1. Field of the Invention
The present invention relates to a near-infrared shield and a display front plate using the near-infrared shield.
2. Description of Related Art
Recently, demands for plasma display panels (PDPs) as display panels for various electronic equipment like large-sized TVs have been increased. A PDP includes two glass plates on which a fluorescent substance is applied, and a gap between the glass plates is filled with a gaseous mixture containing xenon and neon. When a high voltage is applied to the gaseous mixture, ultraviolet radiation is generated. The ultraviolet radiation impinges the fluorescent substance, and thus the fluorescent substance emits light.
At this time however, in addition to the ultraviolet radiation, near-infrared radiation in a wavelength region ranging from 820 nm to 1100 nm, electromagnetic waves and the like are generated as well. Since the near-infrared radiation has a wavelength region overlapping the wavelength region used for near-infrared communications or remote controls of the other electronic equipment, it can cause malfunctions of the equipment. For solving the problem, a near-infrared shield is provided on the front plate of the PDP so as to absorb the near-infrared radiation (see “Characteristics of an antireflection film and optimum design/film formation technology” by Hanaoka et al., first edition, second printing at Technical Information Institute Co., Ltd., Feb. 5, 2002, p. 184).
For the near-infrared shield, for example, a product prepared by dispersing a near-infrared absorption compound in a resin and shaping it into a film is known. Examples of the near-infrared absorption compound include a diimonium compound, a phthalocyanine compound, a cyanine compound and the like. The compounds are known for exhibiting a particularly excellent near-infrared absorption characteristic in a case where two or more of the compounds are used in combination, particularly in a case of combining the diimonium compound and either the phthalocyanine compound or the cyanine compound in comparison with a case of using one of the compounds alone (see JP 11(1999)-316309 A and JP2003-21715 A).
In general however, conventionally-used phthalocyanine compounds do not have the desired solubility in solvents or compatibility with resin. Therefore, various substituents must be introduced when such a compound is used for a near-infrared shield, and this increases the production cost. The conventionally-used cyanine compound is easy to obtain. However, when it is combined in use with a diimonium compound, mutual interaction is generated between the two near-infrared absorption compounds in a long-term storage, and this will cause a problem that both the near-infrared radiation absorptivity and the visible light transmittance change.
Furthermore, since the cyanine compound has a low light resistance in general, both the near-infrared absorptivity and the visible light transmittance of a near-infrared shield using the cyanine alone will change.
In addition, a near-infrared shield used in a display front plate is preferred to have excellent near-infrared shielding property and visible light transmittance, and be capable of enduring long-term storage under a condition of high temperature, high humidity and light irradiation.