Abstract: Described is a device (16) for the selection and detection of at least two spectral regions in a beam of light (14), the device including a selection unit (25) and a detection unit (26). To ensure reliable simultaneous selection and detection of different spectral regions plus high yield and simplest possible construction, the device is designed such that the selection unit (25) comprises means (27) for splitting the light beam (14) up into its spectral components and means (28) for not only masking off a first spectral region (29) but also reflecting at least part (30) of the spectral region which is not masked off, and the detection unit (26) comprises a first detector (31) located in the path of the light in the masked-off first spectral region (29) and a second detector (32) located in the path of the light of the reflected spectral region (30).