Radiation Thermometry

1
Radiation Thermometry

• P M V Subbarao
• Professor
• Mechanical Engineering Department

Non-intrusive Methods of Temperature Measurement
2
Temperature Measurement Using Radiation
A radiation thermometer is an instrument which
collects radiation from a target and produces an
output signal, usually electrical, related to the
radiance, which is used to infer the temperature
of the target.
3
Hemispherical Black Surface Emission
Emissive Intensity
The radiation emitted by a body is spatially
distributed
4
Spherical Black Volumetric Emission
The radiation emitted by a body is spatially
distributed
5
Planck Radiation Law

• The primary law governing blackbody radiation is
  the Planck Radiation Law.
• This law governs the intensity of radiation
  emitted by unit surface area into a fixed
  direction (solid angle) from the blackbody as a
  function of wavelength for a fixed temperature.
• The Planck Law can be expressed through the
  following equation.

h 6.625 X 10-27 erg-sec (Planck Constant) K
1.38 X 10-16 erg/K (Boltzmann Constant) C
Speed of light in vacuum
6
The behavior is illustrated in the figure. The
Planck Law gives a distribution that peaks at a
certain wavelength, the peak shifts to shorter
wavelengths for higher temperatures, and the
area under the curve grows rapidly with
increasing temperature.
7
Emissivity

• A black body is an ideal emitter.
• The energy emitted by any real surface is less
  than the energy emitted by a black body at the
  same temperature.
• At a defined temperature, a black body has the
  highest monochromatic emissive power at all
  wavelengths.
• The ratio of the monochromatic emissive power Il
  to the monochromatic blackbody emissive power Ibl
  at the same temperature is the spectral
  hemispherical emissivity of the surface.

8
Basic Ideas for Radiation Thermometers

• The wavelength of maximum emission varies between
  10.6 mm at 0C and 1.3 mm at 20000C.
• For most measurement applications, radiation is
  emitted predominantly in the visible, near- and
  middle-infrared regions of the electromagnetic
  spectrum.
• A radiation thermometer is an instrument which
  collects radiation from a target and produces an
  output signal, usually electrical, related to the
  radiance, which is used to infer the temperature
  of the target.

9

• The radiant flux, El falling on the detecting
  element of a thermometer in the incremental
  waveband dl will be

where A is the throughput of the optical system,
describing the geometric extent of the beam of
radiation falling on the detector Bl is the
spectral transmission of the optical system Pl,
is the spectral transmission of the medium
between the instrument and the target.
10
For a radiation detector whose responsivity, Rl
is independent of all variables but wavelength
where dV is the output in response to the radiant
flux. dEl
Therefore
and
11
This equation is known as the 'radiometer
measurement equation' and relates the output
signal to the target radiance and hence its
temperature.
In practice, the range of wavelengths
contributing to the output of the thermometer is
restricted by the transmission of the optical
system, the spectral response of the detector
and the nature of the Planck function.
12
Design features

• The basic measurement system for a radiation
  thermometer comprises the following elements.
• (1) The target of measurement.
• (2) An optical system which collects and directs
  the radiation.
• Elements of the optical system may also be used
  to modify the spectral response of the
  thermometer.
• (3) A sensor which produces a signal, usually
  electrical, related to the incident energy flux.
• (4) A reference source which may be physically
  situated in the instrument itself or located in a
  calibration laboratory.
• (5) A means of signal processing and display.

13
Anatomy of Radiation Pyrometers
14
(No Transcript)
15

• The design of the instrument must allow a
  measurement to be made with acceptable accuracy
  and repeatability given all the circumstances of
  the target, the instrument itself and the
  surrounding environment.
• The most important choice which faces the
  designer, and indeed the user is that of the
  operating waveband for the instrument.
• There are several factors, some of them
  conflicting, which need to be considered
  carefully when choosing the span of wavelengths
  to be used.
• First of all, consider an instrument sensitive to
  a narrow waveband dl, centered on wavelength l.
• Using the approximate form of Planck's equation
  which is valid for most practical circumstances

16
we differentiate with respect to T to obtain
The error in measured temperature, dT. created by
an error dIb in measuring Ib can be expressed as
17
Precision of Radiation Thermometers

