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A dispersive optical spectrometer (Fig. 2.1) requires five components. The entrance aperture is usually a slit but sometimes a pinhole or the end of a fiber. The purpose of the aperture is to limit the spatial distribution of the entering light and help define the light cone required to illuminate the system. Except in some rare cases, it is a basic requirement for a well-working spectrometer. For proper functioning, the light arriving at the aperture needs to be a clean cone of light.
The collimating optics can be a mirror or a lens; it ensures that the light will hit the dispersing element in a parallel, collimated fashion because gratings and prisms only work correctly if they are illuminated with collimated light. The optimal situation is reached when the collimator illuminates the disperser fully without sending light to the frame or passing by it. The dispersed light will leave the disperser in parallel bundles of light. Each wavelength leaves under a separate angle, but all light of the same wavelength and spectral order leaves under the same angle.
The dispersed light, traveling under a certain range of angles, will be
collected by the focusing optics, again a mirror or a lens, which refocuses the spectrum in the output plane. At the exit aperture, there can be a slit, a pinhole, or a fiber. In the optimal case, the entrance aperture is re-imaged at the output. If the entrance was illuminated by a spectral interval finer than the system can resolve, the output light interval would be the same except for some losses or changes in polarization. The output plane can also be equipped with an area detector to collect the parallel arriving wavelengths.
If the optical components are perfectly spherical, the foci of the parallel output signals will arrive on the output radius of the focusing output element, i.e., the light will arrive in a curved field. The size Wi (diameter) of the optical components, the angles within the light path, and the quality of the components define the x/y size and quality of the field. Figure 2.1 shows a transmitting setup, which was chosen for easier description. In reality, reflecting grating systems are primarily used instead of transmitting systems because they provide a much wider range of use, more flexibility, and they are prevalent in laboratory use.
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