Page 544 - Sensors and Control Systems in Manufacturing

SpectRx NIR Technology
and the apparent disagreement between Eqs. (10.1) and (10.2) 497
disappears.

10.7.2 Transmission of the Spectroradiometer
Transmission of the spectroradiometer is the ratio of the radiant sig-
nal at the detector to that at the input port, taking into account the
reflectivity of mirrors and transmission through windows, beam-
splitters, and lenses. The description and transmission coefficient of
these elements is shown in Table 10.1. The system transmission T(σ)
(no units) is given by:
n
σ
σ
T() = ∏ ε i () (10.4)
i = 1
where ε (σ) is the spectral transmission efficiency of element i (no
i
units).
For the SpectRx system, the standard beam-splitter is ZnSe, a
nonhygroscopic material well adapted to field applications. The
optical components of the SpectRx systems along with their transmit-
tance is presented in Table 10.1. The total estimated system transmis-
sion is about 30 percent. This transmission can be improved to about
50 percent by using an anti-reflection coating on the protective win-
dows; however, this will have a detrimental effect on the spectral
range coverage. Another way to improve the transmission is to use
different window materials. Unfortunately, most acceptable infrared
window materials (Kbr, Kcl, etc.) are hygroscopic and require more
care than ZnSe.

10.7.3 Throughput
2
The throughput of the SpectRx system is 0.004 cm . The system can be
used without having to reduce the throughput to a spectral resolu-
–1
tion below 1 cm .
10.7.4 Power at the Detector
–1
The power at the detector (W/cm ) is given by:
σ
L()⋅ T()⋅Θ
σ
σ
P detector () = (10.5)
2
where T(σ) = spectral instrument transmission efficiency (no units)
2
L(σ) = spectral radiance from the object view (W/cm cm sr)
–1
2
Θ = instrument throughput (cm sr)
The SpectRx system is equipped with a single input output port
and dual output ports. The evaluation of the power at the detector
takes into account that the energy from a single input port is divided