Standard multi-gas detection

NDIR gas sensors are increasingly being adapted for multi-gas detection, allowing for the simultaneous measurement of multiple gases in a single setup. This capability is particularly valuable in industrial and medical applications. Common approaches are:

• Multiple IR sources: NDIR systems utilize multiple infrared light sources, each tuned to specific wavelengths associated with different target gases, e.g. CO₂, CO and CH₄.
• Multichannel IR detectors: Modern systems often employ multichannel detectors with up to eight channels (see Insights 24/04) by using different bandpass filters each adapted to a specific target gas for a simultaneously signal capturing from multiple wavelengths.

These common NDIR multi-gas sensors face several limitations in performance mainly due to the fixed and equal absorption path length for all channels (Fig. 1). This applies in particular to the dynamic and  measurement ranges as well as the detection limits of the multi-gas sensor.

Adjustable absorption path length

However, each gas has a specific absorption coefficient, is present in different concentrations in a gas mixture, and depending on the application, different emission and exposure limits apply which must be complied with. Therefore, an adjustable absorption path length for each spectral channel increases performance of NDIR multi-gas sensors.

A novel NDIR multi-gas sensor approach enables different and adjustable absorption path lengths in a single optical path (Fig. 2). Single-element detectors are used, each of which is positioned at a reflection point. At each reflection point, a spectral filter transmits a narrow wavelength band (according to a specific gas) to the detector positioned behind it, while reflecting the remaining broadband radiation back into the optical path toward the next reflection point (Fig. 3). The system is modular making it very cost-efficient with a simple adjustment of the path lengths and target gases. In addition, it enables the simultaneous use of different IR detector types, e.g. pyroelectric, thermopile and even photoacoustic detectors, depending on the needs (measurement range, detection limit, costs, etc.).

From tradition to innovation

Traditionally, high-temperature infrared (IR) emitters like the incandescent lamp use a fragile radiating element made of a coiled tungsten wire and a housing made of glass, limiting optical emittance to the short-wavelength infrared (SWIR) or near-infrared (NIR) range. However, a new generation of high-temperature thermal IR emitters (Fig. 1) using robust metal-sheet filaments and sapphire windows extend this range up to a wavelength of 6 microns, covering both SWIR and mid-wavelength infrared (MWIR) spectrums.

A new generation of high-temperature IR emitters

Our unique and patented metal-sheet filament technology (see Insights 24/02)  provides a flat and free-standing IR emitter filament of high robustness and efficiency. The large light-emitting surface can be easily customized to different sizes and geometries by using standard MEMS fabrication processes. A key feature for high-temperature operation is the hermetic housing that is provided by our SOLIDSEAL® technology. Besides available standard glass packages this enables the IR emitters to be equipped with a soldered sapphire window to extend the spectral emittance to significantly higher wavelengths (Fig. 2). Sapphire's hardness and resistance to environmental factors make the emitter robust, especially in harsh conditions with low ambient pressures and temperatures.

Bring unparalleled performance to your projects

In traditional analytical applications, such as infrared spectroscopy, where optical imaging of the light-emitting surface is required, the metal-sheet filament offers a stable and reproducible filament position, eliminates the need for time-consuming lamp positioning and provides a hot spot of high optical output power to ensure reliable and highly accurate measurements.

However, the exceptional performance of this new class of IR emitters enables new possibilities in non-analytical applications, like enhanced imaging and infrared tracking, a contact-less vaporization of liquids (Fig. 3) or even a non-contact pocket lighter with the right optical focus (Fig. 1).

Experience the future of infrared technology today!

A common way to measure gas concentrations

Non-dispersive infrared (NDIR) gas analysis (Fig. 1) is a widely used technique for detecting and quantifying gas concentrations in various industrial, environmental, and medical applications. However, NDIR gas sensors have faced limitations in sensitivity and accuracy, particularly when measuring low gas concentrations. These limitations are primarily due to the performance constraints of the infrared (IR) source and detector components. Conventional infrared sources, such as wire filament and Si-MEMS emitters, have limited optical output power and signal stability, resulting in lower signal-to-noise ratios and reduced measurement sensitivity. 

High-performance IR emitters

INFRASOLID has developed the HISpower series, a range of high-performance thermal infrared emitters in a standard industrial TO-8 housing specifically designed for highly accurate NDIR gas analysis. To demonstrate the unmatched performance an NDIR demonstrator has been built (Fig. 2), that utilizes INFRASOLID’s HIS2000R-CWC300 IR emitter and the world's first eight-channel pyroelectric detector, the InfraTec LRM-278. The IR source’s exceptionally high radiation power generates high detector signals and, therefore, eliminates the need for additional signal amplification (Fig. 3). This allows the analog detector signals to be converted directly into a digital signal for further signal processing. Electronics and signal processing are reduced to a minimum, thus eliminating further sources of noise and reducing manufacturing costs. The wide wavelength range from (2…20) µm enables its use in a broad range of industrial, environmental, and medical applications. 

Pushing the limits of NDIR gas analysis

High-performance IR components play a crucial role in NDIR gas analysis. With an optimized combination of IR source and IR detector the boundaries of NDIR gas analysis can be pushed. In this way, applications that require the measurement of lowest gas concentrations, such as emission monitoring and leakage detection, can meet new legal regulations. 

