Infrared waves, which are part of the electromagnetic spectrum, have longer wavelengths than visible light. This allows infrared radiation to pass through some materials that are opaque to visible light. So can infrared radiation see or detect gases?
What is Infrared Radiation?
Infrared radiation, sometimes called infrared light, is a type of electromagnetic radiation with wavelengths longer than those of visible light. The infrared part of the electromagnetic spectrum covers the range from roughly 700 nanometers, just longer than red light, to 1 millimeter, just shorter than microwaves.
Infrared waves were discovered in 1800 by astronomer Sir William Herschel. He found that when he put a thermometer just beyond the red end of the visible spectrum, it registered a temperature increase. This heat radiation became known as infrared.
Infrared radiation is divided into three categories based on wavelength:
- Near-infrared: wavelengths from 700 nanometers to 1.4 micrometers
- Mid-infrared: wavelengths from 1.4 micrometers to 4 micrometers
- Far-infrared: wavelengths from 4 micrometers to 1 millimeter
Infrared radiation has a variety of applications. It is used in astronomy to view distant stars and galaxies, in meteorology to detect clouds and water vapor, and in thermal imaging to create night vision and heat sensing capabilities.
Infrared Interactions with Matter
When infrared radiation encounters matter, a number of interactions can occur depending on the molecular structure of the material and the wavelength of the infrared waves. Some key interactions include:
- Absorption – Molecules can absorb specific infrared wavelengths that match their natural vibration frequencies. This causes the molecules to increase vibration and rotation.
- Reflection – Smooth surfaces like metals will reflect a portion of the infrared waves that hit them.
- Refraction – Infrared waves passing into certain materials will bend or change direction.
- Scattering – Materials like clothing can scatter infrared waves in multiple directions.
- Transmission – Some materials like many plastics allow a high percentage of infrared waves to pass through them.
These interactions allow infrared sensors and cameras to detect the thermal signatures of objects based on how they emit, transmit, absorb or reflect infrared radiation.
Can Infrared Detect Gases?
Yes, infrared radiation can be used to detect the presence of many gases. This is because the molecules of gases tend to absorb infrared wavelengths that match their unique rotational and vibrational frequencies.
By measuring the absorption of specific infrared wavelengths as they pass through a gas, it is possible to identify the gas based on these molecular absorption patterns. Even gases that are invisible to the human eye, like carbon dioxide, can be detected in this manner using infrared.
Principles of Infrared Gas Detection
Infrared gas sensors operate based on the following key principles:
- Molecules of gases tend to absorb infrared radiation at specific wavelengths related to inter-molecular interactions.
- The amount of absorption at these characteristic wavelengths can indicate the concentration of the gas present.
- Absorption follows Beer-Lambert Law – absorption increases logarithmically as gas concentration increases.
- Tune infrared emitters and detectors to wavelengths absorbed by target gas molecules.
By leveraging these principles, even very small concentrations of gases can be accurately detected using infrared sensors. The sensors can be tuned to the specific absorption wavelengths of the gases of interest.
Infrared Gas Detection Methods
There are a few primary methods used to detect gases with infrared radiation:
- Non-Dispersive Infrared (NDIR) – Shines a broadband infrared light through a gas and uses a simple filter to detect absorption at the target wavelengths.
- Dispersive Infrared (DIR) – Uses a prism or diffraction grating to split infrared into multiple wavelengths for analysis.
- Fourier Transform Infrared (FTIR) – Uses interference patterns to analyze absorption of infrared at multiple wavelengths.
- Photoacoustic Infrared – Measures acoustic waves generated by infrared absorption in the gas.
NDIR is the simplest and most common method used in infrared gas sensor designs. It offers an affordable way to detect many common gaseous air pollutants and toxic gases.
Applications of Infrared Gas Detection
Detecting gases with infrared radiation has many useful applications including:
- Air quality monitoring – detecting CO, CO2, VOCs, etc.
- Natural gas detection – methane, propane, etc.
- Anesthetic gas detection
- Combustible gas detection – methane, butane, propane, etc.
- Automotive emissions testing
- Gas leak detection
- Industrial safety gas monitoring
- Breath alcohol and breath analyzers
Infrared gas detection is used in both handheld instruments and fixed installation monitors. Police and military organizations also leverage infrared cameras and optics to visualize gases.
