Cold-cathode discharge lamp assembly

Abstract: A cold-cathode electric discharge lamp assembly includes a cold-cathode discharge lamp unit (1) and a supplemental light source (12) for emitting light to a discharge space between a pair of non-pre-­ heat electrodes (3a, 3b) of the discharge lamp unit (1) for producing initial electrons in the discharge space.

Description: The present invention relates to a cold-cathode electric discharge assembly, and more particularly to such an assembly having an improved darkness characteristic. A cold-cathode discharge lamp is used as a light source in many applications, e.g. in the field of office machines, such as copying machines. The cold-cathode discharge lamp has the merit of compactness of size, but has the disadvantage of a lack of ability to start its discharge in darkness (this ability will be referred to as the darkness characteristic hereafter). As is well known, the cathode of the cold-cathode discharge lamp is not pre-heated at the start of the discharge. The cold-cathode discharge lamp used in a copying machine is mounted at a dark position in the machine. In this case, the lamp cannot rely on environmental light to assist starting. Thus, the cold-cathode discharge lamp must have a supplemental means for producing initial electrons therein or near the lamp. In conventional cold-cathode discharge lamps, it is known to provide a radioactive element, e.g. a salt compound of promethium 147 Pm, in the lamp bulb. Radioactive rays emitted from the salt compound of promethium 147 Pm produce electrons in the lamp bulb. Then, the discharge lighting can start immediately with the aid of the radioactive rays, even if the cold-cathode discharge lamp is located in a dark place. Thus, the darkness characteristic of the cold-cathode discharge lamp is improved. In this conventional cold-cathode discharge lamp, a relatively small amount of the radioactive element is sealed in the lamp bulb so as not to cause a serious problem to the human body. For example, about 10⁻¹⁶ grams of the salt compound of promethium 147 Pm is sealed in the lamp bulb. If a large amount of the radioactive element is used, serious problems can be caused on the human body by the radioactive rays emitted outside the bulb. If a small amount of the radioactive element is used in the lamp bulb, the darkness characteristics can become poor. From US-A-3.828.214 it is also known to arrange an additional discharge lamp around the main lamp in a common envelope to start the discharge in the main lamp. Since the additional lamp surrounds the main lamp, light from the second lamp disturbs the light distribution of the main lamp. It is an object of the present invention to provide a cold-cathode discharge lamp with an improved darkness characteristic. The present invention provides a cold-cathode electric discharge lamp assembly comprising an electric discharge lamp having a light permeable sealed elongate envelope containing a discharge gas and a pair of discharge electrodes spaced apart in the envelope to define a discharge space therebetween, which electrodes, in operation, are non pre-heated, at least one end of the envelope being mounted in a lamp base, and means for starting an electric discharge between the electrodes, characterised in that the discharge starting means comprises a light source mounted in the lamp base in such manner that, when energised, light is directed from the light source to the discharge space. In order that the invention may be more readily understood, it will now be described, by way of example only, with reference to the accompanying drawing which shows diagrammatically one embodiment of the invention. A cold-cathode electric discharge lamp assembly comprises an electric discharge lamp 1 and means 6 for starting a discharge in the lamp. The lamp has an elongate envelope 2 in the form of a slender glass tube with the thickness of around ten millimeters. An inner surface of the envelope 2 is coated with a fluorescent film which is not illustrated in the drawing. Opposite ends 7a and 7b of the envelope 2 are hermetically sealed and the envelope is filled with a conventional discharge gas, e.g. xenon gas, krypton gas, or mercury vapour. Discharge electrodes 3a and 3b are provided in the lamp envelope 2 supported at the sealed ends 7a and 7b, respectively. These discharge electrodes 3a and 3b are coated with electron emissive material, such as barium nitride BaN, etc. These discharge electrodes 3a and 3b are connected to power supply conductors 8a and 8b, respectively, through lead wires 9a and 9b, which are soldered at 10a and 10b to the respective conductors. The discharge lamp unit 1 has an illumination surface A defined in the envelope 2 for illuminating an object, e.g. a light sensitive drum D of a copying machine. Lamp bases 4a and 4b are provided for mechanically coupling the cold-cathode discharge lamp 1 to a prescribed apparatus, e.g. a copying machine. The lamp bases 4a and 4b are fixed to the sealed ends 7a and 7b, respectively. The lamp bases 4a and 4b also house the lead wires 9a and 9b and respective ends of the power supply conductors 8a and 8b. Thus, they are protected by the lamp bases 4a and 4b. A discharge stabiliser 5 is provided for depressing flickering of a positive column (not shown) which occurs between