Research Directory of the Brandenburg University of Applied Sciences

Fig. 3.13: Thermograms
at 50
Hz frame rates of
a free falling ball
(heated to a temperature
of 70°C,
start at a height
of 1 m) measured
with a LW bolometer
camera SC
2000 (left) and a
MW InSb camera
SC 6000 (right)
with 1 ms integration
time.
mounted on a vibration insulated optical table. The
sample holder was completely adjustable (sample
movement in x-y-z-direction, rotation and tilt) in front
of the microscope objective. Results have been analyzed
using the RTools software.
Characteristics of Microscopic and HIGH-
SPEED Thermography
a) Time resolution
In data sheets of imaging systems time resolution is
usually just characterized by the frame rate as the relevant
camera parameter. Mostly this value is assumed
to be connected with the time resolution for the imaging
analysis. A simple experiment demonstrates that
the frame rate alone will not give the time resolution of
a camera. For this experiment a free falling rubber ball
with a diameter of 3 cm was used. The rubber ball was
heated to about 70°C. Starting at a height of 1m above
the ground the free falling ball was analyzed by thermal
imaging with IR cameras for an object distance of
about 4 m. Fig. 3.13 depicts the results of the measurement
using a bolometer camera SC2000 and the InSb
camera SC6000 both operating with a frame rate of 50
Hz.
With increasing velocity of the ball during the free fall
motion the SC2000 image of the ball becomes blurred
due to the increasing speed of the ball. Fig. 3.14 depicts
the results of a temperature profile measurement
along the falling line of the ball which for this image
had already fallen about 92 cm. Two results are obvious.
The measured temperature of the ball decreases
to about 44°C and we observe nonzero temperature
differences in a 16 cm range of the measured line pro-
Wissenschaftliche Beiträge – Fachbereich Technik
Scientific Articles – Department of Engineering
file although the falling ball had only 3 cm in diameter.
This behavior is caused by the time constant of the
bolometer detectors in the SC2000 camera. Thermal
detectors as bolometers exhibit a time constant within
the ms range. The measured temperature line profile
corresponds to the temperature rise and decay process
of the bolometer sensors of the camera. It is not caused
by the rolling read-out process of the bolometer
FPA. If the camera is rotated by 90° the signal will be
essentially the same.
From the falling distance s the speed of the ball v can
be calculated using v = gt with gravitational acceleration
g = 9.81 m/s 2 . Therefore the falling distance shown
in Fig. 3.13 can be transformed into a time scale using
s = 1/2gt 2 , see Fig. 3.14, bottom.
At the lower end of the ball at the largest falling
distance in the IR image i.e. for the largest falling time,
the respective bolometer detector signal starts to rise
due to IR radiation from the ball. Behind the actual
position of the ball in the image i.e. at those detectors,
who no longer receive IR-radiation from the ball, there
is a drop in the signal, characterized by the detector
time constant. Therefore the observed signal decay on
the small time side of the signal directly reflects this
time constant. Fig. 3.14, bottom depicts this detector
signal rise and decay. For easier analysis, the signal
was normalized to the maximum temperature difference
�T max .
One can estimate an exponential time dependence
with a time constant of about 10 ms. The time resolution
of the camera is limited by this time constant.
To get a correct temperature reading one needs at
least a 99% signal measurement. This corresponds to
a measurement time of 5� = 50 ms. At a frame rate of
Forschungsbericht Research Report 2007 – 2010 81