To identify compounds or investigate sample composition, engineers often turn to infrared (IR) spectroscopy. Correlation tables can be found in various resources. One is available online at en.wikipedia.org/wiki/Infrared_spectroscopy_correlation_table.
IR spectrometry works because molecules can absorb energy at specific frequencies determined by the shape of the molecular potential energy surfaces, the masses of the atoms, and the associated vibrational coupling. Diatomic molecules with their single covalent bond can only elongate and contract, producing a single vibrational mode, with harmonics. More complex molecules, with more bonds, have more complex signatures. Families of related organic molecules can be identified by their similar signature absorption spectra (Fig. 1).
It’s common to perform IR spectroscopy on gases by using a single sensor that monitors a beam of infrared light that passes through a sample of the gas. From this, a transmittance or absorbance spectrum can be produced and analyzed. Solids can be analyzed reflectively or by looking at their combustion products.
A single sensor is adequate if the application simply involves identifying the presence or absence of certain chemicals. For example, it could be water in a stream of gas used in some manufacturing process or a contaminant in a stream of anesthetic in a hospital operating theater. Obviously, a 2D or 3D array of sensors offers opportunities for actual imaging.
SINGLE SENSORS
Cal Sensors, a company deeply steeped in single-sensor lore, recently introduced a pair of lead-salt IR arrays. The company specializes in thin-film sensors made from lead salts, specifically lead sulphide (PbS) and lead selenide (PbSe). Lead-salt sensors have less sensitivity (and cost less) than indium-gallium-arsenide (InGaAs) sensors, but are responsive to wider spectra: roughly 1 to 3 µm for PbS and 1 to 5 µm for PbSe.
According to Brian Elias, director of engineering, much of the market for his company’s single sensors is in checking for the presence of dangerous but odorless gases, such as carbon monoxide and dioxide, or oxides of nitrogen, or the presence of water vapor. He added that a strong new application comes from the recycling industry. Many recycled plastics look alike but require different handling at the recycling center. IR makes it possible to separate them readily.
A great deal of information about lead-salt detectors is available online. David A. Kondas notes in “Technical Report ARFSDTR- 92024: Introduction to Lead Salt Infrared Sensors” (www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA260781&Location=U2&doc=GetTRDoc.pdf) that, by the end of World War I, research was being conducted on various lead-salt materials for IR detection and communications applications. In the 1930s, there was considerable research in Germany on lead-salt infrared detectors for military applications. Eventually, the United States military began to study these detectors.
Infrared devices may be either “thermal” or “photon” (quantum) detectors. The most common type of thermal detector is the bolometer. Two types of photon detectors exist: photoconductors and photovoltaics (see the table).
PbS and PbSe detectors are photoconductive. Fabricated using thin-film techniques, they undergo a change in conductivity when exposed to radiation. When incident photons with energy levels in the infrared region bombard the surface of the thin-film semiconductor, they collide with electrons that have energy levels within the valence band of the detector material. This kicks the electrons across the material's energy bandgap and generates electron-hole pairs that can produce a current in the presence of an external electrical field.
If there’s an external voltage across the detector, changes in current can be detected. In simple terms, the more photons, the more current, but only if the photons are at the wavelength to which the material is sensitive. Incidentally, the photoconductor materials in the table are intrinsic semiconductors. They don’t need doping.
More concretely, there’s a cutoff wavelength for photons below which valence electrons can no longer acquire sufficient energy to promote the production of electron-hole pairs. That wavelength is given by ?cutoff = hc/EG, where h is Planck’s constant, c is the speed of light, and EG is the bandgap energy of the photoconductor. For a PbS detector with an energy bandgap of 0.42 eV at 295 K, the cutoff wavelength would be 2.9 µm. Similarly, for a PbSe detector with an energy bandgap of 0.23 eV at 195 K, the cutoff wavelength is 5.4 µm, as stated by Elias.
According to Kondas, some IR detector systems directed at a fixed target may include a center-spun reticule or chopper wheel somewhere between the path of incoming radiation and the detector. There may also be a detector cooler to increase the responsivity, dark resistance, and detectivity at longer wavelengths.
Detectivity, or D*, is the reciprocal of noise equivalent power (NEP), normalized to unit area and unit bandwidth. It’s expressed in units of cm × vHz/W. NEP is the signal power that gives a signalto- noise ratio (SNR) of 1 for an integration time of half a second.
Most lead-salt detectors are designed to operate at three temperatures: ambient (295 K), intermediate (193 K), and low (77 K). The latter two correspond to the boiling point of Freon 13 and the boiling point of liquid nitrogen, respectively.
Generally, Elias said, resistance decreases consistently with temperature. For PbSe, the rate is 2.7%/°C, from –30°C to –80°C. For PbS, the rate is 3.4%/°C, from 23°C to –40°C (Fig. 2).
As photoconductive devices, the sensors require an external bias voltage. Typically, supplying it is straightforward. The sensor is the lower resistor in a voltage divider, and the signal at the top of the voltage divider is applied to a unity-gain buffer amplifier. In general, Elias said, the sensor signal is linearly related to bias voltage (up to approximately the detector maximum bias voltage).
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