Analytical Instrumentation

Exploring Tunable Diode Laser Spectrometer (TDLS): Components, Principles, and Industrial Applications for Gas Measurement

  • Tunable Diode Laser Spectrometer (TDLS) is an essential tool used for gas measurement in process ducts. 
  • It operates on the principle of absorption spectroscopy, where the amount of laser light absorbed by the gas is measured to determine its concentration. 
  • This technique is particularly valuable in industrial settings for monitoring gasses like oxygen and other process-related constituents.
  • Absorption spectroscopy is founded on the interaction between light and matter, particularly gas molecules, wherein these molecules absorb light at different wavelengths. 
  • This absorption phenomenon stems from the quantum mechanical nature of molecules, as they transition to higher energy states upon absorbing incident light. 
  • Beer’s law governs the intensity of absorption, dictating that the amount of light absorbed is directly proportional to the concentration of the absorbing species and the path length of the light through the substance.
Exploring Tunable Diode Laser Spectrometer
  • The TDLS system employs a laser to generate infrared light, serving as the probing signal for gas measurement.
  • The laser emits a monochromatic beam of light with precise control over wavelength and intensity.
  • It provides the energy required for interaction with gas molecules, initiating the absorption process.
  • Optical lenses play a crucial role in the TDLS system by focusing the laser light through the gas sample.
  • By ensuring efficient interaction between the laser light and the gas molecules, the lenses optimize the absorption process.
  • They help in collimating the beam and directing it towards the gas sample, enhancing the sensitivity and accuracy of measurements.
  • A detector is tasked with capturing the transmitted light after it interacts with the gas sample.
  • This detector quantifies the amount of absorbed light, providing essential data for determining gas concentration.
  • Various types of detectors can be utilized, including photodiodes or photomultiplier tubes, depending on the specific requirements of the TDLS system.
  • The choice of detector depends on factors such as sensitivity, response time, and spectral range.
  • Electronic components within the TDLS system govern the operation of the laser and process the detector signal.
  • These components translate the detected signal into a representation of gas concentration, typically through signal amplification, filtering, and digitization processes.
  • Additionally, the electronics facilitate the analysis and interpretation of absorption patterns, allowing for accurate determination of gas concentration.
  • The electronics also handle communication between different components of the TDLS system, ensuring seamless operation and data integration.
  • In TDLS systems, a transmitter-receiver control unit is often integrated to manage the transmission and reception of signals.
  • This unit controls the timing and synchronization of the laser emission and detection processes.
  • It ensures proper coordination between the laser, optical components, and detector, optimizing the performance and reliability of the TDLS system.
  • The transmitter-receiver control unit may also include features for calibration, diagnostics, and system monitoring, enhancing the overall functionality and usability of the TDLS system.
  • In many TDLS systems, particularly those operating in harsh or contaminated environments, the optical components such as lenses and windows are susceptible to contamination or degradation over time.
  • To mitigate this issue, a purge gas, often a clean and dry inert gas such as nitrogen, is employed to create a protective environment around the optical components.
  • The purge gas is introduced into the optical chamber or around the optical path, displacing potentially corrosive or contaminating gasses that may be present in the surrounding environment.
  • By maintaining a clean and stable environment, the purge gas helps to prevent the accumulation of dust, moisture, or other particles on the optical surfaces, thereby preserving the integrity and performance of the optics.
  • Additionally, the purge gas can contribute to temperature stabilization, reducing the risk of thermal fluctuations that could affect the accuracy of optical measurements.
  1. Laser Emission: The TDL analyzer consists of a laser that emits infrared light.
  2. Gas Interaction: The emitted laser light is directed through the gas to be measured.
  3. Absorption: Gas molecules in the sample absorb specific wavelengths of the laser light, known as absorption lines, based on Beer’s Law.
  4. Detector Signal: The amount of absorbed light is detected by a photodetector.
  5. Signal Translation: Electronics within the analyzer control the laser and process the detector signal.
  6. Gas Concentration Determination: The processed signal is translated into a signal representing the gas concentration.
Beer's Law Formula


  • I is the radiation intensity after absorption.
  • I0 is the initial radiation intensity.
  • E is the extinction coefficient.
  • G is the gas concentration.
  • L is the path length of the measurement area.
Oxygen Sensor Formula (Example)


  • EO2  is the extinction coefficient for oxygen.
  • GO2 is the concentration of oxygen gas.
  • L is the path length of the measurement area.

Note: TDL analyzers can measure gasses beyond oxygen, with the same principles, but variations in analyzer design or components may apply for different gasses.

