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FIGURE 10.21 SpectRx NIR Technology 521
Shearing of the n
secondary beam in
a tilted plate with φ
parallel faces.
δ

t

The attenuation factor caused by the shear is:
Sin(2πσθδ )
M = 2πσθδ (10.54)
S
–1
where σ = wave-number of interest (cm )
θ = beam divergence half angle (radians)
δ = shear given by Eq. (10.34) (cm)

Parallel-face protective windows are included in SpectRx systems
right after the telescope and before the output optics. The level of
channel spectrum generated by such a window is calculated using
Eq. (10.33) with M = 0, and Eqs. (10.34) and (10.35). A certain degree
T
of wedging of this window would further reduce the channel spectrum
but at the cost of introducing the phenomenon of ghost images. Ghost
images are generated when reflections from the wedge’s surfaces (of
an off-axis source, which can be many times more intense than the
source of interest (on-axis)), reach the detector and give an erroneous
signal. This is illustrated schematically in Fig. 10.22.
Ghost images are especially destructive and must be avoided if
possible.

FIGURE 10.22 Intended scene Unwanted scene
Ghost image
phenomenon
encountered with a
wedged substrate.

Ghost image