Detectors used in IR Spectroscopy
• Thermocouple
• Bolometers
• Thermistors
• Golay Cells
• Pyroelectric detectors
Dispersive IR
FTIR
Thermocouple
• Thermocouples are formed by joining two dissimilar metals (e.g., bismuth and
antimony) that create a voltage at their junction forms thermocouples
• IR radiation heats the blackened hot junction, creating a temperature difference.
• This produces a voltage between the wire ends, which is amplified and measured.
• The cold junction is kept away from IR, with a fast response of about 60 msec.
Bolometers
• It consists of thin metallic conductor, its resistance changes due to increase in temperature when IR radiation
falls on it.
• It is a electrical resistance thermometer which can detect and measure feeble thermal radiation.
• The electrical resistance increases approx 0.4% for every celsius degree increase of temperature .
• The degree of change in resistance is regarded as the measure of the amount of IR radiation falling on it.
• A bolometer is made of two platium strips, covered with lamp black, one strip is sheilded from radiation and
one exposed to it. The strips formed two branches of wheatstone bridge
When IR radiations falling on the exposed
strip would heat it, and change the
resistance, this causes current to flow, the
amount of current flowing is a measure of
intensity of IR radiation
Thermistors
• It is made up of metal oxides which functions by changing resistance when heated.
• It consists of two closely placed thermistor flakes, one of the 10 um is an active detector, while the
other acts as the compensating / reference detector.
• A steady voltage is applied, due to the temperature increase there is change in resistance which is
measured and this gives the intensity of the IR radiation
Golay Cell Detector
• 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
• Response time is 20 m/sec
Pyroelectric detector
• Pyroelectric material is sandwiched in the form of single crystalline wafers between two electrodes, one of
which is IR transparent, a temperature dependent capacitor is formed.
• On absorption of IR, temperature changes, and alters the charge distribution across the crystal. This change in
charge distribution can be detected as current in an external electric circuit connecting the two sides
of the capacitor.
• Pyroelectric crystals lose their residual polarization when heated to a temperature called the Curie point. For
TGS, Curie pt is 47deg Celcius
• Temperature fluctuations produce a charge change on the surface of pyroelectric crystals, which produces a
corresponding electrical signal.
• This temperature gradient can be created by the absorption of light.
• Materials with special electric and thermal properties eg. DTGS (Deuterated Triglycine Sulphate), LiTaO3
Lithium tantalate, LiNbO3 Lithium Nibonate, Barium titanate
When temperature of the crystal changes (increases), the polarization decreases, and current
flows though the ammeter until a new equilibrium is reached.
Application of IR Spectroscopy
• Identifies functional groups and determines molecular structure
• Provides unique molecular "fingerprint" for substance identification
• Used in quality control and measurement of materials
• Key tool in forensic science for analyzing fibers, residues, and fluids
• Characterizes pharmaceuticals, proteins, and biopharmaceuticals
• Monitors environmental pollutants and atmospheric gases
• Detects contaminants and analyzes composition in the food industry
• Assists art conservation by identifying pigments and materials
• Probes protein structure and conformational changes in life sciences
• Supports research in semiconductors, catalysts, and fast kinetic studies
• Non-destructive, rapid, and applicable to solids, liquids, and gases
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