Why Must CO Detection Use 4.26m? The Wavelength Logic Behind Infrared Gas Sensing

Why Must CO Detection Use 4.26m? The Wavelength Logic Behind Infrared Gas Sensing

Image: https://ecdn6.globalso.com/upload/p/4932/image_other/2026-07/1100x620-independent-website-cover-image-template-1.jpg

Anyone working with gas sensing knows that infrared detection is one of the most accurate and interference-resistant methods available today.

But ask them this: why 4.26m for CO, 3.3m for methane, 4.67m for CO-where do these wavelengths come from, and can you pick a different one? Most can't answer. They'll say, "It's just industry practice."

It's not practice. It's physics. And understanding this is the only way to properly read an infrared gas detection filter datasheet-and avoid costly mistakes during selection.

Every Gas Has Its Own "Infrared Fingerprint"

Gas molecules exhibit a fundamental physical behavior in the infrared band: when incident light frequency matches the molecule's intrinsic vibrational or rotational frequency, the molecule strongly absorbs that specific frequency. This is called resonant absorption.

Different molecular structures have different intrinsic vibrational frequencies, and therefore different absorption peak positions in the infrared spectrum. CO has a strong bending vibration mode, with its absorption peak centered at 4.26m. Methane (CH) has its C-H stretching vibration absorption near 3.3m. CO absorbs at 4.67m. These are inherent characteristics determined by molecular structure-they do not change regardless of the sensor or filter you choose.

The fundamental principle of infrared gas sensing is simple: pass infrared light of a specific wavelength through the target gas, measure the attenuation of light intensity at the absorption peak, and calculate gas concentration. This method is called Non-Dispersive Infrared (NDIR) detection. It offers high sensitivity, strong immunity to cross-interference, no consumable replacement requirements, and excellent long-term stability-which is why it has become the mainstream gas detection technology in industrial and environmental monitoring.

Image: https://p3-sign.toutiaoimg.com/tos-cn-i-6w9my0ksvp/fb2999f1067846e09e56d9b696064417~tplv-tt-origin-web:gif.jpeg?_iz=58558&from=article.pc_detail&lk3s=953192f4&x-expires=1783994880&x-signature=w1yc%2BbFk3wcRCxuvYe%2BcnGR04Rk%3D

What Problem Does the Filter Actually Solve?

The light source in an NDIR system is typically a broadband infrared emitter-it doesn't emit only the target wavelength, but rather a broad spectrum covering the entire mid-infrared band. Without wavelength selection at the receiver end, the sensor picks up a mixture of all wavelengths, and no meaningful absorption information from the target gas can be extracted.

The role of an infrared gas detection filter is to perform precise wavelength selection right in front of the detector-allowing only the narrow band centered at the target gas absorption peak to pass, while rejecting everything else. Only then can changes in the detector signal be unambiguously attributed to changes in target gas concentration.

One detail is often overlooked: the reference channel.

Image: https://p3-sign.toutiaoimg.com/tos-cn-i-6w9my0ksvp/1b236ab586db4e6796f0e883e9693297~tplv-tt-origin-web:gif.jpeg?_iz=58558&from=article.pc_detail&lk3s=953192f4&x-expires=1783994880&x-signature=yp4QyvKb3HzuczDNe%2FcxnPak0dI%3D

A well-designed NDIR system typically uses two channels. One detection channel is aligned with the target gas absorption peak. A second reference channel is positioned at a wavelength where the target gas does not absorb. By taking the ratio of the two signals, systematic errors-such as source aging, window contamination, and temperature drift-are effectively canceled out. Each channel requires a precision narrowband filter, and the center wavelength, bandwidth, and transmittance consistency between them directly determine final measurement accuracy.

Image: https://p3-sign.toutiaoimg.com/tos-cn-i-6w9my0ksvp/f1152b7078e644298af4ade096d74eb7~tplv-tt-origin-web:gif.jpeg?_iz=58558&from=article.pc_detail&lk3s=953192f4&x-expires=1783994880&x-signature=Sn9H5LGddsv8UrpWFEmzx6i6fDQ%3D

Three Pitfalls in Filter Selection That Actually Matter

1. Center Wavelength Tolerance

The absorption peak of CO at 4.26m is not very wide. If the filter's center wavelength drifts, the transmitted light only covers the edge of the peak rather than the peak maximum. The resulting absorption signal is weaker, and detection sensitivity drops. In low-concentration applications-such as indoor air quality monitoring where target levels are in the 400-5000ppm range-this can make the sensor essentially unusable at the low end.

