the 
the 
focal length (mm) 
Effective diameter (mm) 
Objective (Wide FOV) I 
25.4 
5.6 
Objective (Narrow FOV) 
76.2 
12.5 
Condenser 
3.23 
4.35 
Table 2. Prototype lens dimensions (objective and condenser) 
(iv) The interference filters located between the two lenses can be switched by rotating the filter wheel with a 
step-by-step motor (Figure 2). 
(v) In addition, a zinc-sulfide filter is located on the optical path in order to block radiation with wavelength 
larger than 30 |J.m. 
(vi) The output voltage of the detector can range between ± 3 mV according to the extreme temperatures of the 
various targets. The amplification line consists of a low-noise preamplifier (gain 1000), a voltage-frequency 
converter, and a micro-controller which counts the pulses (it also operates the mirror tilt and filter 
commutation). 
(vii) Measurements will be shown on a liquid crystal display, direct acquisition on a micro-computer will be 
possible through a serial port and a cartridge is available for data storage when the instrument is used in 
autonomous mode (Figure 2). 
D: Detector 
P.A: Preamplifier 
C. V/N: Voltage-frequency convener 
MC: Micro controller 
To,Td: Objective and detector temperatures 
Output: 
1) LCD display 
2) RS232 for direct acquisition 
3) Data storage cartridge 
Figure 2. Amplification line of the radiometer 
(viii) In front of the optical head, a concealable gold-deposited mirror enables comparisons between target 
radiation and reference radiation of the instrument cavity which contains the detector (and is not temperature 
controlled). If the cavity temperature is constant, the difference between the two signals will only be a function 
of the target radiation. The temperature of the cavity is monitored by two platinum thermal probes. Based on 
a maximum thermal drift criterion, the micro-processor activates a motor which operates the mirror in order to 
perform a new reference measure. Self-calibration is planned during field operation using an internal 
blackbody whose temperature will be shifted 10°C from ambient. 4 
4 - RESULTS AND DISCUSSION 
The conception and performances of the instrument were established during an earlier stage first by taking into 
account the characteristics of the components that were available on the market and later by comparing it to the 
few existing instruments [3,4]. Prior to the construction of a prototype, a bread board was built in order to 
perform an experimental survey of the instrument design and to control the main computed performances of 
the instrument. Finally, a prototype was built and calibrated.