NOVELTY - Color purity improving sheet (I), comprises a light-emitting layer (11) which improves purity of a color in a target wavelength range by absorbing light in a specific wavelength range other than the target wavelength range and converts the absorbed light to emitted light in the target wavelength range, and a reflective layer (12).    USE - (I) is useful in optical apparatus, liquid crystal display and solar cell (claimed). (I) is useful in image display which is useful in office equipment (e.g. desktop personal computers (PC), a notebook PC, and a copy machine), portable devices (mobile phone, a watch, a digital camera, a personal digital assistant and a handheld game machine), home electric appliances (a video camera, a television set and a microwave oven), vehicle equipment (including a back monitor, a monitor for a car-navigation system, and a car audio), display equipment (an information monitor for stores), security equipment (including a surveillance monitor and care) and medical equipment (including a monitor for health care and a monitor).    ADVANTAGE - (I) can improve color purity, while preventing brightness from degrading due to the absorption of lights of necessary colors.    DETAILED DESCRIPTION - Color purity improving sheet (I), comprises a light-emitting layer (11) which improves purity of a color in a target wavelength range by absorbing light in a specific wavelength range other than the target wavelength range and converts the absorbed light to emitted light in the target wavelength range, and a reflective layer (12), where: the light-emitting layer has a transmittance of at least 70% at a maximum absorption wavelength of the light-emitting layer; the light-emitting layer and the reflective layer are laminated together in this order in a direction from which light of a backlight is incident; the reflective layer reflects part of light which is in the specific wavelength range other than the target wavelength range and that has been transmitted without being absorbed by the light-emitting means contained in the light-emitting layer; the reflected light enters the light emitting layer again and part of the light that has again entered the light-emitting layer passes through the light emitting layer; and the maximum absorption wavelength of the light-emitting layer is included in a reflection wavelength range in which the reflective layer selectively reflects light. An INDEPENDENT CLAIM is included for an optical apparatus comprising a back light and (I).    DESCRIPTION OF DRAWING(S) - The figure shows color purity improving sheet.    Light-emitting layer (11)    Reflective layer (12)    Color purity improving sheet. (100)