Infrared Filters Gain Attention as a Key Component in NDIR Gas Detection
Hangzhou City, Zhejiang Province,China-September 22, 2026
Why NDIR Gas Detection Cannot Do Without That Infrared Filter？
One of the most precision components in a gas analyzer is, surprisingly to many, the filter. It determines what the instrument can measure and how accurately—even before the sensing chip does.
This story starts with the principle of NDIR.
The Physics of Absorption: One Formula Explains It All
NDIR (Non-Dispersive Infrared) gas measurement relies on the absorption of specific infrared wavelengths by gas molecules. Different molecules have their own “fingerprint” vibrational frequencies: CO₂’s O=C=O antisymmetric stretch falls at 4.26μm, CH₄’s C-H stretch near 3.3μm, and CO’s C≡O stretch at 4.67μm.
Absorption follows the Lambert-Beer law: absorbance A = lg(I₀/I) = εcl, which means transmittance T = I/I₀ = 10^(−εcl). The higher the concentration c and the longer the optical path l, the more light is absorbed. Measure the transmittance, back-calculate the concentration—that is the entire logic of NDIR.
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This law holds strictly only at a single wavelength. Real light sources emit broadband infrared, and the absorption coefficient varies across wavelengths. Applying the law directly would break the linear relationship between absorbance and concentration. That is why NDIR uses narrowband filters to approximate quasi-monochromatic light, and then relies on calibration to correct the residual error.
Why a Filter Is Essential
A receiving element such as a thermopile cannot distinguish wavelengths—it sees a mixed signal. Without filtering, the CO₂ signal would be severely interfered with by water vapor and other gases.
The filter “frames” the target gas’s absorption line: only the segment near 4.26μm is allowed through, while other bands are blocked from the receiver. Without it, NDIR cannot obtain a “clean” signal.
There is also an engineering detail: CO₂ has another combination band near 2.7μm, with an intensity about two orders of magnitude weaker than the 4.26μm fundamental band. That region happens to overlap with a strong water vapor absorption band, so the signal is easily swamped. CO₂ detection uses 4.26μm because the fundamental absorption is strong, the signal-to-noise ratio is high, and water vapor interference is much lower.
How to Read the Parameters: FWHM and OD
For gas detection filters, two parameters are critical. FWHM (Full Width at Half Maximum): for a center wavelength of 4.26μm, 1% means a bandwidth of about 43nm—just enough to frame CO₂’s main absorption peak. Blocking depth is expressed as OD (Optical Density). OD3 means out-of-band transmittance does not exceed 0.1% (10⁻³).
The narrower the bandwidth, the less energy reaches the receiver; the wider the bandwidth, the greater the influence of stray light on the signal. Good coating design balances selectivity and signal strength.
The System Behind a Single Filter
A filter does not exist in isolation.
In an NDIR detection system, an infrared source (MEMS thermal emitter, miniature incandescent lamp, etc.) emits radiation, which passes through a gas cell and is absorbed by the gas. It is then selected by a filter integrated in front of the thermopile window and converted into an electrical signal for processing.
Some instruments also include a reference channel, with a filter set at a wavelength the target gas does not absorb (CO₂ systems often use around 3.9μm). This channel reflects only source aging, dust accumulation, and device drift. Using the ratio of the measurement channel to the reference channel for correction is more stable than simple subtraction. The thermal stability of the filter directly determines the instrument’s long-term drift performance.
At this level, MULTI IR’s BP series gas detection filters cover mainstream requirements: BP4260 for CO₂, BP3400 for CH₄, BP4700 for CO, BP7300 for SO₂, BP9600 for O₃, and BP10600 for SF₆ power industry detection. FWHM is within 1% of the center wavelength, and out-of-band blocking reaches OD3.
Filters also suffer from temperature drift and angular drift during use. These issues are often overlooked but play a decisive role in whether filter detection is accurate. MULTI IR has extensive experience in solving temperature drift and angular drift in infrared filters and can provide corresponding solutions.
MULTI IR – National-level “Little Giant” Specialized and Sophisticated Enterprise. Over 10,000 types of infrared sensitive components in stock. Top 3 globally in comprehensive strength. Product portfolio covers infrared filters, optical coatings, infrared sensors, and other core categories. Lead drafter of the infrared filter industry standard.
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