An optical instrument comprising a monolithic spectrometer and a light source and/or relay for providing light to an entry surface of the spectrometer, wherein the monolithic spectrometer is arranged for spectrally resolving the light, the spectrometer comprising a body of solid material having optical surfaces arranged to guide the light along an optical path (E1,E2,E3,E4) inside the body, the optical surfaces comprising - the entry surface arranged to receive the light to enter into the body as an entry beam directed along a first part of the optical path; - a collimating surface arranged to receive the entry beam directed along the first part of the optical path and to reflect said entry beam as a collimated beam directed along a second part of the optical path; - a grating surface arranged to receive the collimated beam directed along the second part of the optical path and to reflect a refracted beam directed along a third part of the optical path according to a wavelength dependent refraction angle (b); - a focusing surface arranged to receive the refracted beam directed along the third part of the optical path and to focus said refracted beam directed along a fourth part of the optical path for imaging a wavelength component (λ) of the light onto a position along a spectral axis in an imaging plane outside the body; and - an exit surface arranged in the optical path between the focusing surface and the imaging plane to have the light exit the body; wherein the collimating surface and the focusing surface are part of a single surface having a continuous optically functional shape, wherein the surfaces (3,4,5,6,8) of the body are arranged to have the third or fourth part (E3,E4) of the optical path between the grating surface and the exit surface cross with the first part of the optical path between the entry surface and the collimating surface, wherein the collimating surface and the focusing surface share a common area arranged therein between to accommodate different passes of the light along the optical path, wherein an entry beam traveling along the first part of the optical path from the entry surface and projected onto the common area is reflected along the second part of the optical path towards the grating surface, while a refracted beam traveling along the third part of the optical path from the grating surface and projected onto the common area is reflected along the fourth part of the optical path towards the exit surface, wherein an incidence angle (a) of the collimated beam with respect to the surface normal of the grating surface is at least 30 degrees, wherein the grating surface is arranged to refract the collimated beam backwards with respect to the surface normal of the grating surface wherein an absolute value of the refraction angle (b) is larger than the incidence angle (a) of the collimated beam with respect to the surface normal. - the entry surface arranged to receive the light to enter into the body as an entry beam directed along a first part of the optical path; - a collimating surface arranged to receive the entry beam directed along the first part of the optical path and to reflect said entry beam as a collimated beam directed along a second part of the optical path; - a grating surface arranged to receive the collimated beam directed along the second part of the optical path and to reflect a refracted beam directed along a third part of the optical path according to a wavelength dependent refraction angle (b); - a focusing surface arranged to receive the refracted beam directed along the third part of the optical path and to focus said refracted beam directed along a fourth part of the optical path for imaging a wavelength component (λ) of the light onto a position along a spectral axis in an imaging plane outside the body; and - an exit surface arranged in the optical path between the focusing surface and the imaging plane to have the light exit the body; wherein the collimating surface and the focusing surface are part of a single surface having a continuous optically functional shape, wherein the surfaces (3,4,5,6,8) of the body are arranged to have the third or fourth part (E3,E4) of the optical path between the grating surface and the exit surface cross with the first part of the optical path between the entry surface and the collimating surface, wherein the collimating surface and the focusing surface share a common area arranged therein between to accommodate different passes of the light along the optical path, wherein an entry beam traveling along the first part of the optical path from the entry surface and projected onto the common area is reflected along the second part of the optical path towards the grating surface, while a refracted beam traveling along the third part of the optical path from the grating surface and projected onto the common area is reflected along the fourth part of the optical path towards the exit surface, wherein an incidence angle (a) of the collimated beam with respect to the surface normal of the grating surface is at least 30 degrees, wherein the grating surface is arranged to refract the collimated beam backwards with respect to the surface normal of the grating surface wherein an absolute value of the refraction angle (b) is larger than the incidence angle (a) of the collimated beam with respect to the surface normal.