PRESENTATION BY K.CHARMILA ( 13131D0405)
INFRARED RADIATION
SIR FREDERICK WILLIAM HERSCHEL found the infrared radiation in
1800
First called as CALORIFIC RAYS and then renamed as INFRARED RAYS
or Infrared radiation
Infrared radiation lies between the visible and microwave
portions of the electromagnetic spectrum.
INFRARED RADIATION
Infrared waves have wavelengths longer than visible and shorter
than microwaves, and have frequencies which are lower than visible
and higher than microwaves.
Infrared is broken into three categories:near, mid and
far-infrared.
NEAR-INFRARED: part of the infrared spectrum that is closest to
visible light
FAR-INFRARED : part that is closer to the microwave region.
MID-INFRARED: is the region between visible light and microwave
region
CHARACTERISTICS OF INFRARED RADIATION
INVISIBLE TO HUMAN EYES : This is useful for security
applications but sometimes makes measurement and optical systems
difficult
SMALL ENERGY : Infrared radiation energy is equal to vibrational
or rotational energy of molecules hence it can easily identify
molecules
LONG WAVELENGTH : This means infrared radiation is less
scattered and offers better transmission through various medium
EMITTED FROM ALL KINDS OF OBJECTS
INVISIBLE TO HUMAN EYES : This is useful for security
applications but sometimes makes measurement and optical systems
difficult
SMALL ENERGY : Infrared radiation energy is equal to vibrational
or rotational energy of molecules hence it can easily identify
molecules
LONG WAVELENGTH : This means infrared radiation is less
scattered and offers better transmission through various medium
EMITTED FROM ALL KINDS OF OBJECTS
INFRARED THERMOGRAPHY
Infrared thermography is the process of using a thermal imager
to detect infrared radiation (heat) that is emitted by an
object.
The technology allows operators to validate normal operations
and, more importantly, locate thermal anomalies (abnormal patterns
of heat invisible to the eye) which indicate possible faults,
defects or inefficiencies within a system or machine asset.
An Infrared Camera Senses Infrared Radiation
A Processor In The Camera assigns color to the Infrared
Radiation-different color equals different temperature enabling us
to visualize the thermal world
INFRARED IMAGE
Color Palette Range Is Chosen For Clarity
Color Palette 1
White = T>85FBlack = T95F
Black = TT0.
If the incoming power Psignalchanges and Pbiasstays constant the
temperature T will change.
A bolometer works by measuring this change of T with a
thermometer which is directly attached to the absorber.
THERMOPILE DETECTOR
A thermopile is a serially-interconnected array of
thermocouples, each of which consists of two dissimilar
materials
The thermocouples are placed across the hot and cold regions of
a structure and the hot junctions are thermally isolated from the
cold junctions.
In the hot regions, there is a black body for absorbing the
infrared, which raises the temperature according to the intensity
of the incident infrared.
THERMOPILE DETECTOR WORKING
Thermopile works on the principle of Seebeck effect.
The ends are connected to a galvanometer which is represented as
G.
One set of junctions ie.1,3,5, is blackened to absorb completely
the thermal radiation falling on it.
The other set of junctions (2,4) called cold junction is
shielded from the radiation.
When thermal radiation falls on one set of junctions
(1, 3, 5) a difference in temperature between the junctions is
created and a large thermo emf is produced.
The deflection in the galvanometer is proportional to the
intensity of radiation.
PYROELECTRIC DETECTORS
Pyroelectric produce a signal in response to a change in their
temperature.
Below a temperature Tc known as the Curie point, ferroelectric
materials such as TGS or Lithium Tantalate, exhibit a large
spontaneous electrical polarisation.
If the temperature of such a material is altered, for example,
by incident radiation, the polarisation changes.
This change in polarisation may be observed as an electrical
signal if electrodes are placed on opposite faces of a thin slice
of the material to form a capacitor.
When the polarisation changes, the charges induced in the
electrodes can be made to produce a voltage.
The sensor will only produce an electrical output signal when
the temperature changes; that is, when the level of incident
radiation changes.
At the heart of every PIR detector is the pyroelectric
crystal.
Typical detectors use materials, such as triglycine sulfate
(TGS) or lithium tantalite.
They are ferroelectric crystals, which have a maximum
pyroelectric sensitivity at room temperature and therefore do not
require the cooling for detection of temperature changes.
