A concave mirror focuses the light from a light source onto the entry slit. The spectral band at the grating is reflected by means of a second concave mirror onto the adjustable measuring slit. A third concave mirror and a plane mirror tramsmit the measuring beam to the sample being measured. This sample beam is also split and transmitted to the reference detector. The reflectance detector measures the light scattered by the sample, the transmission detector measures the light which is trasmitted through the sample.