aluminum cans with partially oxidized surfaces and a strip of black
electrical tape. The can on the left is hot, middle can is ambient and
right can is cold.
is an extremely important concept. Thermographers see targets exhibiting
this emissivity contrast behavior every day. It could be an insulated
electric cable with a bare metal bolted connection. It could be a bare
metal nameplate on a painted surface such as an oil filled circuit
breaker or load tap changer. It could be a piece of electrical tape
placed by the thermographer on bus bar to enable a decent reading. The
list is long.
turns out that for opaque objects, the emissivity and reflectivity are
complementary. High emissivity means low reflectivity and vice versa.
The conservation of energy law shows us that:
letters for e, r and t are typically used where emissivity is e,
reflectivity, r and transmissivity, t. For opaque targets, t = 0 and the
equation reduces to:
1.2 is a powerful result. In simple terms it says that a high emissivity
means a low reflectivity. A low reflectivity means a high emissivity.
Thermographers like the emissivity to be as high as possible. They then
get the most accurate reading as most of the radiosity is due to radiant
energy emitted by the target. Modern IR cameras correct for emissivity
with a modicum of user input. But the uncertainty in the measurement
increases with decreasing emissivity. Our calculations show the
measurement uncertainty gets unacceptably high for target emissivities
below about 0.5.
tables abound. But emissivity is a slippery slope. Above, we discussed
emissivity as a material surface property. It is that, and more. The
shape of an object affects its emissivity. For semi-transparent
materials, the thickness will affect emissivity. Other factors affecting
emissivity include: viewing angle, wavelength and temperature. The
wavelength dependence of emissivity means that different IR cameras can
get different values for the same object. And they would both be
correct! We recommend measuring the emissivity of your key targets under
conditions they are likely to be monitored during routine surveys. A
quality IR training course can teach you how. It is not difficult.
is a blackbody, a graybody, a realbody?
blackbody is a perfect radiator. It has zero transmittance and zero
reflectance. According to Kirchhoff’s law, then, the emissivity of a
blackbody is one. Blackbodies were first defined for visible light
radiation. In visible light, something that doesn’t reflect or
transmit anything looks black. Hence the name. A graybody has an
emissivity less than one that is constant over wavelength. A realbody
has an emissivity that varies with wavelength. IR cameras sense infrared
radiant energy over a waveband. To get temperature, they compare results
explained above with a calibration table generated using blackbody
sources. The implicit assumption is the target is a graybody. Most of
the time this is true, or close enough to get meaningful results. For
highly accurate measurements, the thermographer should understand the
spectral (wavelength) nature of the target.
Planck is credited for developing the mathematical model for blackbody
radiation curves. The accompanying figure shows the magnitude of emitted
radiation due to an object’s temperature vs. wavelength for various
temperatures. Note the sun has a peak wavelength in the middle of our
visible light spectrum.
curves are nested. They do not cross each other. This means a blackbody
at a higher temperature will emit more radiation at every wavelength
than one at a lower temperature. As temperature increases, the
wavelength span of radiation widens, and the peak of radiation shifts to
shorter and shorter wavelengths. Note, the peak of infrared radiation at
300K (about 27C, 81F) is about 10 mm.
Also, an object at 300K emits radiation only down to about 3 mm.
Since our eyes are not sensitive beyond about 0.75 mm,
we cannot see this. But if we warm the object up to about 300C, we can
just begin to see it glow faintly red.
can’t infrared film be used for thermal imaging applications?
is a question that people have been asking for over twenty years.
Understanding the Planck curves discussed above makes this an easy
question to answer. An object must be hot enough to radiate at short
enough wavelengths to expose infrared films that have special emulsions
sensitive from 0.5 to 0.9 mm. Or, the object must reflect the radiation
from a hot object. The latter has been the classic use of IR film. It
was initially developed during World War II to detect camouflaged gun
emplacements. The enemy had done a good job of creating camouflage that
looked like trees and bushes, difficult to detect analyzing visible
light aerial photographs. But healthy vegetation reflects sunlight in
the near infrared quite strongly. Enemy camouflage did not. Infrared
film was a real breakthrough and made the air photo interpreter’s job
photographic emulsions can be used to study the distribution of objects
that are hot enough to emit infrared energy just below red heat levels
such as stoves, engine parts, high-pressure boilers, etc. The range of
temperatures that can be recorded is from approximately 250°C to 500°C
(482°F to 932°F). In comparison, electronic thermography can be used
on objects with temperatures ranging from -40 °C to more than 1500 °C
(-40 °F to > 2730 °F). So if you wanted to see heat loss from your
house with infrared film, it would have to be on fire! If you want to
capture and image of your house in infrared under ambient, non-fire,
conditions, infrared film will not work. You must use a thermal infrared
can Infrared Thermography be used?
thermography is such a valuable and versatile tool that we cannot
possibly list all the applications. New and innovative ways of using
the technology are being developed everyday.
can be applied in any situation where a problem or condition can reveal
itself by means of a thermal difference. For many situations, this is
quite easy to apply; a thermal condition can be seen because the process
involves release of thermal energy. An example of this is inspecting the
condition of electrical distribution equipment. When electrical current
passes through a resistive element, heat is generated. If the target
emissivity is high enough, we can see that heat with an infrared camera.
Sliding and bolted connections can become resistive through loosening,
corrosion, etc. This increase in electrical resistance usually results
in increase in heat generation and the camera can quickly pick it up.
Sounds simple, and often it is. Frequently, it is not simple due to the
nature of heat transfer. Good training is the key to successful
application of infrared technology.
example is the inspection of concrete bridges. As many of us know,
concrete can develop delaminations, which can lead to potholes. When a
pothole develops, it is quite easy to detect; usually your tire and
wheel “find” the hole and you end up with an unpleasant repair bill.
Wouldn’t it be great if we could find these before they cause
problems? By cleverly using the sun’s energy as a heating medium, and
viewing with an infrared camera; we find that subsurface delaminations
have a different heating effect than the sound parts of the deck
structure, so the camera can see it. This example shows that even though
the bridge deck doesn’t generate heat it can still be analyzed with
thermography given the proper conditions.
is another example of an application where we can use passive heating or
cooling. Recently developed composite aircraft materials are extremely
sturdy and lightweight. These materials are vital to aircraft
performance and airworthiness. However, the honeycomb structure of this
material presents a potentially dangerous problem: water ingress.
has been discovered that certain control surfaces tend to absorb water
in the honeycomb structure, for reasons that are not fully understood.
The problem is aggravated by the effects of lightning and hail, which
cause barely visible impact damage. The water enters the honeycomb and
freezes when the aircraft is at high altitude. As the ice expands it
breaks down the cells in the structure. This condition grows like a
cancer and eventually jeopardizes the entire structural integrity of the
recently, the only effective method of diagnosing the problem was
through radiography. While this is still the most accurate way, it has
several disadvantages: it is expensive in time, equipment, and manpower,
and can expose maintenance personnel to hazardous ionizing radiation.
can be an indispensable tool for inspecting planes for this problem.
After the plane has landed, the ice remains at 0 C while it is melting.
The rest of the plane has warmed to ambient temperatures on the
approach. This provides an ideal opportunity to search for the ice
pockets with a thermal imaging system while the plane is being serviced.