A waveguide for conveying light with an input end and an output end to supply for an electromagnetic spectrometer includes: an input end having a convex envelope of a cross-section of the waveguide at the input end, which envelope defines a circular shape or a shape of a regular polygon with n1 corners, wherein n1 is a natural number bigger than 3; an output end having a cross-section that defines a slit shape; and a plurality of filaments, wherein an arrangement of the plurality of filaments defines the cross-sections at the input and output ends, wherein each filament includes a core and a reflective coating covering a lateral area of the core, wherein the core includes an optically transparent material.