Instrumentation & Measurement Magazine 24-4 - 5

256 pixels) and records infrared radiation between 3 and 5 μm
with a full frame rate fcam
of up to 250 Hz. The nominal thermal
resolution of the camera is 0.017 °C. For this study, we used
the standard 25 mm focal length lens provided by the camera
manufacturer. A frame grabber card (PCIe-1427, National
Instruments) was used to transfer the infrared images in real
time to a computer and to synchronize the infrared camera
and the stimulation source, respectively (Fig. 1a). The personal
computer sets the stimulation frequency via the frame
grabber card, while both the infrared camera and the stimulation
source are working in slave mode. The setups depicted in
Fig. 2 used a quantum detector-based infrared camera. Quantum
detectors transformed absorbed photon energy directly
into released electrons. Most commercial cameras are operating
in the photovoltaic mode where photo-excited charge
carriers are collected by a diode junction. Thermal detectors
convert the far-IR radiation into heat which causes electrically
measurable changes in the electrical resistance of a bolometer.
They are called microbolometers.
Quantum-based sensors demonstrate better performances
than microbolometers in terms of sensitivity and speed, but
their costs are prohibitive, and microbolometers are therefore
widely used in industry. As they are functioning in the rolling
frame mode (meaning the image acquisition cannot be triggered),
they have to act as the master clock synchronizing the
stimulation source.
LIT Algorithm
Many algorithms can be found in the literature to compute
the phase and amplitude image. In synchronous narrow twochannels
correlation, two factors are used to approximate the
sine and cosine function:
S xy
0
,
S x y
90
,

 


nN2sin
1
ij
 
11
nN2cos
1
ij
 
11
Nn
Nn
n
 j Tij, xy
21
,

 


n
 j Tij, xy
21
,
(1)
frame rate (n ≥ 10), synchronous narrow two-channels correlation
is the optimal digital demodulation algorithm in terms
of accuracy and computing time. This method demonstrates
the advantage that it may be performed in real time and does
therefore not require any storage of the infrared images. The
first image T0,0
(x, y) is usually subtracted before performing the
demodulation.
To perform the synchronous narrow two-channels correlation,
we implemented a custom software (LabVIEW, National
Instruments) using the Vision Acquisition Software (VAS, National
Instruments). The demodulation is achieved online and
processes the images in real time. The use of the Vision Development
Module (VDM, National Instruments) allows an easy
handling of the images and their subsequent processing.
Sensitivity
One advantage of LIT is the averaging nature of the method
that allows the detection of very small temperature gradients
buried in a noisy background. For a LIT experiment, the theoretical
performance of the setup is given by the sensitivity S,
representing the minimal temperature difference that can be
measured. It is defined as:
S 
NETD
ft
(5)
cam acq
(2)
where Ti,j (x, y) are the time-dependent infrared images, n is the
number of measurements per stimulation cycle, and N is the
number of stimulation cycles. 
S0 xy
,
S0 xy
,
and 
90
S xy
,

and 
90
S xy
,
age ϕ(x,y) can be calculated [15]:

xy,
are the " inphase "
and " in-quadrature " images equivalent to the real and
imaginary image resulting from a discrete Fourier transform.
From 
the amplitude A(x, y) and phase imA
, ,,xy S xy S xy
N
90
  tan1 S xy
,
90
S xy
0
,

 




2 0 22

  
(3)
(4)
When the stimulation source is synchronized with the infrared
camera, and if the pulsation ω0
June 2021
where NETD is the Noise Equivalent Temperature Difference
or the minimal temperature difference that the infrared camera
can resolve in a single image. The desired sensitivity can
be obtained by increasing the measurement time or choosing
a camera with higher frame rate or lower NETD. The setups
presented in Fig. 2 are based on a quantum infrared detector
exhibiting a NETD of 17 mK. An alternative setup, including a
standard microbolometer camera, for example the PI-460 (Optris)
based on 288 × 382 pixels bolometer array, demonstrates
a NETD of 40 mK for a full frame rate of 80 Hz. As a result,
based on (5), we have to measure 17.3 times longer using the
microbolometer camera-based setup to achieve the same sensitivity
as with the setup using the InSb sensor.
Diffusion Length
When the sample is subjected to a periodic thermal stimulation,
highly damped thermal waves will propagate into
the medium in all directions on a distance referred to as the
thermal diffusion length [15] Λ, which depends on the thermophysical
properties of the medium and on the pulsation:
Λ 
2
 0
(6)
where α is the medium thermal diffusivity. As a result, if the
thermal stimulation frequency ω0
is chosen high enough,
is low compared to the
LIT has the ability to prevent lateral (and longitudinal) heat
spreading from the infrared images, resulting in sharp images,
which can be particularly interesting when using high magnification
objective lenses or to determine the spatial location of
NPs in a solid sample. The operating range of the camera in the
IEEE Instrumentation & Measurement Magazine
5

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