10 Hardware of Near-Infrared Spectroscopy
249
Fig. 10.20 Optics of a VPH
grating
λ 0 = 2 ·
10
6
N
· sin α
(10.11)
λ min =
10
6
N
· (sin α + sin β 1 )
(10.12)
λ max =
10
6
N
· (sin α + sin β 2 )
(10.13)
Higher-order diffraction does not exist in Bragg diffraction. Therefore, a cut filter
is not necessary in a VPH spectrometer to eliminate second and higher order diffraction. The VPH grating is easy to handle due to its sandwich structure. The surface of
the VPH grating can be wiped or polished. The typical optical alignment of a VPH
grating spectrometer is shown in Fig. 10.21.
10.2.3 Hadamard Spectrometer
In a Hadamard spectrometer, an image of the spectrum is focused onto the exit slit
position of a grating spectrometer. The light passing through a multi-aperture plate
deployed at the exit slit position is detected by a single detector (Fig. 10.22). This
measurement is repeated more than n times for different multi-aperture plates. The
signal intensity of the n signals corresponding to the different multi-aperture plates
can be expressed in terms of Eq. 10.14.