Spectral imaging device comprising: - a slit for directing part of a source light beam inside said spectral imaging device, said slit having a slit length A; - a first concave mirror; - a convex reflective diffraction grating comprising a reflective surface on which a diffractive grating is formed; - a second concave mirror; - first matrix detection means; said slit and said first concave mirror being configured such that said part of a source light entering the spectral imaging device from said slit is directed towards said first concave mirror; said first concave mirror and said convex reflective diffraction grating being configured such that said first concave mirror is able to reflect a light beam originating from said slit towards said convex reflective diffractive grating; said convex reflective diffraction grating and said second concave mirror being configured such that said convex reflective diffraction grating is able to reflect and diffract incoming light from said convex reflective diffraction grating towards said second concave mirror; said second concave mirror and said matrix detection means being configured such that said second concave mirror is able to reflect incoming light from said second concave mirror towards said first matrix detection means; wherein said reflective surface of said convex reflective diffraction grating is defined by a complex invariant shape class; wherein said first concave mirror, said convex reflective diffraction grating, and said second concave mirror are configured such that a spectrally dispersed image of the slit with said slit length A can be produced on the detection means with an image length B, said slit length A and image length B defining a demagnification such that: demagnification factor = A B said demagnification factor being higher than 1.3, characterized in that: said slit having a slit F-number of 3.6 and said first matrix detection means having a detection means F-number equal to 2.1. - a slit for directing part of a source light beam inside said spectral imaging device, said slit having a slit length A; - a first concave mirror; - a convex reflective diffraction grating comprising a reflective surface on which a diffractive grating is formed; - a second concave mirror; - first matrix detection means; said slit and said first concave mirror being configured such that said part of a source light entering the spectral imaging device from said slit is directed towards said first concave mirror; said first concave mirror and said convex reflective diffraction grating being configured such that said first concave mirror is able to reflect a light beam originating from said slit towards said convex reflective diffractive grating; said convex reflective diffraction grating and said second concave mirror being configured such that said convex reflective diffraction grating is able to reflect and diffract incoming light from said convex reflective diffraction grating towards said second concave mirror; said second concave mirror and said matrix detection means being configured such that said second concave mirror is able to reflect incoming light from said second concave mirror towards said first matrix detection means; wherein said reflective surface of said convex reflective diffraction grating is defined by a complex invariant shape class; wherein said first concave mirror, said convex reflective diffraction grating, and said second concave mirror are configured such that a spectrally dispersed image of the slit with said slit length A can be produced on the detection means with an image length B, said slit length A and image length B defining a demagnification such that: demagnification factor = A B said demagnification factor being higher than 1.3, characterized in that: said slit having a slit F-number of 3.6 and said first matrix detection means having a detection means F-number equal to 2.1. said slit and said first concave mirror being configured such that said part of a source light entering the spectral imaging device from said slit is directed towards said first concave mirror; said first concave mirror and said convex reflective diffraction grating being configured such that said first concave mirror is able to reflect a light beam originating from said slit towards said convex reflective diffractive grating; said convex reflective diffraction grating and said second concave mirror being configured such that said convex reflective diffraction grating is able to reflect and diffract incoming light from said convex reflective diffraction grating towards said second concave mirror; said second concave mirror and said matrix detection means being configured such that said second concave mirror is able to reflect incoming light from said second concave mirror towards said first matrix detection means; wherein said reflective surface of said convex reflective diffraction grating is defined by a complex invariant shape class; wherein said first concave mirror, said convex reflective diffraction grating, and said second concave mirror are configured such that a spectrally dispersed image of the slit with said slit length A can be produced on the detection means with an image length B, said slit length A and image length B defining a demagnification such that: demagnification factor = A B said demagnification factor being higher than 1.3, characterized in that: said slit having a slit F-number of 3.6 and said first matrix detection means having a detection means F-number equal to 2.1.