1. Field of the Invention
The present invention relates to a circular fluorescent lamp having a tube with a small outer diameter, and a lighting fixture using the lamp.
2. Description of the Related Art
Generally, it is known that the luminous efficacy of a fluorescent lamp changes according to the mercury-vapor pressure ratio of the lamp. The mercury-vapor pressure is controlled by the temperature of a cold spot, which is the coldest portion of the fluorescent lamp during the lamp operation. When the temperature of the cold spot becomes high, more mercury evaporates, so that the luminous flux of the fluorescent lamp can increase. If the temperature of the cold spot becomes too high, then the luminous flux decreases, because, the in excess evaporated mercury absorbs ultraviolet rays generated in the fluorescent lamp, which are changed to visible light.
A circular fluorescent lamp, having an outer tube diameter of about 29 mm and an overall circular outer diameter of 225 mm, can appropriately maintain the cold spot temperature. However, recently, fluorescent lamps having a small tube outer diameter have become available. The temperature of the fluorescent lamp tends to increase because of the small volume of the tube, so that the cold spot can not be appropriately maintained at the proper temperature in the fluorescent lamp. Accordingly, the cold spot can not control the mercury-vapor pressure of the lamp, so that the luminous efficacy may be reduced.
In order to maintain the cold spot of the fluorescent lamp at the proper temperature, Japanese Laid Open Patent Application HEI 11-3682 discloses a circular fluorescent lamp having long and short stems, which seal opposite ends of the tube of the fluorescent lamp. That is, one stem including conductive wires and filament is longer than the other stem. As a result, the longer stem side of the fluorescent lamp has the cold spot. Since the filament generating heat near the long stem is far from the end of the tube as compared with that of the short stem, the end of the long stem of the tube is easily cooled during the lamp operation as compared with the other portions of the tube.
Such circular fluorescent lamp will be described in more detail by way of example shown in FIG. 8 which shows an enlarged longitudinal section around the ends of a conventional fluorescent lamp. The circular fluorescent lamp 30 is provided with a circular tube 31 having a tube outer diameter of 16.5 mm. A pair of stems 32, 33 seal respective ends of the tube 31, which are accommodated by a lamp base 36 having pins 37. Each of stems 32, 33 comprises conductive wires 35, and a filament 34 connected between conductive wires 35. A length H1 of one stem 32 is formed longer than a length H2 of the other stem 33. The lamp base 36 can rotate around the center axis of the circular tube 31. In this case, when the in fluorescent lamp lights, the cold spot 38 occurs at the sealing portion associated with the stem 32, because, the filament 34 generating heat is further apart from the sealing portion for the stem 32.
The conductive wires 35 extended outwardly from the stem 32 are longer than those of the stem 33. Furthermore, the outer conductive wires 35 of the stems 32, 33 are loosely connected to the pins 37. Accordingly, when the lamp base 36 is rotated about within +15 degrees to xe2x88x9215 degrees around the center axis of the tube 31, each of the conductive wires 35 moves with the lamp base 36. As a result, the conductive wires 35 occasionally touch each other. In particular, the touching occurs easily at the side of longer stem 32 because of the looseness of the long outer conductive wires 35. As a result, conductive wires 35 are shorted. If a short circuit occurs, the electrical ballast may be damaged.
According to one aspect of the invention, a circular fluorescent lamp comprises a light-transmitting circular tube, filled with a discharge gas including mercury and a rare gas, having an outer tube diameter in the range of about 14 mm to about 18 mm. A phosphor layer is coated on the inner surface of the light-transmitting circular tube. Each of the stems, sealing opposite ends of the light-transmitting circular tube, holds a pair of conductive wires, of which one end of each is connected to a filament, and the other end of each extends outwardly from the circular tube. A lamp base, arranged between the ends of the light-transmitting circular tube so as to rotate slightly around the center axis of the circular tube, fixes conductive pins which are connected to the conductive wires. An insulator, arranged between the conductive wires, limits the movement of the conductive wires.
According to another aspect of the invention, a lighting fixture comprises the circular fluorescent lamp. A ballast supplies the electric power to the circular fluorescent lamp. The circular fluorescent lamp and the ballast are arranged in a body.
These and other aspects of the invention will be further described in the following drawings and detailed description of the invention.