TECHNICAL ARTICLE | How Does IRLAM™ Achieve High-Accuracy Gas Measurement?
Abstract
HORIBA’s Infrared Laser Absorption Modulation (IRLAM™) technology combines a wavelength-selective Quantum Cascade Laser (QCL), compact multi-pass Herriott Cell and proprietary signal-processing algorithms to measure low concentrations of exhaust gas components. This article explains how these technologies work together to improve sensitivity, reduce interference and support rapid measurement during vehicle and engine development, aftertreatment evaluation, certification preparation and on-road emissions testing.
Original article written by Joshua Israel, Applications Engineering and Technical Marketing Manager, HORIBA Instruments, Inc.
Accurately measuring vehicle and engine exhaust requires more than detecting whether infrared light has been absorbed. Samples can contain water vapor and numerous components with overlapping absorption characteristics. Concentrations may be low at the tailpipe yet change rapidly during acceleration, deceleration, cold-start operation and other transient test conditions.
A measurement system must distinguish the target component from interfering gases, maintain sufficient sensitivity at low concentrations and calculate concentration quickly enough to follow changes in the exhaust stream. These requirements affect laboratory certification testing as well as powertrain development, catalyst evaluation and Real Driving Emissions (RDE) measurements conducted outside the test cell.
IRLAM was developed to address these challenges by combining purpose-designed optical hardware with a proprietary concentration calculation method. Rather than treating the laser, gas cell and calculation algorithm as separate elements, HORIBA developed them as three connected technology pillars.
IRLAM is based on infrared absorption spectroscopy. Gas molecules absorb light at characteristic wavelengths, and the resulting absorption signal can be used to determine the concentration of a target component.
IRLAM applies this principle through three technologies developed specifically for gas measurement:
Together, these technologies support sensitive, selective and rapid measurement without requiring a large optical cell or a high-performance external computer. This combination allows the same underlying measurement principle to be incorporated into laboratory analyzers and more compact systems intended for portable or on-board testing.
Figure 1: The three technology pillars of IRLAM™ and the measurement characteristics they support.
Figure 2: Schematic representation of a target gas absorption feature and absorption from a potentially interfering gas.
Each gas component has characteristic infrared absorption features. Selecting a wavelength region associated with the target gas is therefore fundamental to absorption-based concentration measurement.
IRLAM uses a Quantum Cascade Laser whose wavelength is modulated around a selected absorption feature of the target component. Structural and temperature control of the laser allows the emitted wavelength to be precisely adjusted within the mid-infrared region (~3-100 nm), where many gases relevant to emissions measurement exhibit strong absorption.
This targeted approach limits the wavelength range used in the calculation. It can therefore improve selectivity and reduce the influence of nearby absorption from coexisting gases compared with methods based on a broader light source.
HORIBA develops, designs and manufactures the QCL used in IRLAM specifically for gas measurement. The laser incorporates technologies intended to reduce optical-interference noise and limit wavelength changes caused by variations in ambient temperature. These characteristics help maintain a stable relationship between the selected wavelength and the absorption response used to calculate concentration.
For vehicle and engine emissions testing, wavelength-selective measurement is particularly relevant to NO and NO2. Both gases have absorption features in the mid-infrared region. IRLAM assigns an appropriate light source to the component being measured, enabling direct measurement without first converting NO2 to NO. This provides separate NO and NO2 information for activities such as engine calibration, catalyst development, aftertreatment evaluation and emissions-system validation.
Measurement sensitivity is influenced not only by the selected wavelength but also by the distance the light travels through the sample gas. A longer optical path increases interaction between the light and absorbing molecules, improving the system’s ability to resolve low concentrations.
A conventional approach to increasing optical path length would require a longer physical cell. The Herriott Cell instead uses opposing mirrors to reflect the laser beam repeatedly through the sample. This creates a multi-pass optical path that is considerably longer than the physical dimensions of the cell.
The high directivity of the QCL allows the beam to follow this extended path while maintaining a usable signal at the detector. The result is a compact optical system that supports sensitive measurement without requiring a correspondingly large analyzer enclosure.
This balance between optical path length and physical cell size is particularly important in mobility applications. Laboratory systems must follow transient engine and vehicle operation, while portable and on-board systems must also account for equipment size, sample volume and installation constraints.
Figure 3: Internal structure and exterior of the IRLAM Herriott Cell. Multiple reflections extend the optical path within a compact cell.
Optical measurement produces a detector signal, but that signal must still be converted into an accurate gas concentration. The IRLAM concentration calculation algorithm performs this task by extracting selected features from the measured absorption signal.
When the QCL wavelength is modulated around the target gas absorption feature, the detector output signal, represented as D(t), changes in response to the sample’s absorption spectrum. A logarithmic transformation is applied to the detector output, and the direct-current component is removed to obtain the absorption-modulated signal A(t).
The algorithm then correlates A(t) with predetermined feature reference signals Fᵢ(t) over the modulation period T. This produces a set of feature quantities Sᵢ describing selected characteristics of the measured absorption signal.
