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Everyone in gas sensing knows that infrared detection is the go-to method. But most people only remember the wavelength numbers: CO\u2082 at 4.26μm, methane at 3.3μm, CO at 4.67μm.<\/p>\n
Few can explain where these numbers come from, whether they can be changed, or what happens if you try.<\/p>\n
This article starts from molecular physics and lays it all out.<\/p>\n
Why Do Molecules Absorb Infrared Light?<\/p>\n
For a molecule to absorb electromagnetic radiation, one condition must be met: the frequency of the incident light must match the natural frequency of some internal motion mode within the molecule. Resonance must occur for energy to be absorbed.<\/p>\n
For gas molecules, these motion modes are molecular vibrations and rotations. Chemical bonds within molecules are not rigid rods—they behave like springs, oscillating periodically around equilibrium positions. Bond lengths stretch and contract, bond angles bend, and the entire molecule is in constant motion. Each of these vibrational modes has its own natural frequency, determined by atomic masses and bond strengths.<\/p>\n
When infrared light’s frequency matches the natural frequency of a vibrational mode, the molecule resonantly absorbs photons, increasing its vibrational amplitude. Macroscopically, this manifests as a measurable attenuation of infrared radiation at that frequency after passing through the gas.<\/p>\n
CO\u2082 is a linear molecule with three atoms—two oxygen atoms symmetrically arranged on either side of a central carbon. It has three vibrational modes. One of them—the antisymmetric stretch—has a natural frequency that falls precisely at 4.26μm (approximately 2349 cm\u207b¹). This vibrational mode induces a change in the CO\u2082 molecule’s dipole moment, allowing it to efficiently absorb infrared light at this frequency. This is the physical origin of CO\u2082’s characteristic absorption peak.<\/p>\n
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Methane (CH\u2084) has a tetrahedral structure. Its C–H stretching vibrations produce a primary absorption band near 3.3μm (approximately 3016 cm\u207b¹), with a weaker absorption band around 7.7μm. CO has a strong absorption at 4.67μm (approximately 2143 cm\u207b¹) from its C≡O triple bond stretch. NO\u2082 absorbs at 6.2μm, and SO\u2082 near 7.3μm.<\/p>\n
These numbers are not industry conventions. They are physical constants determined by molecular structure. They cannot be changed. The peak positions remain essentially unchanged under any temperature, pressure, or concentration conditions—though temperature and pressure do affect peak width and intensity, they do not shift the peak position.<\/p>\n
Why Can’t You Use a Different Wavelength to Detect the Same Gas?<\/p>\n
The short answer is: outside the characteristic absorption peak, the target gas barely absorbs infrared light at all. The signal change at the detector approaches zero, and concentration measurement becomes impossible.<\/p>\n
The deeper reason lies in the Lambert-Beer Law, which describes the relationship in NDIR gas detection: the attenuation of light intensity is proportional to gas concentration and optical path length, with the proportionality coefficient being the molar absorption coefficient of the gas at that wavelength. At the characteristic absorption peak, the molar absorption coefficient is at its maximum, giving the sensor its highest sensitivity. Move just a few tens of nanometers away from the peak, and the molar absorption coefficient drops sharply—sensitivity falls by orders of magnitude.<\/p>\n
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Take CO\u2082’s 4.26μm peak. The molar absorption coefficient here is hundreds of times higher than at adjacent wavelengths. This means that if a filter drifts by even 30–50nm—landing on the shoulder of the peak rather than its apex—the sensor’s response at low concentrations will be too weak to distinguish from noise. This is why the center wavelength tolerance on infrared gas detection filters is so stringent. It is not a manufacturing overreach—it is a physical requirement imposed by system sensitivity.<\/p>\n
The Filter in an NDIR System: More Than Just “Letting Light Through”<\/p>\n
The basic structure of an NDIR gas detection system includes a broadband infrared source, a gas cell (the optical path through which the sample gas flows), a narrowband filter, and a detector.<\/p>\n
What the filter does here is extract the narrow band corresponding to the target gas’s absorption peak from the broadband radiation and deliver it to the detector. Without a narrowband filter, the detector receives a superimposed signal from the entire broadband spectrum, making it impossible to isolate the target gas’s absorption information.<\/p>\n
A well-designed NDIR system uses two channels: the detection channel’s filter is aligned with the target gas absorption peak (e.g., 4.26μm for CO\u2082), while the reference channel’s filter is aligned with an adjacent band where the gas does not absorb (e.g., 3.95μm). By taking the ratio of the two channel signals, systematic errors—such as source intensity drift, window contamination, and temperature variation—are effectively canceled out. These errors affect both channels identically, and the ratio calculation automatically removes them, leaving only the gas-absorption-induced difference.<\/p>\n
