Discharge lamp

Abstract: A discharge lamp comprises a substantially straight glass bulb (1) filled with a discharge gas (2) and electrodes (5, 6) provided at each longitudinal end portion (10, 11) on the outer surface of the bulb (1). A high frequency voltage is applied across the electrodes (5, 6). The outer surface area per unit length at each end portion (10, 11) of the bulb (1) is larger than that at the light emitting portions of the bulb. The outer surface may be formed uneven or in the shape of ridges and furrows (10a) at the end portions (10, 11) for enlarging the surface area, the electrodes (5, 6) being adapted to the ridges and furrows (10a). A fluophor may be applied to the inner surface of the bulb (1) except for the inner surface portions opposite to the electrodes (5, 6).

Description: The present invention relates to a discharge lamp according to the preamble of claim 1. FIG.1 is a longitudinal cross-sectional view of a fluorescent lamp according to the preamble of claim 1 disclosed in JP-A-60-12660 and FIG.2 an enlarged transverse cross-sectional view thereof. In the figures , the glass bulb 1 is charged with mercury vapor and rare gas 2, and a phosphor 3 is applied to the entire inner wall of the bulb. On the outer circumferential surface of the glass bulb 1 are disposed a pair of electrodes 5,6. The high frequency generating circuit 7, connected to a power source through a switch 11 and a plug 13, applies a high frequency voltage across the electrodes 5,6. The high frequency voltage applied causes discharge 8 in the bulb 1. This discharge excites the mercury atoms to thereby develop ultraviolet rays which in turn cause the phosphor to emit a visible light. Such type of fluorescent lamp suffers from a problem that strong impact by electrons and ions causes deterioration of the phosphor at the portions 4a,4b on the inner surface of the bulb opposite to the electrodes 5,6, causing the amount of light to decrease with time. Therefore the life-time of the discharge lamp will be shorter in optical information apparatuses, particularly a facsimile apparatus where a change in light output with time can be a problem. An object of the present invention is to provide a long-life fluorescent lamp but not having significant change in the amount of light emitted with time. This object is met by the invention as set out in the characterising part of claim 1. Further features and details of the present invention will be apparent from the following description of a specific embodiment which is given by way of example with reference to the accompanying drawings, in which: FIG.1 is a longitudinal cross-sectional view of a prior art fluorescent lamp; and FIG.2 is an enlarged transverse cross-sectional view taken along the line D-D of the prior art fluorescent lamp in FIG.1. FIG.3A is a diagram for showing an embodiment of the invention; FIG.4B is a cross-sectional view taken along the line B-B of FIG.3A; FIG.3 shows an embodiment of the invention and FIG.4 illustrates a cross-section taken along the line B-B of FIG.3. When the high frequency voltage is applied across the electrodes, the discharge 8 takes place as shown. A high electrode drop is developed at the inner surface of a discharge glass bulb opposite to electrodes mounted on the outer circumferential surface thereof. The electrons and ions, accelerated by this voltage, impinge on the inner wall of the glass bulb, causing damage to the phosphor applied. In this embodiment, the phosphor 3 applied on the inner surface of the bulb has apertures 9a,9b, diametrically opposite to each other as shown in FIG.4 and extending longitudinally of the glass bulb. On the outer surface of the bulb 1 are provided electrodes 5,6 at locations opposite to the apertures 9a,9b, where the phosphor does not exist. The electrodes 5,6 also extend longitudinally of the glass bulb. By this arrangement, rapid deterioration of the phosphor 3 due to impact of the electrons and ions is prevented. Additionally, forming the electrodes 5,6 from a high reflection material provides more reflection from the apertures 9a,9b, thus allowing effective use of light emitted.

Claims: A discharge lamp comprising a substantially straight glass bulb (1) filled with discharge gas (2), a phosphor (3) applied to the inner surface of the bulb (1), and electrodes (5, 6) mounted on the outer surface of the bulb (1) for connection to a high-frequency voltage source (7), characterised in that the areas (9a, 9b; 9c, 9d) of the inner surface of the bulb (1) opposite said electrodes (5, 6) are free of the phosphor (3). The lamp of claim 1, wherein said electrodes (5, 6) are disposed longitudinally of the bulb (1) and opposite each other with said bulb (1) therebetween. The lamp of claim 1, wherein said electrodes (5, 6) are disposed at both longitudinal end portions of said bulb (1).

Inventors: ANZAI YOSHINORI C O MITSUBISHI, SAIKATSU TAKEO C O MITSUBISHI, SAKURAI TAKEHIKO C O MITSUBISH, ANZAI, YOSHINORI C/O MITSUBISHI DENKI K. K., SAIKATSU, TAKEO C/O MITSUBISHI DENKI K. K., SAKURAI, TAKEHIKO C/O MITSUBISHI DENKI K. K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1989
Main classification: H01J65
IPC codes: H01J65