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Gateway to Spectroscopy > Spectrometers > Design concepts > Luminosity

Luminosity

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): 

 

Optical Components

Energy Through
put (%) 
Grating Cross Dispers. Mirror Prism Lens Filter
Echelle in UV   1   1   5   0   1   0   8.6
Echelle in VIS   1   1   3   1   4   0   3.8
Czerny Turner in UV   1   0   2   0   1   0 33.2
Czerny Turner in VIS   1   0   3   0   1   1 29.9
Light Loss % Factors UV 50 32 15  -   8  -  
VIS 50 84 10   8   8 10