Optical radiation from a sample is received by the slit and it is passed through an aperture in a reflective plane of a folding mirror towards a curved reflective surface of a collimating mirror. The slit and the curved reflective surface have a common optical axis. The reflective plane and the curved reflective surface face each other. The optical radiation passed through the folding mirror is collimated by the curved reflective surface. The collimated optical radiation is directed to the reflective plane of the folding mirror by the curved reflective surface. The collimated optical radiation is reflected in a direction other than the common optical axis of the slit and the curved reflective surface by the reflective plane.