1 April 2002
A spectrum of IR perspectives
By Ellen Fussell
Spectrometry, process control, plastics manufacturing, preventive maintenance, and nuclear liquid-level detection are only a few areas that have reaped the benefits of infrared (IR) technology.
IR is an invisible energy with a longer wavelength than visible light, according to Infrared Inc., a Reno, Nev.–based company specializing in IR imaging systems, surveillance, and security platforms. While only objects at a very high temperature emit visible light energy, all objects at ordinary temperatures emit IR energy. This wavelength is based on the electromagnetic spectrum (see story below).
Electro Optical Industries Inc., a Santa Barbara, Calif.–based worldwide provider of IR test and calibration instrumentation, described IR as that region of the spectrum that extends from the visible region to about 1 millimeter (in wavelength). IR detectors come in two classes: thermal and quantum. Thermal IR detectors include responsivity with little dependence on wavelength and operation at room temperature. Quantum detectors have high detectivity and fast response speed.
Ideal infrared instances
IR or noncontact temperature sensors measure heat, movement, and objects out of reach, as Karen Ackland explained in a June 1998 InTech article, "Selecting the right infrared temperature sensor."
"An infrared temperature sensor collects radiation from a target in the field of view defined by the instrument’s optics and location," she said. "The infrared energy is isolated and measured using photosensitive detectors. The detectors convert the infrared energy to an electrical signal, which is then converted into a temperature value based on the instrument’s internal algorithms and the target’s emissivity"—a term referring to the emitting qualities of the target’s surface.
Semiconductor manufacturers rely on laser IR techniques in analytical systems to monitor moisture contamination in corrosive gases. In a September 2001 SensorTech article, Tiger Optics’ Wen-Bin Yan explained the features of his company’s advanced IR-based analyzer, the MTO-1000, which analyzes ultratrace gas impurities in semiconductor manufacturing.
"Compared to the current conventional moisture analyzers, like an electrolytic-based analyzer, this is extremely fast because it’s laser based," Yan said. "The speed of response is within less than a second, and that means you can monitor any change quickly so you can take corrective action sooner. Otherwise, by the time you see the change it’s too late, and the damage is already done."
Another form of IR—Fourier transform IR (FT-IR) and Fourier transform near IR (FT-NIR)—sees use in chemical, petrochemical, pharmaceutical, and certain polymer manufacturing plants. Midland, Mich.–based Dow Chemical uses FT-IR and FT-NIR to measure polymers—small molecules strung together and the basis of all plastics—and to control its polymerization process online.
"When you’re talking about control, you normally think of measuring temperature, pressure, or flow rate," said Rich Harner, a senior research specialist at Dow. "With online analyzers we have another control point: monitoring the chemical composition as easily as you monitor the temperature. And the chemical composition is what the process is after, ultimately."
Harner said one advantage of near IR is the ability to use fiber optics, "which gives us a nice system for going into hazardous process areas. If you can put the probe directly into the process pipe, you have a real-time measurement of the composition in that pipe."
By using near IR in polymerization and in the chloralkali process (sodium hydroxide and sodium chloride, or measuring salt and hydroxide in water), manufacturers can make process control decisions based on analytical or compositional data that’s occurring in real time, speeding up the response time to process changes, Harner said.
With FT-NIR plus fiber-optic interface, Dow manufacturers have actually moved the lab equipment out into the process area "so the sample hasn’t changed when you get back to the lab," Harner said. "We can actually measure immediate composition in the pipe. So there is virtually no time delay except maybe a couple of seconds vs. a sample system and gas chromatograph that could get your results back in minutes or maybe as long as a half hour. In fact, running back to a lab could take hours and perhaps days," he said.
That’s significant because "if time is critical in the control [chemical conversion] of the process, then the faster you get the composition result back, the better you can do process control."IT
What’s the electromagnetic spectrum?
The electromagnetic spectrum, as explained by Electro Optical Industries (www.electro-optical.com), is a continuum of all electromagnetic waves arranged according to frequency and wavelength. The spectrum includes visible light, ultraviolet, IR, microwave, radio, and gamma waves. All electromagnetic energy passes through space at the speed of light in the form of sinusoidal waves.
In the electromagnetic spectrum, wavelength and frequency define radiation. The spectrum of waves divides into sections based on wavelength. The shortest waves are gamma rays. The longest are radio waves. Visible light is a particular band of electromagnetic radiation the human eye can see and sense. This energy consists of the narrow portion of the spectrum. The IR ranges start at the end of the red spectrum.