4.1. Preliminary studies 
4.1.1. Numerical simulations. The optical components of the Kohler arrangement needed to be precisely 
adjusted in order to optimize the power incident upon the detector. The Monte-Carlo method was used to 
define the optimal location of the detector and lenses on the optical axis and to find an adequate diameter for 
the entrance diaphragm. In addition, the sensitivity of the instrument performances to variations of these 
parameters was characterized. 
4.1.2. Control of components. Measurements were performed with an IR spectrometer and a CO 2 laser (10.6 
urn) in order to characterize the optical properties of the lenses and filters and to control the responsivity of the 
detector. 
4.1.3. Bread board study. A bread board consisting of only the detector, lenses and the preamplifier was built 
It allowed an experimental assessment of the sensitivity and noise of the planned prototype to be performed. 
The obtained values were observed to be in reasonable agreement with the optimum theoretically predicted 
values. 
4.2. Prototype study 
The prototype is a 4-channel instrument (three narrow bands and one broad band). It consists of all the 
elements shown in Figures 1 and 2. The filter wheel is equipped with three different interference narrow band 
(1-p.m wide) filters centered on 8.7, 10.5 and 11.5 pm, respectively. The spectral range of the broadband 
channel (8-14 pm) results from the combination of the lenses, Ge window and ZnS filter. 
4.2.1. Sensitivity. The sensitivity of the prototype is determined by comparing the difference c (expressed in 
counts, ct) between instrument output signals obtained by looking successively at its own cavity and a standard 
blackbody, and the corresponding computed difference in radiative power incident to the detector w (pW). c is 
given by 
where L(T) is the blackbody radiance at temperature T, A is the detector surface area, and O. is the solid angle 
corresponding to the FOV of the detector. The radiance L(T) is given by 
where B^_(T) is given by Planck’s law at the temperature T, x(k) is the transmittance of the instrument optics. 
The transmittances of the different channels are shown in Figure 3. Sensitivity S is given by the slope of the 
calibration curve 
The calibration procedure is the following: the instrument is aimed at a temperature-controlled 
reference blackbody. Before looking at the blackbody, the instrument response is obtained in the mirror mode. 
The temperature of the detector and the blackbody are given by platinum probes. The responses of the four 
channels are obtained successively. 
c = C( V- C( V’ 
( 1 ) 
where C(T) are the counts given when looking at a blackbody at temperature T. w is computed by 
" = W(Tcn) - w (Td) = 1 L(Tcn) ■ UTd) l A. Cl, 
( 2 ) 
(3) 
(4) 
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