The silicon carbide (SiC) globar

A silicon carbide (SiC) globar is the most commonly used infrared (IR) light source in measuring devices for IR spectroscopy. It features a high emissivity and operates at high temperatures typically ranging from 1200 K to 1600 K, resulting in a high optical output signal in the mid and far IR range. However, these ceramic-based IR sources are not ideal black-body emitters especially in the far IR and terahertz (THz) range with wavelengths greater than 10 µm (Fig. 1).

Black-body radiation

High optical output of an infrared emitter is achieved through a combination of high emissivity, a large emitting area, and high temperature, as outlined by the Stefan-Boltzmann law. However, it is important to note that according to Planck's law of radiation and Wien's displacement law, an increase in operating temperature results in a shift of the peak intensity of black-body radiation towards shorter wavelengths with a low impact on increasing optical output at longer wavelengths. Hence, to achieve optimal performance in the far IR and terahertz (THz) range, maximizing emissivity and ensuring a substantial radiating area are of utmost importance.

Al₂O₃ ceramic with black coating

In order to increase the emissivity of ceramics in the far IR and THz range a novel black coating has been developed. It can be applied on both sides of a ceramic like Al₂O₃ (Fig. 2) and features an emissivity close to that of a black-body (see Fig. 1). Operating temperatures of 1200 K and more are feasible with this black coating and will lead to higher signals compared to a standard SiC globar (Fig. 3). Additionally, the higher emissivity allows a reduction of the operating temperature which is accompanied by several benefits.

Benefits in FT-IR spectroscopy

In measuring instruments like a FT-IR spectrometer a lower operating temperature of the IR emitter has many advantages: lower temperature drift, higher stability and lifetime, faster measurements, little-to-no risk of fire, no sample heating relevant for biological applications and many more.

Tungsten light bulbs – the broadband standard

The tungsten (halogen) lamp is commonly used as a light source for absorption spectroscopy in the near-infrared wavelength range due to its broad radiation spectrum (Fig. 1). It is also employed in high-volume gas sensing applications, such as measuring CO₂ and hydrocarbons. However, the fragile and thin wire filament does not meet all the requirements of an ideal light source. To ensure reliable and highly accurate measurements, a stable and reproducible filament position is crucial. Achieving this stability involves complex and high-precision manufacturing of light bulbs. Shocks and vibrations might also cause the lamp intensity to flicker and limit the accuracy of measurements. In practical applications, time-consuming effort is often required to position the incandescent lamp correctly.

Improving the filament

The photometric properties of the filament are largely determined by its geometry. The preferred light source is a flat filament with a square light-emitting surface. Most of the emitted light radiates perpendicularly from the flat surface of the filament, aligning with the collection optics for maximum efficiency. INFRASOLID’s unique emitter technology provides free-standing metal-sheet filaments made of a monolithic high-melting metal by emitting a broad spectrum of light in the near- and mid-infrared range (Fig. 2). Unlike the tungsten wire filament that can move in all spatial directions during vibration and shock, the metal-sheet filament experiences only very limited movement along one axis, similar to a clamped sheet of paper. This provides higher mechanical and optical stability, enabling more accurate measurements in harsh environments and with hand-held devices. It also eliminates the need for time-consuming lamp positioning.

Customization

Metal-sheet filaments can be manufactured in different sizes and geometries to easily adapt to customer-specific applications. The larger area of the metal-sheet filament results in higher optical output (see Fig. 1). As the availability of broadband emitting LEDs in the near- and mid-infrared spectral range is very limited thermal emitters like the light bulb will continue to be the standard light source for absorption spectroscopic applications.

SMD-packaged infrared emitters

SMD stands for Surface Mount Device and refers to electronic components that are mounted directly onto the surface of printed circuit boards (PCBs). These packages are designed to be smaller and more efficient than traditional through-hole packages, which are still the standard of infrared (IR) optical components. However, SMD packages are the dominant choice in modern electronics manufacturing due to their advantages in size, cost, performance, and ease of automation in assembly.

INFRASOLID’s unique and patented infrared emitter technology allows the fabrication of highly efficient and miniaturized thermal IR emitters in different SMD packages. As shown in Fig. 1, the high degree of miniaturization enables a compact arrangement of IR emitter matrices with individually controllable elements. The SMD-packaged IR emitters feature a broadband radiation spectrum and can be equipped with different filter windows allowing to emit different wavelengths, i.e. to display different colors.

Automation and high-volume applications

SMD technology increases the efficiency and automation of PCB assembly, leading to higher production rates, fewer errors, less waste, and reduced costs. SMD components tend to be more robust to physical shock and vibration because of their small size and larger surface area in contact with the PCB. The small size enables a higher density of components on a PCB, leading to smaller, more compact electronic devices. SMD-packaged infrared emitters will therefore pave the way to completely new applications, like in hand-held, portable, and wireless devices for gas sensing, material analysis, and remote sensing.

Remote sensing applications

A SMD IR emitter array, e.g. with 3x3 elements, can generate different IR characters, as shown in Fig. 2, for the communication and identification in remote sensing applications as well as for precise alignment of optical systems. It enables also a detection in challenging visual conditions, harsh environments, and over long-distances.

Let's roll the infrared dice together and ask for our SMD IR emitters in your future applications!