the discharge electrodes 3a and 3b in the discharge lighting. The discharge stabiliser 5 comprises a strip or line 11 coated on the outer surface of the lamp envelope 2. In use, a prescribed potential is applied between the stabiliser electrode 11 and one of the discharge electrodes, e.g. the discharge electrode 3b, so that flickering of the positive column is depressed. One end of the stabiliser electrode 11 is coupled to a power supply conductor 8c through a lead wire 9c. The lead wire 9c is connected to the power supply cord 8c by a soldered joint 10c. The discharge starter 6 comprises a small light source 12, e.g. an incandescent lamp, an LED (Light Emitting Diode), an EL (Electro Luminsecent) lamp, etc. A pair of power supply conductors 8d, 8e are connected to the lamp by way of a pair of lead wires. The light source 12 is mounted in one of the lamp bases 4a and 4b and is arranged so that light emitted therefrom falls on the discharge path between the electrodes. Now the operation of the cold-cathode discharge lamp device of the drawing will be described. When the power supply means is activated, the light source 12 immediately starts to emit light. The light emitted from the light source 12 makes the discharge gas around the discharge electrode 3a in the lamp envelope 2 activate. Thus, a photoelectric conversion takes place in the discharge gas by the illumination. According to the photoelectric conversion, some amount of initial electrons are produced around the discharge electrode 3b. The initial electrons are accelerated by the electric field between the discharge electrodes 3a and 3b. The accelerated electrons collide with molecules of the discharge gas sealed in the envelope 2. Thus, further electrons, e.g. secondary electrons, are produced according to the collision. The initial electrons and the secondary electrons again produce further electrons one-by-one. Thus, electrons propagate rapidly in the envelope 2. When the electrons propagate in a sufficient amount, a discharge takes place between the discharge electrodes 3a and 3b. According to the embodiment of the present invention as described above, the light radiated from the discharge starter 6, i.e. the light source 12, produces electrons in the discharge lamp unit 1. Thus, initial electrons as a seed necessary for starting the discharge lighting are securely obtained. As a result, the cold-cathode discharge lamp device can securely and rapidly start the discharge lighting, although the device is located in a dark place. In other words, the cold-cathode discharge lamp device according to the present invention has an improved darkness characteristic. The discharge starter 6 faces the discharge electrode 3b, but the starter 6 is housed in the lamp base 4b. Light radiation from the discharge starter 6 to the outside of the cold-cathode discharge lamp device is restrained. In some office machines, light other than the light emitted from the discharge lamp unit is required to be masked. For office machines, the embodiment of the cold-cathode discharge lamp device has good adaptability. The light source 12 can be de-activated after the discharge lighting has taken place in the discharge lamp unit 1. The timed operation of the discharge starter 8 can be made in a known manner, such as by use of a delay device. In case of the discharge electrode 3b being of plate-shape, the discharge starter 6 can be aligned with the surface of the plate-shaped discharge electrode 3b. In the illustrated embodiment, the discharge starter 6 is provided in the lamp base 4b, but the present invention is not limited to this arrangement. The discharge starter 6 may be provided at any position facing the discharge space of the discharge lamp unit 1, e.g. on the glass tube 2. Further, a plurality of discharge starters, e.g. two discharge starters, can be provided in the pair of lamp bases, respectively.

Claims: A cold-cathode electric discharge lamp assembly comprising an electric discharge lamp (1) having a light permeable sealed elongate envelope (2) containing a discharge gas and a pair of discharge electrodes (3a, 3b) spaced apart in the envelope to define a discharge space therebetween, which electrodes, in operation, are non pre-heated, at least one end of the envelope being mounted in a lamp base (4a), and means (6) for starting an electric discharge between the electrodes, characterised in that the discharge starting means (6) comprises a light source (12) mounted in the lamp base (4a) in such manner that, when energised, light is directed from the light source to the discharge space. A lamp assembly as claimed in Claim 1, characterised in that the discharge starting means (6) additionally comprises a further light source mounted in a further lamp base (4b). A lamp assembly as claimed in Claim 1 or 2, characterised in that the or each light source (12) is an incandescent lamp. A lamp assembly as claimed in Claim 1 or 2, characterised in that the or each light source (12) is a light emitting diode. A lamp assembly as claimed in Claim 1 or 2, characterised in that the or each light source (12) is an electro-luminescent lamp.

Inventors: YOSHIIKE YOSHIJI C O PATENT DI, YOSHIIKE, YOSHIJI C/O PATENT DIVISION
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1988
Main classification: H01J61
IPC codes: H01J61