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November 2017 VISION SYSTEMS DESIGN www.vision-systems.com 18
However, in the SWIR, they appear different from one another, espe-
cially against the background where it’s commonly desirable for epox-
ies to match the substrate in the visible.
Wavelength is a function of temperature
Imaging is conducted by either reflectance, where the object is illumi-
nated with an external light source, or, if it is hot enough, by measuring
the objects’ emission of light. When an object is illuminated the object
reflects photons from an external light source to the camera. The angle
of the light, wavelength of the light, and its composition will directly
affect the reflective properties of the object.
Alternatively, objects emit light at all wavelengths but the amount
of light is dependent on the object’s temperature. A blackbody is con-
sidered a perfect emitter of radiation. Assuming an object as a perfect
blackbody, Planck’s law describes the emission as:
Bλ(λ, T) = 2hc2 λ5 1 hc
Where l is wavelength, c is the speed of light, h is the Planck Constant,
k B is the Boltzmann constant, and T is temperature in degrees Kelvin.
SWIR thermal imaging
Figure 3 is a plot of emitted light by wavelength for different temper-
ature blackbodies. Since all objects are really imperfect blackbodies,
the amount of emitted light is tempered by the emissivity, which is a
function of wavelength.
Since emissivity is a function of wavelength and the physical properties of an object, when an object such as a coal glows red, it’s obviously hot and should not be touched. As seen in Figure 3, a 500°C
perfect blackbody emits enough energy at 700-750nm in the red to
be seen by human eyes. For temperatures below 500°C, though the
object may still be too hot to touch, it is not visible to human eyes
because the emission at red combined with the emissivity is too low
for it to appear red.
Using infrared cameras that detect light beyond 750nm, enables
easy hot-object detection. Typically, this is done with long wave infrared (LWIR) or mid-wave infrared (MWIR) cameras but it can also be
accomplished with SWIR cameras, depending on the object’s temperature. For example, Figure 4 is a soldering iron glowing in the SWIR. At
approximately 250°C, it glows enough to illuminate a person in a dark
room, essentially acting as a light bulb in the SWIR.
SWIR thermal imaging is commonly conducted at temperatures
Figure 2. Images of various epoxies of different colors on an Aluminum background in the SWIR (left) and visible (right). The reflectivity in the
SWIR is very different from epoxy to epoxy as well as against the background. The color of the epoxy even if it is clear is not relevant in the SWIR.