An optical sheet for a backlight unit, and backlight unit using the same

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PROBLEM TO BE SOLVED : To improve the luminance a liquid crystal display device without increasing the consumed power of a back light unit by using the back light unit having a wavelength conversion function which converts rays at wavelengths except for the wavelengths of visible light into visible rays. SOLUTION : An optical sheet 1 consists of a sheet part 2 and a plurality of prisms 3 arranged parallel and adjacent to each other on the surface of the sheet part 2. The optical sheet 1 contains a powdery optical glass 5 dispersed in a base material 4 prepared from a synthetic resin. By incorporating a wavelength converting substance which converts rays at wavelengths except for the wavelengths of visible light into visible rays into the optical glass 5, the glass has a wavelength conversion function to convert the transmitting rays at wavelengths except for wavelengths of visible light into visible rays. As for the wavelength converting substance, rare earth ions which change into fluorescent active ions by accepting rays at wavelengths except for the wavelengths of visible light are used. As for the synthetic resin to form the base material 4, for example, polyethylene terephthalate and an acrylic resin can be used.