Fluorescent lamps are well-known in the art and are used for a variety of types of lighting installations. Such lamps are characterized as low pressure discharge lamps and include an elongated envelope, whose interior surface is coated with a layer of phosphor, and an electrode at each end of the envelope. The envelope also contains a quantity of an ionizable medium such as mercury, and a starting gas at a low pressure, generally in the range of 1 to 5 mm Hg. The starting gas may consist of argon, neon, helium, krypton or a combination thereof.
One of the most commonly used methods for introducing mercury into such lamps is a mechanical dispensing unit which forms part of a so-called exhaust machine. Mercury is dispensed by the action of a slotted plunger passing through a reservoir of mercury and into the closed exhaust chamber housing the exhaust tube. The mercury falls through the exhaust tube into the lamp. This method of dispensing mercury has many drawbacks. In the first place, the mercury dispensing unit complicates the exhaust machine. In the second place, the amount of mercury introduced into the lamp envelope by this method can not be precisely controlled. The lamp during processing is at a high temperature and is in open communication with the exhaust machine. As a result, it is inevitable that a portion of the introduced mercury evaporates and disappears from the lamp, or a portion of the filling gas is driven out of the lamp. Furthermore, the introduction of mercury through the exhaust tube involves the risk of mercury getting stuck in the exhaust tube so that after lamp sealing, the lamp contains too little or no mercury at all. For these reasons a large excess of mercury, namely a multiple of the quantity required by the lamp is generally introduced. Finally, working with mercury on the exhaust machine requires additional safety Precautions on medical grounds.
An alternative method of dispensing mercury is to place inside the lamp a mercury compound that is inert under lamp processing conditions but can later be activated to release mercury. Disadvantageously, this method releases impurities, which then require special gettering. Moreover, this method requires a relatively long period of time to activate the mercury compound (e.g., 20 to 30 seconds). As a result, this method of dispensing mercury does not readily lend itself to high speed production machinery.
Another method of introducing mercury into an arc discharge lamp is set forth in U.S. Pat. No. 4,553,067 which issued to Roche et al on Nov. 12, 1985 and is assigned to the same Assignee as the present Application. Therein a mercury dispensing target is located within an exhausted lamp having a coil at each end of the lamp. The dispensing target is affixed to a lead-in wire adjacent one of the coils. During processing, the mercury target is heated by bombarding the target with a directed stream of electrons produced by one of the coils which causes the contained mercury to be released. Although this method reduces mercury release times to 3.5 seconds, it is desirable to obtain further reductions.
According to the teachings of the present invention, portions of the mount structure at a single end of the lamp are coated with an insulating coating except for a portion of the mercury target so as to focus the directed stream of electrons during the mercury releasing process on the main body of the mercury target (i.e., the portion containing the mercury). As a result, the time needed to release mercury into the lamp is significantly reduced.
Insulating coatings have been used in fluorescent lamps without mercury releasing targets for the purpose of reducing or preventing end discoloration. Generally, all non-electron emissive internal metal parts of the lamp which are exposed to ion bombardment during normal lamp operation can be coated. For a.c. lamp operation, portions of the mount structure at both ends of the lamp are provided with the insulating coating. U.S. Pat. No. 2,769,112, for example, which issued to Heine et al on Oct. 30, 1956, teaches the application of a refractory insulating oxide (e.g., zirconium oxide) to all metal parts except the cathode in an effort to cure the brown patch or end band problem. U.S. Pat. No. 3,069,580, which issued to Waymouth, Jr. on Dec. 18, 1962, teaches the use of refractory insulating cements over the lead-in wires so that the cathode coil will take all of the electron current during the anode half cycle. U.S. Pat. No. 3,706,895, which issued to Martyny et al on Dec. 19, 1972, teaches the use a coating of a polyimide plastic containing zirconium oxide on portions of the lead-in wires. As stated therein, the coating retards end darkening and will reduce or eliminate brown patches which are caused by ion induced sputtering of the lead wire. U.S. Pat. No. 4,204,137, which issued to Roy on May 20, 1980, teaches coating the portions of the electrode support wires which are exposed to ion bombardment within the body of the lamp with a refractory material such as boron nitride. As stated in this patent, the coating improves the life of lamps by reducing end blackening.