Instrumentation & Measurement Magazine 24-4 - 60

defects, photothermal radiation excitation was employed as
the active heat source here. This also has the advantages of being
low cost, having good controllability and being easy to
realize. A hot air gun, a single xenon flash lamp and a 2 × 2 matrix
array of xenon flash lamps were therefore introduced so
that their excitation effects could be compared (Fig. 1). The single
flash lamp was controlled by discharging one of the lamps
in the 2 × 2 matrix array.
An InSb infrared camera with an internal circulating Stirling
cooler and a CdHg Te (MCT) focal plane detector was
utilized as the data acquisition equipment. This has a spectral
range of 2.0 - 5.7 μm and a resolution of 640 × 512 pixels. The
surface excitation temperature for the hot air gun was set at
200 °C. Each flash lamp can be infinitely driven by a 3 kJ power
supply. Washable black paint was evenly sprayed on the surface
of an aluminum plate to reduce light reflection. A box
cover was then installed around the lamps to avoid interference
from external light sources and suppress the noise. In the
infrared camera's field of view, the distance from the surface to
the imager was set at 30 cm and 50 cm, with the excitation duration
time being set at 5 s and 10 s, respectively.
The infrared thermal images are shown in Fig. 2. The upper
images show the grayscale values of the thermal distribution
of a test sample with 600 × 600 matrix elements processed by
MATLAB. The local light intensity motivated by the hot air
gun (Fig. 2a) was concentrated and the uniformity was poor.
The opposite was the case with the 2 × 2 matrix flash lamp
array (Fig. 2c). The lower images in Fig. 2 show the ordinate
grayscale values recorded along the length of the aluminum
plate (Row 200 and Row 300). The maximum difference in the
grayscale value excited by the hot air gun reached 100 (Fig. 2a).
It reached 70 with the single flash lamp (Fig. 2b). The variation
in the excitation induced by the 2 × 2 matrix flash lamp array
was rather small (Fig. 2c). Sharp changes corrupted by noise
occurred in these images.
Thermal uniformity was established to evaluate the effects
of the three excitation methods. This is accomplished in the following
way. First, an infrared thermal image at a certain time is
selected to calculate the average change in the grayscale value
of the aluminum plate before and after heating:
n
 i 1
M
Mi
n
(1)
where M is the average change in the grayscale value; n is the
total number of pixels; and ΔMi
is the difference in the change
of grayscale value.
The uniformity of the thermal excitation C of the aluminum
plate can then be expressed as:

 
C  
1



n
i


MM
n

M
where the greater the value of C, the better the uniformity of
the thermal excitation.
60
i
100%
(2)
The thermal uniformity of the images is presented in
Table 1. This varied according to the thermal excitation device,
testing distance, and duration time. The uniformity improved
as the duration time and radiation distance increased in the
camera's field of view. Comparing the three thermal excitation
IEEE Instrumentation & Measurement Magazine
June 2021
2
Fig. 2. The grayscale values of the aluminum plate excited by: (a) the hot air
gun; (b) the single flash lamp; (c) the 2 × 2 matrix flash lamp array.

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