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 a first component is capable of absorbing light at a first wavelength region w less than or equal to lambda 1 less than or equal to x, and acts as a sensitizer in the medium, and a second component is capable of emitting light at a second wavelength region y less than or equal to lambda 2 less than or equal to z.    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 has a modified, e.g. more narrow wavelength range in comparison to the incident light. The device is useful for film-formation over large areas, hence enables the production of tailor-made opto-electronic devices and a spectrum concentrator.    ADVANTAGE - The device shows a great versatility with respect to the radiation wavelengths involved, both incident and emitted and which may therefore be tailor-made depending on the desired excitation and/or emission characteristics. The device allows efficient use of low intensity and/or low spectral power density light sources, such as the sun, LED (broadband and non-coherent) and others.    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 a first component is capable of absorbing light at a first wavelength region w less than or equal to lambda 1 less than or equal to x, and acts as a sensitizer in the medium, and a second component is capable of emitting light at a second wavelength region y less than or equal to lambda 2 less than or equal to z, and acts as an emissive component in the medium, where lambda 1 less than or equal to l2, and 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, and the first component and the second component are organic compounds, where 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 0.59 less than or equal to 10-3 Pa.s, where 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 comprising 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, 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 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.