Fast microbolometer-based infrared camera system 1 Introduction 2 ...
Fast microbolometer-based infrared camera system 1 Introduction 2 ...
Fast microbolometer-based infrared camera system 1 Introduction 2 ...
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DIAS Infrared GmbH – Publications – No. 20 2<br />
The frequency dependence of the responsivity R(ω) of a bolometer can simply be expressed by /3/:<br />
( ω)<br />
R<br />
R<br />
=<br />
1<br />
1 + ω τ<br />
2 2<br />
0 th<br />
where R0 is the DC responsivity. That means that the output voltage U(t) of a pixel follows an input<br />
step exponentially:<br />
U() t<br />
( →∞)<br />
U t<br />
= 1 −e<br />
t<br />
−<br />
τ th<br />
Figure 1 shows the output signal after an input step. After a time > 3τth the output becomes 95 % of<br />
its final value. This time is usually called response time (see table 1). The response time is the minimum<br />
time to be waited for after the object temperature is changed. After t > 6τth 99 % of the final<br />
value are reached.<br />
output<br />
1<br />
0,9<br />
0,8<br />
0,7<br />
0,6<br />
0,5<br />
0,4<br />
0,3<br />
0,2<br />
0,1<br />
2.2 Frame rate<br />
7 ms<br />
15 ms<br />
0<br />
0 10 20 30 40 50<br />
time in ms<br />
Figure 1: Output signal after an input step (Parameter: time constant τth)<br />
The maximum of the frame rate is specified by the ROIC. Usually, the analog output amplifier of<br />
the ROIC gives the maximum of the pixel frequency. Typical pixel frequencies for different frame<br />
rates and usable focal plane array sizes are given in table 2. Practically, the pixel frequencies are<br />
slightly higher due to extra clocks for the ROIC. For large FPAs, two analog outputs are usable.<br />
Frame rate<br />
Size 50 Hz 100 Hz 200 Hz<br />
320 × 240 3.84 MHz 7.68 MHz 15.36 MHz<br />
384 × 288 5.53 MHz 11.1 MHz 22.1 MHz<br />
640 × 480 15.36 MHz 30.72 MHz 61.44 MHz<br />
Table 2: Pixel frequency<br />
(2)<br />
(3)