Luminosity is a measure of the 'light-collecting power' of a spectrometer. In an ideal instrument, it depends on the product of the area of the entrance slit and the solid angle subtended by the dispersing element (e.g. by the diffraction grating). It is assumed that all other elements in the spectrometer, e.g. lenses and detectors, are made large enough so as not to reduce the luminosity.
In real spectrometers, the number and quality of optical components can have a dramatic effect on luminosity. The table below compares the effect on luminosity of the optical components in an Echelle grating spectrometer and a Czerny Turner monochromator in both the Visible (VIS) and ultraviolet (UV):
里面的数字代表此种光学元件的数量
注：这个表是基于中阶梯光栅光谱仪，光路是C-T型的，如下图
其中：
Echelle 表示中阶梯光栅 Czerny Turner 表示光路类型是C-T型 Grating：光栅 Cross Dispers，不知道是什么 Mirror，反射镜 Prism：棱镜，有些仪器采用双单色器，棱镜预分光 Lens：透镜 Filter：滤光片 Energy Through put (%) ：假设入射光束能量100%,这里的值就是最后出单色器的光能量。
Cross-dispersion is used to separate the the echelle orders on the detector. For bHROS, the cross-dispersion is provided by a set of 60o fused silica prisms. This method was chosen because prism cross-dispersion is about a factor of two more efficient than grating cross-dispersion (owing to the absence of parasitic orders) and also conveys a number of other advantages (i.e. being virtually stray light free and providing more uniform order separation).
The cross-dispersion system provides a minimum order separation of 3.00 mm in the camera focal plane. This separation provides sufficient inter-order spacing for (1) adjacent sky and object spectra in dual-fiber mode or (2) one broad object spectrum in single-fiber mode. The minimum order separation occurs at a wavelength of approximately 780 nm, with the order separation being larger than this at longer and shorter wavelengths.
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