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This dual-channel design imposes specific requirements on the two filters: the center wavelength accuracy, transmittance, and temperature stability of the detection and reference channels must be closely matched, and batch-to-batch consistency must be traceable. If the two filters are mismatched, the ratio calculation will introduce systematic errors, and measurements will drift with temperature changes or source aging—errors that cannot be corrected by calibration algorithms.<\/p>\n
MULTI IR’s gas detection narrowband filter series covers the major target gas bands—CO\u2082 (4.26μm), methane (3.3μm), CO (4.67μm), NO\u2082 (6.2μm), refrigerant R454B, and more. Center wavelength tolerance is held within ±20nm (for mid-IR bands), with peak transmittance above 85%. Detection and reference channels are paired and supplied as matched sets, ensuring dual-channel parameter consistency. These filters are currently shipping in volume across applications including industrial gas leak monitoring, indoor air quality sensing, and online carbon emission monitoring.<\/p>\n
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Blocking Depth: Another Critical Parameter Often Overlooked<\/p>\n
Gas detection imposes stricter requirements on filter blocking depth than many other industrial applications.<\/p>\n
The reason lies in the complexity of gas mixtures. In real industrial environments, the target gas is rarely a single component—CO\u2082, water vapor, methane, and CO may all be present simultaneously, each with its own characteristic absorption at different wavelengths. If a CO\u2082 detection filter does not block deeply enough around 3.3μm, methane absorption can leak through the sideband into the CO\u2082 detection channel, causing artificially high CO\u2082 readings in high-methane environments.<\/p>\n
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Water vapor is the most common interferer in NDIR systems. Water molecules have extensive absorption bands across the mid-infrared spectrum, partially overlapping with the absorption peaks of CO\u2082 and methane. In high-humidity environments, the filter’s blocking performance directly determines whether the system can maintain stable readings as humidity fluctuates.<\/p>\n
This is why MULTI IR pushes its gas detection filters from OD3 toward OD4 blocking depth specifications. Each additional OD reduces transmittance at the corresponding band by a factor of 10—and the crosstalk from sideband leakage is reduced by the same factor. In multi-component gas mixtures, this difference shows up directly in measurement selectivity.<\/p>\n
CO\u2082 responds to 4.26μm not because someone decided it should—but because the CO\u2082 molecule’s antisymmetric stretch vibration has its natural frequency right there. That frequency has existed since the beginning of the universe. It will not change as sensor technology evolves.<\/p>\n
Understanding this is the only way to truly appreciate why center wavelength accuracy matters so much in gas detection filters, why blocking depth cannot be compromised, and why dual-channel parameter consistency is the physical foundation of measurement accuracy.<\/p>\n
Molecular spectroscopy writes the rules. Optical components execute them with precision.<\/em><\/p>\n Media Contact<\/span> Everyone in gas sensing knows that infrared detection is the go-to method. But most people only remember the wavelength numbers: CO\u2082 at 4.26μm, methane at 3.3μm, CO at 4.67μm. Few<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[],"_links":{"self":[{"href":"http:\/\/www.northcarolinaheadlines.com\/news\/wp-json\/wp\/v2\/posts\/543154"}],"collection":[{"href":"http:\/\/www.northcarolinaheadlines.com\/news\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/www.northcarolinaheadlines.com\/news\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/www.northcarolinaheadlines.com\/news\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"http:\/\/www.northcarolinaheadlines.com\/news\/wp-json\/wp\/v2\/comments?post=543154"}],"version-history":[{"count":0,"href":"http:\/\/www.northcarolinaheadlines.com\/news\/wp-json\/wp\/v2\/posts\/543154\/revisions"}],"wp:attachment":[{"href":"http:\/\/www.northcarolinaheadlines.com\/news\/wp-json\/wp\/v2\/media?parent=543154"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/www.northcarolinaheadlines.com\/news\/wp-json\/wp\/v2\/categories?post=543154"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/www.northcarolinaheadlines.com\/news\/wp-json\/wp\/v2\/tags?post=543154"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}
Company Name:<\/strong> HANGZHOU MULTI IR TECHNOLOGY CO., LTD.<\/a>
Email:<\/strong> Send Email<\/a>
Country:<\/strong> China
Website:<\/strong> https:\/\/www.miroptech.com\/<\/a><\/p>\n<\/p>\n","protected":false},"excerpt":{"rendered":"