When a food is stored in a refrigerator or the like for a long time, the food decays and generates a gas or gases. This can facilitate growth and cultivation of bacteria and fungi (referred to as “germ” hereinafter), generate an offensive smell, and adversely affect other foods in the refrigerator. To avoid such situations, a certain measure is necessary for disinfecting the growing (or grown) germ in the refrigerator. An ultraviolet lamp is often used for this purpose.
One of conventional (common) ultraviolet lamps used for this purpose is a low-pressure mercury lamp. In recent years, use of an excimer lamp is studied because the excimer lamp has a better luminous efficacy than the low-pressure mercury lamp and emits light at a higher light intensity (higher optical output intensity) than the low-pressure mercury lamp. The excimer lamp, however, emits vacuum ultraviolet light which has a wavelength equal to or shorter than 200 nm although it somewhat depends upon a light emitting gas sealed in the lamp. For example, when a xenon gas is sealed in the excimer lamp as the light emitting gas, the excimer lamp emits vacuum ultraviolet light having a dominant wavelength at 172 nm. If this vacuum ultraviolet light is used in the refrigerator as it is, the vacuum ultraviolet light creates ozone in the air inside the refrigerator, and adversely affects a human body. Thus, the vacuum ultraviolet light cannot be used as it is.
In view of such fact, it is preferred that a fluorescent substance or phosphor is disposed on an inner surface of an electric discharge vessel of the excimer lamp such that the vacuum ultraviolet light having a wavelength equal to or shorter than 200 nm, which is generated upon excimer light emission in the electric discharge space, is converted to ultraviolet light having a wavelength between 230 nm and 250 nm, which is longer than the wavelength of the vacuum ultraviolet light, and the resulting light is emitted to the outside. By doing so, no ozone is generated in the air inside the refrigerator. The ozone is harmful to the human body. Also, it is possible to generate light having a peak wavelength between 230 nm and 250 nm which is effective for disinfecting colon bacilli, Staphylococcus aureus and the like.
However, if the fluorescent substance is provided on the inner surface of the electric discharge vessel of the excimer lamp, the fluorescent substance is exposed to the spark plasma. As a result, a problem arises, i.e., gas impurities and/or moisture is generated from the fluorescent substance, and they may remain in the electric discharge vessel. If the gas impurities and moisture remain in the electric discharge space, the irradiance steeply drops and therefore the disinfection capability decreases. Thus, a problem arises, i.e., sufficient disinfection is not performed. In addition, if sufficient disinfection should be performed, a disadvantage arises, i.e., the disinfection time should be extended.
On the other hand, there is a technique to remove gases and moisture by means of a getter disposed in the excimer lamp. For example, Japanese Patent Application Laid-Open Publication No. 2006-228563 (Patent Literature 1) discloses a technique to provide an inner tube in an electric discharge vessel and provide a getter in the inner tube. This conventional technique, however, requires provision of the inner tube inside the electric discharge vessel among other requirements, and therefore the electric discharge vessel has a complicated inner structure. It is troublesome to fabricate such electric discharge vessel. In an alternative configuration, a separate getter chamber is provided in the electric discharge vessel, which is separate from the light emitting space. In this configuration, the electric discharge vessel has to have a long length, and the inner structure of the electric discharge vessel becomes complicated.
In order to perform the disinfecting treatment in the refrigerator, downsizing of the lamp is necessary, and the lamp has to have a simple structure. The above-mentioned structures are not suitable for use in the refrigerator. Thus, there is a demand for an excimer lamp that can be used in the refrigerator, i.e., an excimer lamp having a small and simple structure, and causing no steep decrease in the irradiance of the emitted light.