Figure 4: Conceptual measurement principle for feature-quantity extraction and interference correction.
Reference feature quantities are established in advance for the target gas and relevant interfering gases. By comparing the quantities obtained from the sample with these reference relationships, the system can estimate the contribution of each component and calculate the concentration of the target gas.
This approach differs from conventional spectral curve fitting. A conventional method may compare hundreds of measured spectral data points with reference spectra and solve a correspondingly large system of equations. IRLAM compresses the relevant information into a much smaller set of feature quantities before calculating concentration.
Figure 5: Comparison of conventional spectrum-fitting and IRLAM feature-based concentration calculation.
Reducing the amount of data required for the calculation lowers the computational workload. According to HORIBA’s development data, the feature-based method can reduce a calculation involving several hundred simultaneous equations to approximately ten equations, shortening calculation time by a factor of approximately 10 to 100.
This makes rapid calculation possible with an embedded processor rather than a large external computing system. The algorithm therefore contributes both to interference correction and to compact analyzers capable of following changing exhaust concentrations during transient test cycles and on-road operation.
Learn more about the IRLAM concentration calculation algorithm
Infrared measurement technologies differ in how they generate light, select wavelengths and convert measured absorption into concentration. Broadband methods analyze a wider range of spectral information, while IRLAM uses a component-specific QCL and a limited wavelength window around a selected absorption feature.
This focused quantum cascade laser gas analysis approach reduces the amount of spectral information that must be processed. When combined with feature-based calculation, it allows the analyzer to distinguish the target signal from relevant interference without performing conventional full-spectrum fitting.
Figure 6: Simplified comparison of infrared gas measurement approaches, including IRLAM™.
These approaches are not interchangeable in every automotive application. The appropriate method depends on the regulated or emerging components being investigated, required concentration ranges, exhaust conditions, response requirements and the number of gases that must be measured simultaneously.
IRLAM also differs from conventional chemiluminescence-based NOx measurement. Chemiluminescence detector systems commonly use an NO2-to-NO converter when determining total NOx. IRLAM measures NO and NO2 directly from their infrared absorption characteristics, removing the converter from the measurement chain.
An SAE technical paper compares IRLAM and conventional CLD measurements using certification-quality NOx data from diluted light-duty vehicle exhaust.
The interaction between IRLAM’s optical hardware and calculation algorithm supports several characteristics relevant to vehicle and engine emissions measurement.
IRLAM-equipped analyzers support the measurement of multiple exhaust components across laboratory, portable and on-board mobility applications.
Components Measured | Mobility Applications |
|---|---|
Available Components depend on the analyzer, optical configuration and intended application.
| Available configurations support applications throughout vehicle and engine development:
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HORIBA applies IRLAM™ technology across laboratory, portable and on-board measurement systems for vehicle and engine emissions testing.
Quantum Cascade Lasers emit light in the mid-infrared region, where many exhaust gas components have strong absorption features. By tuning the laser around a selected feature, the measurement system can target a specific component while limiting the influence of nearby absorption from coexisting gases.
A Herriott Cell reflects the laser beam multiple times through the sample gas. This creates a long optical path within a compact physical space, increasing interaction between the laser light and gas molecules without requiring a correspondingly large measurement cell.
IRLAM combines component-specific wavelength selection with feature-based signal processing. The calculation algorithm compares selected characteristics of the measured absorption signal with reference features for the target and interfering gases, helping estimate each component’s contribution to the signal.
Sensitivity depends on several factors, including the target gas and selected absorption feature, laser performance, optical path length, sample pressure, measurement range, detector performance and signal-processing method. Detection limits should therefore be evaluated for the specific analyzer and configuration.
Quantum cascade laser measurement can support rapid analysis because it targets a limited wavelength region rather than processing a complete broadband spectrum. In IRLAM, the compact gas cell and feature-based calculation method also help the analyzer follow changing concentrations during transient engine cycles and on-road testing. Actual response characteristics depend on the analyzer and sampling configuration.
High-accuracy gas measurement depends on the complete measurement chain: how the light is generated, how it interacts with the sample and how the resulting signal is interpreted.
IRLAM brings these elements together through a wavelength-selective Quantum Cascade Laser, a compact multi-pass Herriott Cell and a feature-based concentration calculation algorithm. The QCL targets an appropriate absorption feature, the Herriott Cell extends the optical path and the calculation method extracts the information needed to determine concentration and correct for interference.
By integrating these technologies, IRLAM supports sensitive, rapid gas analysis for vehicle and engine development. Its architecture enables direct NO and NO2 measurement, wet exhaust analysis and reduced dependence on converter-related components across laboratory and on-road measurement environments.
About the Original Author:
Joshua Israel, Applications Engineering and Technical Marketing Manager, HORIBA Instruments, Inc.
Read the SAE technical paper comparing IRLAM and conventional CLD measurements using certification-quality NOx data from diluted light-duty vehicle exhaust.
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