Tech Briefs Magazine - November 2023 - PIT-7

ber, the more sensitive the detector.
Thermal sensitivity, also called Noise
Equivalent Temperature Difference
(NETD), describes the smallest temperature
difference observed when using a
thermal device. The lower the NETD value,
the better the sensor will detect slight
temperature differences. Integrators
and developers should look for manufacturers
that can provide NETD performance
at the industry-standard 30 °C. A
word of warning: cameras from low-cost
manufacturers may hide poor sensitivity
by taking NETD at 50 °C instead of the
industry-standard 30 °C. The table below
can be used to rate the sensitivity of a
thermal detector generally.
Increased sensitivity makes thermal
imagers more effective at seeing smaller
temperature differences, especially in
scenes with low thermal contrast and
when operating in challenging environmental
conditions like fog, smoke, and
dust. Selecting a more entry-level, essentially
lower-cost thermal camera that features
" acceptable " to " satisfactory " thermal
sensitivity results in an end product
that offers low contrast scenes, resulting
in poorer image quality, reduced detection
range, and limited situational awareness
compared to cameras with greater
sensitivity. Devices with better thermal
sensitivity are ideal for a wide variety of
uses, from search and rescue to industrial
inspection to security.
To illustrate differences in sensitivity,
compare a 50 mK sensitivity thermal camera
to a 20 mK sensitivity thermal camera
in the images below. The top two images
were captured with a 20 mK cooled core
camera of a handprint on a wall immediately
after the hand was removed from the
wall (left), while the right image shows the
thermal handprint's signature after two
minutes. Compared to the corresponding
bottom images captured with a 50 mK
camera, the camera with 20 mK sensitivity
can still see most of the thermal signature
of the handprint, whereas the less-sensitive
camera shows only the partial remains
of the handprint. The value of sensitivity is
apparent when looking for thermal signatures
in such low-contrast scenes.
Cooled or Uncooled?
IR imaging cameras with a cooled detector
provide distinct advantages versus
thermal imaging cameras with an uncooled
detector. A cooled thermal imaging
camera has an imaging sensor integrated
with a cryocooler, which lowers
the sensor temperature to cryogenic
Photonics & Imaging Technology, November 2023
temperatures. This reduction in sensor
temperature is necessary to reduce thermally-induced
noise to a level below that
of the signal from the scene being imaged.
It can result in significantly improved
thermal sensitivity.
However, these performance improvements
come at a cost. Cooled IR cameras
Figure 1: Thermal sensitivity is a key performance metric for low-contrast scenes, including foggy
weather.
Figure 2: The electromagnetic spectrum includes the infrared waveband ranges from 0.75 µm in the
near infrared to nearly 1 mm (1,000 µm) in the far infrared.
Figure 3: Thermal sensitivity of 20 mK provides significantly more sharpness(right) and clarity than the
same camera at 40 mK sensitivity (left).
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Tech Briefs Magazine - November 2023

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