• This relationship indicates that the precision
  with which the output needs to be measured in
  order to achieve a required accuracy increases
  with wavelength.
• For this reason it is advantageous to work with
  the shortest possible wavelength.
• The nature of the Planck's law curve sets a lower
  practical limit on the wavelength which can be
  used at a particular temperature.
• The bandwidth of radiation accepted by the
  instrument must be sufficiently wide to create a
  signal from the detector that can be measured
  with acceptable accuracy, in comparison with the
  system noise.
• Finally, the waveband chosen must be free from
  absorption effects in the sight path of the
  thermometer.
• There is no single solution which is best for
  every application and care must be exercised in
  choosing the correct waveband.

18
Classification of Thermometers

• (1) Partial radiation thermometers
• These use a fraction of the spectrum defined by
  the spectral response of the detector and the
  optical system.
• (2) Total radiation thermometers These use
  virtually the whole of the spectrum.
• (3) Ratio or two-colour thermometers These use
  two distinct wavebands.
• Thermometers of all types may be constructed
  either as portable, hand-held devices or as units
  for permanent installation in a fixed position.

19
Partial Radiation Thermometers

• The advantage of using short wavelengths can be
  conveniently realised by using an instrument
  sensitive to all wavelengths shorter than a
  limiting value which is set by the
  characteristics of the detector or a filter
  incorporated into the optical system.
• Thermometers of this type are widely used in many
  applications for the measurement of temperatures
  above 500C.
• Photon detectors such as silicon and germanium
  photodiodes are often used because their spectral
  response is of an appropriate form, and lies in
  the part of the spectrum where the rate of energy
  emission is high.
• This type of thermometer is the one most
  frequently encountered.

20

• The optical or disappearing filament pyrometer is
  an example of a partial radiation thermometer
  which uses the eye itself as the detector working
  in a comparative mode.
• An electrically heated filament is viewed against
  a background of the target.
• The current through the filament is adjusted
  until its brightness is equal to that of the
  target, at which point it cannot be seen. hence
  the name of the instrument.
• The temperature of the target is inferred from
  the magnitude of the current which current
  flowing through the filament.

21
(No Transcript)
22
Total Radiation Stefan-Boltzmann Law

• The maximum emissive power at a given temperature
  is the black body emissive power (Eb).
• Integrating this over all wavelengths gives Eb.

23
(No Transcript)
24
Ratio Thermometer

• A ratio thermometer is essentially two
  thermometers sensitive to different wavebands
  built into a single body.
• For a thermometer operating at narrow wavebands
  of l2 and l2, the radiances Il1 and Il2 will be
  equal to el1 Ibl1 and el2 Ibl2

25

• The signals from the detectors are processed to
  produce an output which is a function of R.
• If el1 el2 i.e., the body is grey, then the
  output is independent of emissivity.
• Similarly the output will be unaffected by
  partial obscuration of the target provided that
  both channels are equally affected.
• At first sight the ratio thermometer appears very
  attractive.
• It does, however, suffer from some limitations.
• First, very few bodies are exactly grey and the
  deviation from greyness creates a measurement
  error if it is not known accurately.
• Furthermore the inherent accuracy of a ratio
  thermometer is less than that of a
  single-wavelength instrument.
• A single-channel thermometer with wavelength l1
  has

26
Similarly for a ratio thermometer
The ratio thermometer can therefore be considered
to behave in the same way as a single-channel
instrument whose effective wavelength le is given
by
The effective wavelength of the thermometer will
therefore be longer than that of at least one of
the channel and consequently its sensitivity will
be lower than that of a single waveband
thermometer.
27
Anatomy of Ratio Radiation Thermometer
28
Principal components of radiation thermometers

• All radiation thermometers contain the same
  principal elements, namely
• an optical system.
• a detector and
• signal processing facilities.

29
Optical system

• The purpose of the optical system is to collect
  the incoming radiation and direct it onto the
  detector.
• Filters may also be included to restrict the
  waveband used.
• Depending on the nature of the application, the
  desired accuracy of the thermometer and the cost
  that can be tolerated, the optical system may be
  one of the following types
• (1) Aperture optics
• (2) Mirror systems
• (3) Lens systems
• (4) Fiber optics.

