Optical Spectroscopy Instrumentation Design
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between each wavelength. The grating equation specifies the angle required to bring each
wavelength through the exit slit (Figure 5.12):
sinα
β
λ
+
=
−
sin
10
6
kn
(5.6)
where k is the diffraction order, n the grating groove density (grooves per millimeter), and
λ the vacuum wavelength in nanometers.
Equation (5.6) illustrates the presence of overlapping spectral orders at a grating angle
but with higher diffraction orders. For example, if a signal is measured at 750 nm in the first
A
B
D
G
E
C
F
Figure 5.11. A Czerny–Turner monochromator. Light (a) is focused onto an entrance slit (b) and is
collimated by a curved mirror (c). The collimated beam is diffracted from a rotatable grating (d) and
the dispersed beam refocused by a second mirror (e) at the exit slit (f). Each wavelength of light is
focused to a different position at the slit, and the wavelength that is transmitted through the slit (g)
depends on the rotation angle of the grating. (http://upload.wikimedia.org/wikipedia/commons/e/e8/
Czerny-turner.png)
angle
A
angle
B
normal
D ν
entrance
slit
exit
slit
α
β
+
–
Figure 5.12. Graphical representation of the grating equation (Fortin, 2008).