Infrared Detection of Common Gases
Many common gases can be detected using infrared absorption patterns at key wavelengths. Here is an overview of some of the main gases detectable with infrared:
Carbon Dioxide
Carbon dioxide (CO2) is one of the most prevalent gases that absorbs infrared radiation. It has strong, characteristic absorption wavelengths at 4.26 μm in the mid-infrared region.
Infrared gas sensors leverage this absorption peak to measure CO2 concentrations even at low part-per-million (ppm) levels. This allows infrared CO2 sensors to be used in applications like air quality monitoring, climate research, and combustion gas analysis.
Carbon Monoxide
Carbon monoxide (CO) is a toxic gas that absorbs infrared strongly at a wavelength of 4.7 μm. This absorption peak in the mid-infrared range allows NDIR sensors to detect potentially dangerous CO leaks at levels below 1 ppm.
Infrared CO detectors help improve safety by alerting occupants to rising CO levels before they reach hazardous concentrations. They are used in homes as well as industrial environments.
Nitrous Oxide
Nitrous oxide (N2O), also known as laughing gas, exhibits infrared absorption bands between 3.5-4.5 μm. By tuning into these mid-infrared wavelengths, infrared sensors can detect N2O levels down to around 1 ppm.
Infrared N2O detection finds uses in medical applications as well as monitoring emissions from fossil fuel combustion where N2O is a byproduct.
Methane
Methane (CH4) shows multiple absorption peaks in the mid-infrared region from 3-4 μm. These strong, distinct peaks allow reliable detection of methane down to approximately 1 ppm concentration.
Infrared methane detection assists in locating leaks from natural gas pipelines and facilities. It is also used to monitor methane emissions related to agriculture and climate change research.
Hydrogen Sulfide
Hydrogen sulfide (H2S) absorbs infrared radiation around wavelengths of 3-5 μm and has a particularly strong absorption peak at 4.04 μm. This allows infrared hydrogen sulfide detectors to identify dangerous H2S gas leaks at 10 ppb levels or below.
Infrared sensors help protect workers in the oil/gas industry along with waste water treatment where H2S exposure is a hazard.
Ammonia
Ammonia (NH3) displays primary absorption bands between 8-12 μm in the longwave infrared region, providing sensitivities down to around 50 ppm.
Infrared NH3 gas detection is important in industrial refrigeration applications and ammonia-based fertilizer production to protect workers from exposure.
Hydrocarbons
Many hydrocarbon compounds like oils and solvents absorb infrared wavelengths from 8-12 μm. This fingerprint region allows infrared sensors to identify specific hydrocarbons.
Photoacoustic infrared detection is commonly used to monitor hydrocarbon emissions as well as detect hydrocarbon-based gases like propane (C3H8) at ppm levels.
Limitations of Infrared Gas Detection
While infrared absorption provides a very useful technique for sensing gases, it does have some limitations:
- Cannot detect symmetric molecules like O2, N2 that lack dipole moment
- Subject to interference from dust/ dirt accumulating inside the detection chamber
- Water vapor and carbon dioxide can interfere with other gas detections
- Detection limits are typically at ppm or high ppb levels, not extremely sensitive
- Requires a continuous infrared source to function, increasing power consumption
The limitations mean infrared is not well-suited to detecting very low concentrations down to ppb or ppt levels. The technology also will not work for some symmetric molecules. Additional filters and scrubbers may be needed to reduce interference from dust or water vapor.
Other Methods for Detecting Gases
While infrared absorption is one of the most widely used techniques for gas detection, there are some other methods that can be used as well:
- Electrochemical sensors – Detect gases by their redox reactions at an electrode.
- Catalytic sensors – Measure combustible gases based on their heat of oxidation.
- Photoionization detectors – Use UV radiation to ionize gases for detection.
- Semiconductor sensors – Respond to gases changing electrical resistance across thin metal oxide films.
- Colorimetric detector tubes – Use visual color changes due to gas reactions with coated tubes.
These other gas detection methods tend to complement infrared detection with additional capabilities and sensing of different target gases. For example, electrochemical provides excellent sensitivity for toxic gases like carbon monoxide.
Conclusion
In summary, infrared radiation can effectively detect a wide variety of gases due to their unique molecular absorption patterns. By tuning into the specific mid-infrared wavelengths absorbed by gases, it is possible to identify and accurately measure their concentrations down to reasonable ppm levels.
Infrared gas detection serves important functions across many fields including air quality monitoring, leak detection, combustion gas analysis, and industrial safety. While there are some limitations, infrared absorption remains one of the most versatile technologies for sensing common gases.