30
Aperture of a Pyrometer
31
Effect of Lens
32
(No Transcript)
33
Detectors

• The detector is the key component in a radiation
  thermometer, being the means by which the
  incident radiation is converted to a measurable
  parameter.
• parameters by which a detector is selected.
• (i) Spectral responsivity describes in a relative
  sense the manner in which the output of the
  detector varies with the wavelength of the
  incoming radiation.
• It can be expressed in terms either of output
  per unit of incident energy in a given wavelength
  interval or of output per photon arriving in the
  wavelength interval.
• (ii) Detectivity describes the signal-to-noise
  ratio of the detector in relation to incident
  radiant power, and defines the resolving power of
  the detector.
• (iii) Linearity. A linear relationship between
  the output of a detector and the incident
  radiation flux is a useful property.
• (iv) Response time describes the manner in which
  a detector responds to changes in the incident
  radiation.

34
Thermal Detectors

• The incident radiation causes an increase in
  temperature of the detector, thereby creating a
  change in its temperature-dependent properties.
• The measurement of one of these will provide
  information about the temperature of the detector
  and, by inference, the rate of incident energy
  and the temperature of the source of the
  radiation.
• Thermal detectors generally have a spectral
  response which is uniform over a broad band,
  making them particularly useful for total
  radiation and wide-band thermometers.
• The most commonly used thermal detectors are
  thermopiles, bolometers and pyroelectric crystals.

35
Photon Detectors

• Photon detectors are those in which the incidence
  of a photon causes a change in the electronic
  state of the detector.
• The integrated effect of individual photons
  creates a change of measurable magnitude.
• There are numerous photon effects of which the
  photoconductive and photovoltaic effects are
  those most commonly used for the detection and
  measurement of infrared radiation.
• In either case incident photons excite carriers
  in the detector material from a non-conducting to
  a conducting state.
• The photon must have sufficient energy to
  overcome the gap between the bands,

where h is Planck's constant, l is the
wavelength, c is the velocity of light, E is the
energy of the photon and Es, is the excitation
energy.
36
Spectral response of a photon detector
37
Spectral response of commonly used photon
andthermal detectors
38
Non Black Bodies Determining Emissivity

• There are various methods for determining the
  emissivity of an object.
• Emissivity of many frequently used materials in a
  table.
• Particularly in the case of metals, the values in
  such tables should only be used for orientation
  purposes since the condition of the surface can
  influence emissivity more than the various
  materials themselves.

39
Pyrometer with emissivity setting capability

• Heat up a sample of the material to a known
  temperature that you can determine very
  accurately using a contact thermometer.
• Then measure the target temperature with the IR
  thermometer.
• Change the emissivity until the temperature
  corresponds to that of the contact thermometer.
• Now keep this emissivity for all future
  measurements of targets on this material.

40
Reference Target

• At a relatively low temperature (up to 260C),
  attach a special plastic sticker with known
  emissivity to the target.
• Use the infrared measuring device to determine
  the temperature of the sticker and the
  corresponding emissivity.
• Then measure the surface temperature of the
  target without the sticker and re-set the
  emissivity until the correct temperature value is
  shown.
• Now, use the emissivity determined by this
  method for all measurements on targets of this
  material.

41
Black Body Reference

• Create a blackbody using a sample body from the
  material to be measured.
• Bore a hole into the object.
• The depth of the borehole should be at least five
  times its diameter.
• The diameter must correspond to the size of the
  spot to be measured with your measuring device.
• If the emissivity of the inner walls is greater
  than 0.5, the emissivity of the cavity body is
  now around 1, and the temperature measured in the
  hole is the correct temperature of the target.
• If you now direct the IR thermometer to the
  surface of the target, change the emissivity
  until the temperature display corresponds with
  the value given previously from the blackbody.
• The emissivity found by this method can be used
  for all measurements on the same material.

42
Reference Black Coating

• If the target can be coated, coat it with a matte
  black paint.
• "3-M Black" from the Minnesota Mining Company or
  
• "Senotherm" from Weilburger Lackfabrik, either
  which have an emissivity of around 0.95).
• Measure the temperature of this blackbody and set
  the emissivity as described previously.

