NOVELTY - Device for modifying e.g. narrowing the wavelength range of a spectrum of light incident on the device, comprises a medium for photon energy up-conversion comprising at least two components and a matrix in which the components are distributed, where: first component acts as a sensitizer; the second component acts as an emissive component; the first component and the second component are organic compounds; and the device optionally further comprises a light transparent container for containing the medium, or optionally a light transparent substrate for carrying the medium.    USE - The device is useful for modifying e.g. narrowing the wavelength range of a spectrum of light incident on the device, such that light emitted from the device onto an object has a modified, e.g. more narrow wavelength range in comparison to the incident light (claimed). The device is useful: for the production of tailor-made optoelectronic devices; and as a spectrum concentrator.    ADVANTAGE - The device shows a great versatility with respect to the radiation wavelengths. The device allows the use of low intensity and/or low spectral power density light sources e.g. sun and light emitting diode.    DETAILED DESCRIPTION - Device for modifying e.g. narrowing the wavelength range of a spectrum of light incident on the device, such that light emitted from the device has a modified, e.g. more narrow wavelength range in comparison to the incident light, comprises a medium for photon energy up-conversion comprising at least two components and a matrix in which the components are distributed, where: the first component is capable of absorbing light at a first wavelength region of w less than or equal to lambda 1 less than or equal to x; the first component acts as a sensitizer in the medium; the second component is capable of emitting light at a second wavelength region of y less than or equal to lambda 2 less than or equal to z; the second component acts as an emissive component in the medium; lambda 2 is less than or equal to lambda 1; upon absorption of light by the first component at the first wavelength region lambda 1, the emissive component emits light at the second wavelength region lambda 2; the first component and the second component are organic compounds; the first component and the second component are distributed in the matrix, which is a solid, a gel or a fluid with viscosity equal or higher than 0.59x 10-3 Pa.s; the medium optionally comprises further pairs of sensitizer and emissive components, but having different absorption and, optionally, emission wavelength regions; and the device optionally further comprises a light transparent container for containing the medium, or optionally a light transparent substrate for carrying the medium. An INDEPENDENT CLAIM is included for use of the device for modifying, e.g. narrowing the wavelength range of a spectrum of light incident on the device, comprising: arranging the device in such a spatial relation to the object such that radiation irradiated upon the object passes through the device before impinging on the object; and, optionally, irradiating the object, e.g. a solar cell or a window or a window of a greenhouse with light, preferably sunlight.