| 摘要 |
PURPOSE:To prevent any luminance nonuniformity as well as reductions in the optical output of the entire emitter tube by making an amount of an alkali metal corresponding to the reduction in the pressure of alkali metal vapor existing around the positive electrode to be discharged from an alkali metal alloy installed near the positive electrode when said pressure starts to decrease. CONSTITUTION:When the lamp of this invention is lit by direct current for example, sodium charged in an emitter tube 3 makes positive ions which are then gradually drawn toward a negative electrode 6. As the result, the amount of sodium existing around a positive electrode 5 notably decreases and the pressure of sodium vapor existing around the positive electrode 5 starts to decrease. When the pressure of sodium vapor existing around the positive electrode 5 starts to decrease, an amount of sodium corresponding to the decrease is discharged from a sodium alloy 7 installed near the positive electrode 5 and therefore sodium is supplied near the positive electrode 5. Owing to the above structure, apparently sodium vapor is almost homogeneously distributed in the emitter tube 3 thereby preventing any reductions or nonuniformity of luminance. Consequently, light is homogeneously emitted from the entire emitter tube 3. |