43
Merits of Radiation thermometers

• No contact or interference with process
• No upper temperature limit as thermometer does
  not touch hot body
• Accurate and stable over a long period if
  correctly maintained
• Quick response (1 ms to 1 s, according to type)
• Long life
• High sensitivity

44
(No Transcript)
45
Effect of Ambient Conditions
Typical measuring windows are 1.1--1.7 µm, 2
--2.5 µm, 3.5 µm and 8.14 µm.
46
(No Transcript)
47
Point to Field Measurement of Temeprature
48
Optical Imaging for Temperature Measurement

• P M V Subbarao
• Professor
• Mechanical Engineering Department

Simultaneous Measurement of Temperature at
Infinite Locations .
49
Interferometry for Temperature Measurements

• Interferometry is the technique of diagnosing the
  properties of two or more waves by studying the
  pattern of interference created by their
  superposition.
• The instrument used to interfere the waves
  together is called an interferometer.
• Interferometry is an important investigative
  technique in the fields of astronomy, fiber
  optics, engineering metrology, optical metrology,
  oceanography, seismology, quantum mechanics,
  nuclear and particle physics, plasma physics, and
  remote sensing.
• In an interferometer, light from a single source
  is split into two beams that travel along
  different paths.
• The beams are recombined to produce an
  interference pattern that can be used to detect
  changes in the optical path length in one of the
  two arms.
• Here we discuss about the use of the
  Mach-Zehnder interferometer in measurements of
  the index of refraction.

50
Idealized Interferometer
Case 1
Physical distance traveled by beam A1, xa1
Physical distance traveled by beam B1, xb1
Beam B1
Beam A1
Beam B2
Physical distance traveled by beam A2, xa2 lt
Physical distance traveled by beam B2, xb2
Beam A2
Case 2
51
Idealized Interferometer
Case 1
Optical distance traveled by beam A1, n1 la1
Physical distance traveled by beam B1, n1 lb1
Beam B1
Beam A1
Beam B2
Physical distance traveled by beam A2, n2 la2 lt
Physical distance traveled by beam B2, n2 lb2
Beam A2
Case 2
la1 lb1 la2 lb2
52
Schematic diagram of the Mach-Zehnder
interferometer
53
Theory

• In the measurement of the index of refraction
  using the Mach-Zehnder interferometer, a sample
  of thickness d with index of refraction n0 is
  inserted in one of the arms of the
  interferometer.
• The insertion of this sample increases the
  optical path length in this arm due to the fact
  that light travels more slowly in a medium' as
  compared to air.
• The optical path length in the sample is equal to
  n0d.
• When the temperature of the sample changes, the
  index of refraction will change to n.
• This corresponds to a change in the optical path
  length of (n - n0)d.
• This will result in a shift of the fringe pattern
  by Dm fringes where

54
Index of Refraction of Water

• The dependence of the index of refraction n of
  water on wavelength, temperature and density
  hasrecently been studied by Schiebener et.
• Using a large number of experimental data sets
  published between 1870 and 1990 they arrived at
  the following formula

where
55

• r is the density, l is the wavelength, T the
  absolute temperature, a0 to a7 are dimensionless
  coefficients, and
• lr and luv are the effective infrared and
  ultraviolet resonances respectively.

The equation holds for the following ranges
56
Index of Refraction of Air

• The index of refraction n of dry air at 15 C and
  a pressure of 1.01 3 x 105 Pa has been calculated
  from the expression

where s 1/lac and lac is the wavelength in
vacuum of the laser beam in mm. This equation is
valid for wavelengths between 200 nm and 2 mm.
For pressures and temperatures different from
the indicated values, the value of (n -1) has to
be multiplied by
57
Experimental Set-up
Test Field
58
Rayleigh Benard convection
59
Interference Pattern
60
Signs of Pure Conduction
61
Onset of Convection
Ra 5.0 X 104
Ra 1.4 X 104
62
High Rayleigh Number RBC
63
Transition to Turbulence
64
Natural Convection Fringes