Infrared energy falling on a material leads to increase in its thermal energy. The thermally induced changes can be monitored basically by three different types of detectors:
Thermocouples and Bolometers
Thermocouples comprise of a pair of junction of two different metals . IR radiation falling on one junction raises its temperature whereas the other junction is maintained at a constant temperature .The temperature difference between the two junctions results in a potential difference between the two which is in proportion to the temperature difference between them.Several thermovouples connected in seriesare called a thermopile.
Bolometers on the other hand are pure metal strips whose resistance changes significantly with change in temperature e.g. platinum. Resulting changes in resistance are proportional to the radiation reaching the bolometer.
The advantages of thermocouples and thermopiles are the usefulness over a wide range of wavelengths with a stable response but suffer from slow response time and low sensitivity.
Pyroelectric detectors are most popular in IR spectroscopy. These are made from a single crystalline wafer of a pyroelectric material such as Deutrated Triglycerine Sulfate (DTGS) and Lithium tantalate . (LiTaO3).On application of IR radiation the polarisation of the crystal lattice. changes The change depends on rate of temperature change
Absorption of IR radiation results in a decrease in the electrical resistance which can be monitored. The response and response time ,linearity and sensitivity of photoconductive detectors such as Mercury Cadmium Telluride (MCT) is higher than pyroelectric detectors such as DTGS. High-speed scanning applications requiring higher sensitivities such as hyphenated techniques particularly FT-IR coupled with GC give excellent sensitivity and speed of scanning.
Photoconductive detectors require cooling with liquid nitrogen to reduce thermal noise. A brief reference of useful range of common FTIR detectors is provided below:
|DETECTOR||TYPE||APPLICATION RANGE (\(cm^-^1\))|
|MCT – B (Wide Band)||Photoconductive||12500-400|
|MCT – A (Narrow Band)||Photoconductive||12500-650|
|DTGS||Pyroelectric||Mid to far – IR depending on window material|
|Lithium Tantalate||Pyroelectric||Mid to far – IR depending on window material|
|PbS||Photoconductive||10000 – 3200|
|In Sb||Photoconductive||10000 – 1500|