  • TDLS offers high sensitivity, allowing for precise measurement of gas concentrations even at low levels, making it ideal for critical industrial processes where accurate monitoring is essential.
  • It provides real-time data on gas concentrations, enabling prompt response to any deviations from desired levels, improving the  process control and safety.
  • TDLS can selectively measure specific gasses of interest, offering targeted analysis without interference from other constituents, which is crucial for process optimization and quality control.
  • TDLS provides accurate measurements due to its precise control over laser wavelength and intensity, ensuring reliable data for process optimization and compliance.
  • It is a non-invasive technique that does not require direct contact with the gas sample, minimizing sample contamination and simplifying installation in industrial settings.
  • TDLS systems can cover a wide dynamic range of gas concentrations, from ppm to % levels, offering adaptability to a range of industrial uses.
  • Many TDLS systems support remote operation and monitoring, allowing for flexibility and accessibility in industrial environments where direct access may be limited.
  • TDLS systems can be expensive to procure and maintain, particularly for applications requiring high-performance instruments with advanced features.
  • The complexity of TDLS systems, including intricate optical and electronic components, may require specialized expertise for installation, alignment, calibration, and troubleshooting.
  • Environmental conditions, such as extreme temperatures or high levels of particulate matter, can impact the performance and longevity of TDLS systems, necessitating additional protective measures and maintenance.
Applications of Tunable Diode Laser (TDL) Analyzer in Process Industries
  • TDLS is used for monitoring gas concentrations in various chemical processes, including petrochemical refineries, pharmaceutical manufacturing, and specialty chemical production.
  • TDLS systems are employed in environmental monitoring applications, such as stack emissions testing and air quality assessment, to ensure compliance with regulatory standards.
  • In power plants, TDLS is utilized for measuring gas concentrations in combustion processes, optimizing fuel efficiency, and minimizing emissions.
  • TDLS plays a crucial role in semiconductor fabrication processes by monitoring gas purity levels and ensuring the integrity of cleanroom environments.
  • TDLS is utilized in oil and gas production facilities for monitoring gas compositions, detecting leaks, and enhancing safety measures during exploration, extraction, and transportation processes.
  • Tunable diode laser spectroscopy (TDLS), also known as tunable diode laser absorption spectroscopy (TDLAS), is a technique used for gas analysis. 
  • It involves the use of a tunable diode laser to measure the absorption of light by gas molecules at specific wavelengths.
  • TDLAS can only measure one gas at a time, limiting its ability to monitor multiple gasses simultaneously.
  • Each laser has a specific range of wavelengths it can detect, necessitating additional lasers for detecting multiple gasses, increasing complexity and cost.
  • TDLAS is effective for only about twenty compounds that absorb light in the near- or mid-infrared range, restricting its applicability.
  • The laser beam can be easily blocked by dust or objects, leading to inaccurate or no measurements.
  • Implementing TDLAS systems can be complex and expensive due to specialized equipment and calibration requirements.

The full form of “TDLS” is “Tunable Diode Laser Spectrometer” or “Tunable Diode Laser Analyzer.”

  • Beer’s Law states that the intensity of light absorbed by a substance is directly proportional to the concentration of the absorbing species and the path length of the light.
  • In TDLS, the absorbed light intensity provides information about gas concentration, following Beer’s Law.

The Tunable Diode Laser Spectrometer (TDLS) is a versatile instrument capable of measuring various gasses in industrial processes. Some of the gasses it can measure include:

  1. Oxygen (O₂): TDLS is commonly used for monitoring oxygen levels in combustion processes, ensuring efficient fuel utilization and minimizing emissions.
  2. Carbon Dioxide (CO₂): TDLS helps track CO₂ concentrations in applications such as greenhouse gas monitoring, fermentation processes, and carbon capture.
  3. Carbon Monoxide (CO): Monitoring CO levels is crucial for safety in industrial environments (e.g., steel production, chemical plants).
  4. Hydrogen Chloride (HCl): TDLS measures HCl concentrations in processes like waste incineration and chemical manufacturing.
  5. Ammonia (NH₃): Used in applications such as fertilizer production and stack emissions monitoring.
  6. Hydrogen Sulfide (H₂S): TDLS detects H₂S in natural gas, petrochemical, and wastewater treatment processes.

Sundareswaran Iyalunaidu

With over 24 years of dedicated experience, I am a seasoned professional specializing in the commissioning, maintenance, and installation of Electrical, Instrumentation and Control systems. My expertise extends across a spectrum of industries, including Power stations, Oil and Gas, Aluminium, Utilities, Steel and Continuous process industries. Tweet me @sundareshinfohe

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