And both TGS and lithium tantalite exhibit a large spontaneous
electrical polarization below their Curie points
QUANTUM DETECTORS
Unlike thermal detectors, quantum detectors do not rely on the
conversion of incoming radiation to heat, but convert incoming
photons directly into an electrical signal.
The decisive difference between quantum detectors and thermal
detectors is their faster reaction on absorbed radiation.
The mode of operation of quantum detectors is based on the photo
effect.
When photons in a particular range of wavelengths are absorbed
by the detector, they create free electron-hole pairs, which can be
detected as electrical current.
Photon (quantum) IR detectors generate an output signal that is
proportional to the number of photons absorbed in the device
material rather than to their total energy.
At the same time the energy of each single photon must be high
enough to cause delocalization of carriers across the device
structure, resulting in increasing the device conductivity (as in
photoconductive detectors) or in generating potential difference
across a junction (as in photovoltaic detectors).
These detectors are characterized by selective energy (or,
wavelength)-dependent response.
The signal output of a quantum detector is very small and is
overshadowed by noise generated internally to the device at room
temperatures.
Since this noise within a semiconductor is partly proportional
to temperature, quantum detectors are operated at cryogenic
temperatures [i. e. down to 77 K (liquid nitrogen) or 4 K (liquid
helium)] to minimize noise.
This cooling requirement is a significant disadvantage in the
use of quantum detectors.
However, their superior electronic performance still makes them
the detector of choice for the bulk of thermal imaging
applications.
Some systems can detect temperature differences as small as
0.07C.
PHOTOELECTRIC DETECTORS
Photoelectric detectors are semiconducting devices that convert
optical signals into electrical signals.
They rely on the quantum nature of matter, whereby incident
photons, ranging from high energy gamma rays through visible light
to infrared rays, excite electrons in matter to produce an
electrical charge.
The operation of a photoelectric detector involves two steps:
(1) conducting charge generation by an incident light photon and
(2) charge transport and/or multiplication by whatever current gain
mechanism may be present. Photoelectric detectors are classified as
photoemission and photoconductive detectors.
PHOTOCONDUCTIVE DETECTORS
Photoconductors are the simplest conceivable optical
detector.
The device consists of a piece of (undoped) semiconductor
material with electrical contacts attached.
A voltage is applied across the contacts. When a photon arrives
in the semiconductor it is absorbed and an electron/hole pair is
created.
Under the influence of the electric field between the two
contacts the electron and the hole each migrate toward one of the
contacts.
The electron to the positive contact and the hole to the
negative contact. Thus the resistance of the device varies with the
amount of light falling on it.
PHOTO EMISSIVE AND PHOTO VOLTAIC DETECTORS
Photoemission occurs when the energy of the photonh is
sufficient to eject an electron from the surface of the solid into
vacuum.
Photoconductivity, sometimes referred to as internal
photoemission, is observed in a solid when the absorbed photon
raises an electron from a non-conducting energy state to a
conducting energy state where it contributes to electrical
conductivity
photovoltaic current is generated as a result of the absorption
of photons of a voltage difference across a p-n junction
(generation of voltage).
In general, photoconductive detectors have a higher frequency
response, however they also have a higher signal to noise
ratio.
ADVANTAGES OF INFRARED THERMOGRAPHY
fast inspection rate (up to a few m2 at a time)
no contact
security of personnel (since there is no harmful radiation
involved, however high power external
results are relatively easy to interpret (you see what you are
inspecting) since they are (often)
obtained in image format, furthermore images can be processed to
extract more information;
wide span of applications
unique inspection tool for some inspection tasks (e.g. as in the
case of some ceramic coatings hardly inspected by other NDT
approaches or in the case of some maintenance surveys).
LIMITATIONS
difficulty in obtaining a quick, uniform and highly energetic
thermal stimulation over a large surface
effects of thermal losses (convective, radiative) which induce
spurious contrasts affecting the reliability of the
interpretation
cost of the equipment
capability of detecting only defects resulting in a measurable
change of the thermal properties
emissivity problems
APPLICATION AREAS OF INFRARED TECHNOLOGY IN NDT
REFRACTORIES
Furnaces and
Boilers
Commercial Facilities
MANUFACTURING
POWER GENERATION
PETROCHEMICAL
HIGH VOLTAGE SUBSTATIONS
METAL REFINING
