Patent GB-703127-A (GB)
Title: Improvements in and relating to electric incandescent lamps
IPC: H01K1/32
Filed: 1952-01-03
Abstract: 703,127. Electric Incandescent lamps. BRITISH THOMSON-HOUSTON CO., Ltd. Jan. 3, 1952 [Jan. 8, 1951], No. 257/52. Class 39 (2) To obtain increased light production at reduced power input, the bulbs of electric incandescent lamps are provided, on at least one surface, with a coating of titanium dioxide, which transmits visible light and reflects a portion of the infra-red radiation back to the filament. Preferably both the interior and exterior surfaces of a spherical bulb are coated with titanium dioxide, and a tungsten filament is disposed substantially at the centre. With a peak infra-red radiation of about 10,000 A to 11,000 A from a tungsten filament, and using titanium dioxide having a refractive index of about 2.5, the thickness t of the oxide film to give maximum reflection at 11,000 A is derived from the formula 2 # t = (2k-1/2) 1, where # is the refractive index; for maximum reflection k=1, and with 1=11,000 A then t=1100Š. An apparatus for coating the interi or surfaces of bulbs with titanium dioxide comprises a container 6 for titanium tetrachloride which is preferably heated to a temperature of about 10‹C. above normal to facilitate its vaporization, the container being provided with a valve-controlled inlet pipe 4 for dry air and an outlet pipe 8 for the mixture of dry air and vaporous titanium tetrachloride. Humidified or moistened air is also introduced into the pipe 8 from a conduit 13 connected to a vessel 11 containing water, this vessel also having a valve-controlled inlet pipe 9 whereby air is introduced above the level of the water. The contact of the moist air with the titanium tetrachloride vapour in the pipe 8 produces a chemical reaction in which the tetrachloride vapour is converted to titanium dioxide, this reaction continuing as the fluids progress through a chamber 14 into a conduit 15 passing through a stopper 17 into the lamp bulb 21. The upper end of the conduit 15 is flared outwardly at 18 so that, in conjunction with a deflector 19 fixed around an exhausting conduit 20 arranged concentrically within the conduit 15, it provides a guide which directs the vapour or gaseous material along the inner periphery of the bulb. The conduit 20, extending downwardly through the chamber 14, is connected to a vacuum pump (not shown), and serves to produce a steady flow of the gaseous reaction product over the inner surface of the bulb, resulting in a uniform transparent film-like deposit of titanium dioxide the thickness of which is dependent on the time of exposure. During the coating process the bulb 16 is heated by a surrounding furnace 22 to a temperature within the range 50‹-250‹C., preferably to about 150‹C. Apparatus for coating the exterior of the bulb with titanium dioxide operates similarly to that shown in Fig. 2, with the difference that the titanuim tetrachloride vapour and moistened air are introduced through separate inlets to a chamber containing a bulb which is supported horizontally and rotated continuously about its horizontal axis. Fig. 4 shows an example of a lamp made according to the invention, with inner and outer coatings 43, 44 of titanium dioxide; and a helical tungsten filament 45 located along a diameter of the bulb.