Collection: Bandpass Filters for CO2 Gas Detection

Why Optical Filters Are Used in Gas Detection

In NDIR gas detection, an infrared (IR) light source emits a broad spectrum of wavelengths through a gas sample. Target gases absorb IR radiation at characteristic wavelengths due to their molecular vibrations. By employing optical bandpass filters that isolate these specific wavelengths, the detector measures the decrease in light intensity caused by absorption, allowing for precise calculation of the gas concentration.

For example, carbon dioxide (CO₂) has a significant absorption band near 4.26 micrometers (µm). Utilizing an optical filter centered at this wavelength ensures that the detection system is specifically responsive to CO₂ levels without interference from other gases or atmospheric components.

Impact of Not Using Optical Filters

Without appropriate optical filters:

• Reduced Sensitivity: The detector receives a mixture of wavelengths, diluting the signal associated with the target gas absorption.
• Increased Noise: Unfiltered ambient light and other IR sources introduce noise, decreasing the signal-to-noise ratio.
• Cross-Sensitivity: Other gases or environmental factors may interfere, causing false readings or inaccurate measurements.

Case Study: CO₂ Gas Detection

In CO₂ gas detection applications, two optical filters are commonly used:

1. Detection Channel Filter
2. Reference (Compensation) Channel Filter

Parameters for the Filters

Detection Channel

Central Wavelength (CWL) 4.26 µm

Bandwidth (FWHM) 180 nm

Peak Transmission >70%

Blocking Range 2–12 µm

Blocking Optical Density (OD) ≥ 4

Reference Channel

Central Wavelength (CWL) 3.90 µm

Bandwidth (FWHM) 90 nm

Peak Transmission >70%

Blocking Range 2–12 µm

Blocking Optical Density (OD) ≥ 4

Importance of the Reference Channel

The reference channel is vital for compensating for:

• Source Intensity Variations: Changes in the IR source output over time or due to temperature fluctuations.
• Detector Sensitivity Changes: Variations in detector performance influenced by temperature or aging.
• Environmental Factors: Dust, humidity, or optical window contamination affecting the measurement.

By comparing signals from the detection and reference channels, the system corrects for these common-mode effects, isolating the absorption due to CO₂ alone.

Conclusion

Optical bandpass filters are crucial for the effectiveness of gas detection systems. They provide selectivity and sensitivity by isolating wavelengths absorbed by the target gas while minimizing interference. Without these filters, detection systems would suffer from reduced performance and accuracy, potentially leading to unreliable measurements and safety risks.