true density of the silica employed is about 2.2 grams per cubic centimetre. Preferably the additive consists of titania, which has a true density of about 4.2. Other suitable additives are barium titanate and zirconia. The silica and the additive may be ground in a air velocity grinder of known type, and are preferably applied to the envelope by an electrostatic process. In this case the envelope 12, Fig. 4, is rotatably supported on an insulating lava chuck 26. its exterior is heated by gas burners 32a, and the coating is applied through an injector nozzle assembly including a probe 66 on a conduit 50 mounted coaxially with a return conduit 68. A D.C. voltage of from 8 to 25 KV is applied between the probe and the burners. The conduit 50 is supplied from the reservoir 36, Fig. 3 of a smoke generator 34. An injector 40 supplied from a compressor 56 through a drier 58 and a pressure regulating valve 42 maintains a circulation of the coating material mixed with air in a conduit 44 leading to a baffle 46 in the upper part of the reservoir 36. A power driven agitator 60 and a pressure gauge 48 are also provided. Alternatively the silica may be deposited by burning ethyl orthosilicate as described in Specification 653,323, or by suspending silica in a solvent of butyl acetate with a nitrocellulose binder. The mixture may be applied to either a clear or internally frosted glass bulb or envelope. When two layers are applied to the envelope a substantially white material, e.g. titania, is applied as a first coating on the internal surface of a normally transparent lamp envelope, and a silica coating applied upon said first coating, the titania having a coating bulk density to true material density ratio of at least 0.051 and a thickness such that its own light absorption has a value of 0.3% to 3.5%. Specification 822,757 also is referred to.