**6.3 Spectroradiometer output optics and detector**

The purpose of the output optics is to couple the dispersed radiation from the monochromator, at the monochromator output slit, onto the detector. To obtain the maximum signal-to-noise at the detector, the detector is often placed as close to the output slit as possible in order to collect all the radiation. If the detector must be placed at some distance from the output slits of the monochromator, some optics will be needed to collect the radiation and re-image it onto the detector. Note that if the beam is focused too intensely onto the detector, local overloading of the conversion process from photons to the output (electrical) signal may occur, causing non-linearity in the detector response.

The spectroradiometer shown in Figure 15 is designed to enable various detectors to be used depending on the wavelength range of interest. Each of these detectors requires different output optics of the spectroradiometer to couple the radiation from the monochromator to the sensitive area of the detector. For the VIS and NIR wavelengths, a silicon (Si) photodiode and a InGaAs photodiode can be mounted directly behind the slits. For the UV and VIS wavelengths, a photomultiplier (PMT) is used with a lens assembly to focus the radiation from the slits onto the PMT cathode. The assembly for the infrared (IR) detectors (Ge and InSb) is a bit more complex since these detectors are often cooled with liquid nitrogen and prefer to be operated in an upright position. In addition, their sensitive areas are usually quite small. An elliptical mirror may be used to change the direction of the radiation beam, to focus the radiation onto the detector, and to avoid the problem of a change in the focal position with wavelength (Gaertner & Boivin, 1995).