MULTI IR's gas detection filters are manufactured with a center wavelength tolerance within plus-minus 20nm (for mid-IR bands) and peak transmittance above 85%. These specifications are validated through real application testing-not arbitrarily chosen.

Image: https://p11-sign.toutiaoimg.com/tos-cn-i-6w9my0ksvp/53b3593150d54ba08bdaf5a84125edf4~tplv-tt-origin-web:gif.jpeg?_iz=58558&from=article.pc_detail&lk3s=953192f4&x-expires=1783994880&x-signature=QSy3k0djCOWxj%2Fdzo3DDjVV%2BO3s%3D

2. Bandwidth Selection

Narrower bandwidth gives better wavelength selectivity and stronger rejection of adjacent-band interference. But if the bandwidth is too narrow, optical throughput drops, reducing the signal-to-noise ratio in systems with limited source power. For single-component gas detection (such as CO alone), a half-bandwidth of roughly 150-200nm is usually sufficient. For multi-component detection or scenarios where adjacent absorption peaks are close together, narrower bandwidth is required to separate channels cleanly.

3. Blocking Depth and Blocking Range Matching

This was touched on earlier, but there is one specific scenario in gas detection that deserves extra attention: water vapor.

Water vapor absorbs across the entire mid-infrared spectrum, including the region around the CO 4.26m peak. If the filter does not block the water vapor absorption bands sufficiently, changes in ambient humidity will leak into the CO signal and cause false readings. In high-humidity environments, this needs to be verified at the selection stage-not discovered during system debugging.

Image: https://p3-sign.toutiaoimg.com/tos-cn-i-6w9my0ksvp/c587c951c1f2423fbfec24127cba9b32~tplv-tt-origin-web:gif.jpeg?_iz=58558&from=article.pc_detail&lk3s=953192f4&x-expires=1783994880&x-signature=qNMqotp2C62oyim%2BvEcVpSdeD4E%3D

A Question Often Asked: Why Not Just Use a Broadband Filter?

Some might think: if the goal is to let the target band through, wouldn't a simple longpass or shortpass filter be easier?

In low-precision applications, maybe. But in NDIR gas detection, usually not. The reason is simple: broadband transmission lets more background radiation reach the detector, raising the signal baseline and reducing the effective signal-to-noise ratio. To maintain sufficient sensitivity at low concentrations, a narrowband filter is unavoidable.

That is why NDIR gas sensors are always designed as highly integrated optical systems-light source, gas cell, filter, and detector-each component parameter serves the final detection accuracy, and every choice affects the rest.

Image: https://p3-sign.toutiaoimg.com/tos-cn-i-6w9my0ksvp/cba8d9d90eb64b59a0b1fc9c30d43cea~tplv-tt-origin-web:gif.jpeg?_iz=58558&from=article.pc_detail&lk3s=953192f4&x-expires=1783994880&x-signature=t9gkbEmPuAzDzK98iF2tbtIYfRw%3D

Gas detection, on the surface, looks like an electronics problem. At its foundation, it is optics and physics.

The specifications on a datasheet are backed by molecular spectroscopy, thin-film optics, and system integration. Understanding these fundamentals prevents you from being misled by a single attractive peak transmittance number.

If you are developing an NDIR gas sensor or evaluating filter solutions from different suppliers, please contact the MULTI IR technical team directly. We carry stock in all major gas detection bands-CO, CH, CO, NO, and more-and offer custom development based on your specific system parameters, complete with measured spectral curves.

Choose right. Measure right.

Media Contact
Company Name: HANGZHOU MULTI IR TECHNOLOGY CO., LTD.
Email:Send Email [https://www.abnewswire.com/email_contact_us.php?pr=why-must-co-detection-use-426m-the-wavelength-logic-behind-infrared-gas-sensing]
Country: China
Website: https://www.miroptech.com/

Legal Disclaimer: Information contained on this page is provided by an independent third-party content provider. ABNewswire makes no warranties or responsibility or liability for the accuracy, content, images, videos, licenses, completeness, legality, or reliability of the information contained in this article. If you are affiliated with this article or have any complaints or copyright issues related to this article and would like it to be removed, please contact retract@swscontact.com



This release was published on openPR.