Fast microbolometer-based infrared camera system 1 Introduction 2 ...

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