Portable reflectometer for the characterization of mirrors of solar collectors more specifically, for the spectral measurement of the specular reflection of mirrors of solar collectors, comprising: - a module that carries out the measurement of the coefficient of the reflection of the mirror comprising a plurality of light emitting diodes as optical sources and a plurality of measurement photodetectors as well at least one reference photodetector for obtaining a reference signal of the light emitted by the light emitting diodes in a different direction to that of light incident on the mirror; - a plurality of diaphragms that limit, in size and aperture, the output light beams from the light emitting diodes which are incident on the mirror, to ensure the size of the area illuminated on the mirror surface, thus limiting the contribution of diffuse reflection in the measurement; - a plurality of lenses located in the reflected light beam path after the specular reflection of the beam on the mirror which are oriented on the optical axis of the reflected beam, and that focus the light beams on the photodetectors in order to have tolerance in curvature and thickness of the mirrors; - an electronic system for acquiring signals output by the photodetectors and for processing these signals to obtain spectral values, the electronic system comprising synchronous detection means using an electronic modulation generator and analog processing or digital processing in combination with an analog-to-digital converter; - a data processing and equipment control system that provides global reflectance values by weighting the obtained spectral values with the according weight of the wavelengths in the solar spectrum; - a system for storing relevant data; - a user interface system; - a system for the communication between the above systems (12,13,14,15,23); - an exterior casing, a carrier piece and lateral casings for holding the carrier piece, wherein optical channels for measuring the reflectance defined by the plurality of measurement light emitting diodes and the plurality of measurement photodetectors are arranged in a line; wherein the light emitting diodes and the measurement photodetectors are arranged on the upper surface of the carrier piece and the lenses and the diaphragms are arranged on the lower surface of the carrier piece, said diaphragms being defined by holes in the carrier piece to provide light paths from the light emitting diodes to the lower surface of the carrier piece; the plurality of lateral support casings comprise rubber O-rings placed along the lower profile of the lateral support casings to ensure the correct support of the reflectometer on the mirror without damaging the mirror; a printed circuit board containing the electronics of the reflectometer; whereby the at least one reference photodetector is supported on the printed circuit board which is arranged above the light emitting diodes and the at least one reference photodetector measures the light beam emitted by the light emitting diodes in an upward direction. - a module that carries out the measurement of the coefficient of the reflection of the mirror comprising a plurality of light emitting diodes as optical sources and a plurality of measurement photodetectors as well at least one reference photodetector for obtaining a reference signal of the light emitted by the light emitting diodes in a different direction to that of light incident on the mirror; - a plurality of diaphragms that limit, in size and aperture, the output light beams from the light emitting diodes which are incident on the mirror, to ensure the size of the area illuminated on the mirror surface, thus limiting the contribution of diffuse reflection in the measurement; - a plurality of lenses located in the reflected light beam path after the specular reflection of the beam on the mirror which are oriented on the optical axis of the reflected beam, and that focus the light beams on the photodetectors in order to have tolerance in curvature and thickness of the mirrors; - an electronic system for acquiring signals output by the photodetectors and for processing these signals to obtain spectral values, the electronic system comprising synchronous detection means using an electronic modulation generator and analog processing or digital processing in combination with an analog-to-digital converter; - a data processing and equipment control system that provides global reflectance values by weighting the obtained spectral values with the according weight of the wavelengths in the solar spectrum; - a system for storing relevant data; - a user interface system; - a system for the communication between the above systems (12,13,14,15,23); - an exterior casing, a carrier piece and lateral casings for holding the carrier piece, wherein optical channels for measuring the reflectance defined by the plurality of measurement light emitting diodes and the plurality of measurement photodetectors are arranged in a line;