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PRINCIPLE:

Thermal detectors contain a small active element on which radiation is
focused.
By blackening and insulating the element and by minimum the size of
the element, temperature change and detector response is maximized.
Temperature change is approximately inversely proportional to the
exposed surface area of the element.
As the intensity of the radiation increases the temperature change on
the element of the detector increases.




                                                                      2
TYPES OF THERMAL DETECTORS:
1. Thermocouples        :          Thermocouple

2. Thermistors          :          Pyroelectric detector

3. Pneumatic devices :             Golay cell.

Merits:
 Used for wide wavelength range.
 Linearity in response is seen.

Demerits:
  Slow response time.
  lower sensitivity.
                                                           3
Thermocouple:




                4
THERMOCOUPLE:

o Two dissimilar metals like bismuth and antimony.

o Two ends are called Hot junction, Cold junction.
o The surface at the junction of the wires is coated with black metallic oxide.
o IR radiation falls on hot junction By heat source change in temperature at
  the junction between the metallic wires causes an electric potential to
  develop between the wires.
o The potential difference between the unjoined ends of the wires is amplified
  and measured.
o Cold junction is not exposed to IR
o Response time is 60 m/sec

                                                                            5
THERMISTORS:

• Thermistors are the devices that have an electric resistance that is higher
   temperature dependent.

• The materials used in thermistors are sintered oxide of Cobalt, Manganese
   , Nickel.

• A constant potential is applied across the thermistor and the difference in
   current flow between an illuminated thermistor and a non-illuminated
   thermistor is measured using a differential operational amplifier.

• As the temperature of mixture increases, its electrical resistance decreases.

• Response time is 80 m/sec.

• It should be operate at a frequency of less than 12Hz.
                                                                          6
PYROELECTRIC DETECTORS:
Pyroelectric detector contains a noncentrosymmetrical crystal, it
exhibits an internal electric field along the polar axis.
Pyroelectric effect depends on the rate of change of the detector
temperature rather than on the temperature itself.
These detectors operate with a much faster response time and makes
the choice for Fourier Transform Spectrometers where the response is
essential.
Materials used in pyroelectric detectors
i.    Triglycine sulfate(TGS)
ii.   Deuterated triglycine sulfate(DTGS)
iii. Lithium niobate(LiNbO3)
iv. Lithium tantalate(LiTaO3).                                     7
Golay cell:




              8
Golay detector for far-ir
UsesGOLAY CELL from gas expansion on
      light pointer
heating.




                                       9
GOLAY CELL:
 Golay cell consists of a small metal cylindrical closed by a rigid blackened
metal plate.

Pneumatic chamber is filled with xenon gas.

At one end of cylinder a flexible silvered diaphragm and at the other end
Infra red transmitting window is present.

When infra red radiation is passed through infrared transmitting window
the blackened plate absorbs the heat. By this heat the xenon gas causes
expand

The resulting pressure of gas will cause deformation of diaphragm. This
motion of the diaphragm detects how much IR radiation falls on metal plate.

Light is made to fall on diaphragm which reflects light on photocell
                                                                          10
Response time is 20 m/sec
References:
1.   Instrumental methods of chemical analysis-by gurudeep, R.Chatwal, Sham

     k. Anand.

2.   Instrumental methods of chemical analysis-by Robert D. Braun 6th edition

     page No. 360-364.

3.   www.wikipedia.com

4.   Instrumental Methods Of Analysis by Willard, Merit, Dean, Settle 6th

     edition Page No. 295-298.



                                                                            11
12

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Thermal Detectors: Principles and Types

  • 1. 1
  • 2. PRINCIPLE: Thermal detectors contain a small active element on which radiation is focused. By blackening and insulating the element and by minimum the size of the element, temperature change and detector response is maximized. Temperature change is approximately inversely proportional to the exposed surface area of the element. As the intensity of the radiation increases the temperature change on the element of the detector increases. 2
  • 3. TYPES OF THERMAL DETECTORS: 1. Thermocouples : Thermocouple 2. Thermistors : Pyroelectric detector 3. Pneumatic devices : Golay cell. Merits:  Used for wide wavelength range.  Linearity in response is seen. Demerits: Slow response time. lower sensitivity. 3
  • 5. THERMOCOUPLE: o Two dissimilar metals like bismuth and antimony. o Two ends are called Hot junction, Cold junction. o The surface at the junction of the wires is coated with black metallic oxide. o IR radiation falls on hot junction By heat source change in temperature at the junction between the metallic wires causes an electric potential to develop between the wires. o The potential difference between the unjoined ends of the wires is amplified and measured. o Cold junction is not exposed to IR o Response time is 60 m/sec 5
  • 6. THERMISTORS: • Thermistors are the devices that have an electric resistance that is higher temperature dependent. • The materials used in thermistors are sintered oxide of Cobalt, Manganese , Nickel. • A constant potential is applied across the thermistor and the difference in current flow between an illuminated thermistor and a non-illuminated thermistor is measured using a differential operational amplifier. • As the temperature of mixture increases, its electrical resistance decreases. • Response time is 80 m/sec. • It should be operate at a frequency of less than 12Hz. 6
  • 7. PYROELECTRIC DETECTORS: Pyroelectric detector contains a noncentrosymmetrical crystal, it exhibits an internal electric field along the polar axis. Pyroelectric effect depends on the rate of change of the detector temperature rather than on the temperature itself. These detectors operate with a much faster response time and makes the choice for Fourier Transform Spectrometers where the response is essential. Materials used in pyroelectric detectors i. Triglycine sulfate(TGS) ii. Deuterated triglycine sulfate(DTGS) iii. Lithium niobate(LiNbO3) iv. Lithium tantalate(LiTaO3). 7
  • 9. Golay detector for far-ir UsesGOLAY CELL from gas expansion on light pointer heating. 9
  • 10. GOLAY CELL:  Golay cell consists of a small metal cylindrical closed by a rigid blackened metal plate. Pneumatic chamber is filled with xenon gas. At one end of cylinder a flexible silvered diaphragm and at the other end Infra red transmitting window is present. When infra red radiation is passed through infrared transmitting window the blackened plate absorbs the heat. By this heat the xenon gas causes expand The resulting pressure of gas will cause deformation of diaphragm. This motion of the diaphragm detects how much IR radiation falls on metal plate. Light is made to fall on diaphragm which reflects light on photocell 10 Response time is 20 m/sec
  • 11. References: 1. Instrumental methods of chemical analysis-by gurudeep, R.Chatwal, Sham k. Anand. 2. Instrumental methods of chemical analysis-by Robert D. Braun 6th edition page No. 360-364. 3. www.wikipedia.com 4. Instrumental Methods Of Analysis by Willard, Merit, Dean, Settle 6th edition Page No. 295-298. 11
  • 12. 12