Infrared Filter Selection Guide for 4500nm Applications
1. Industrial Combustion Temperature Monitoring
Application Scenario
Real-time monitoring of flame and combustion gas temperatures in high-temperature industrial equipment such as boilers and furnaces is critical for optimizing combustion efficiency and ensuring safety. For example, CO₂ exhibits a strong absorption peak near 4.26 microns (4260nm), and its radiation signal often overlaps with the 4500nm band during combustion, requiring precise capture of this spectral range through specialized filters.
Filter Configuration Requirements
- Central Wavelength & Bandwidth:
Select a narrow-band filter with a central wavelength of 4500nm and a bandwidth controlled between 100-200nm. For instance, an industrial pyrometer using a 4500nm central wavelength with 140nm bandwidth effectively isolates CO₂ absorption signals, minimizing interference from other gases like H₂O.
Require ≥80% high transmittance to enhance signal strength, ensuring detection of weak radiation in high-temperature environments.
The cut-off range should cover 400-11000nm with a depth (OD value) >10 to suppress background light and non-target band interference.
Use monocrystalline silicon or germanium substrates, which offer high transmittance in the mid-infrared (3-5μm) and excellent thermal stability to withstand industrial temperatures (400-2000°C).
- Temperature Stability Design:
Optimize film density through Ion Beam Assisted Deposition (IBAD) to reduce central wavelength drift caused by temperature changes (e.g., controlled within 0.139nm/°C), ensuring long-term reliability.
Selection Rationale
- Interference Elimination: Narrow-band design precisely matches CO₂ absorption peaks, avoiding cross-interference with other gases (e.g., CH₄ at 3.3μm), enhancing detection specificity.
- Signal-to-Noise Enhancement: Combined high transmittance and deep cut-off characteristics maximize target signal strength while suppressing environmental noise, enabling clear differentiation between combustion zones and background radiation in flame monitoring.
- Extreme Environment Adaptation: Silicon/germanium substrates and optimized coating processes resist high temperatures and mechanical vibrations. For example, filters maintain stable performance at 2000°C in Furnace Exit Gas Temperature (FEGT) monitoring.
2. Non-Dispersive Infrared (NDIR) Gas Sensors
Application Scenario
Used for quantitative analysis of gases like CO₂ and CH₄ in environmental monitoring or industrial processes, such as optimizing CO₂ levels in agricultural greenhouses or detecting methane leaks in chemical plants.
Filter Configuration Requirements
- Central Wavelength & Bandwidth:
For CO₂ detection, choose a narrow-band filter at 4260nm (adjacent to 4500nm) with ≤100nm bandwidth to accurately capture absorption peaks; for CH₄ detection, use a 3300nm filter (with attention to potential 4500nm band interference).
Require ≥85% transmittance to improve detection sensitivity. For example, an NDIR sensor achieves 0.1ppm CO₂ sensitivity through optimized film system design.
Cut off visible and near-infrared bands (400-2800nm) with OD >4 to eliminate interference from sunlight or artificial light.
Employ calcium fluoride (CaF₂) or chalcogenide glass (e.g., Ge-As-Se) with low infrared absorption coefficients (<0.01cm⁻¹) to reduce optical loss and enhance long-term stability.
- Anti-Interference Design:
Metal-dielectric hybrid metasurfaces (e.g., Al-Ge structures) achieve high transmission efficiency (80%) and narrow bandwidth (FWHM=0.4μm), while minimizing temperature drift.
Selection Rationale
- Precision Detection: Narrow-band filters isolate target gas absorption signals from complex spectra. For instance, a 4260nm filter avoids overlap with H₂O absorption (2.7μm) in CO₂ measurement.
- High Sensitivity: High transmittance and low-scattering design improve sensor response speed. An NDIR system using metasurface filters reduces CO₂ detection limits to 400ppm.
- Long-Term Reliability: Chalcogenide glass substrates and IBAD coatings enhance moisture and aging resistance, suitable for outdoor or high-humidity environments.
3. Selection Summary
Industrial Temperature Monitoring Key Specifications
- Central Wavelength: 4500nm
- Bandwidth: 100-200nm
- Peak Transmittance: ≥80%
- Cut-off Range: 400-11000nm, OD >10
- Substrate Material: Monocrystalline Silicon/ Germanium
- Critical Design: High-temperature stability, deep cut-off characteristics
Gas Detection Key Specifications (CO₂/CH₄ Example)
- Central Wavelength: 4260nm (CO₂)/ 3300nm (CH₄)
- Bandwidth: ≤100nm
- Peak Transmittance: ≥85%
- Cut-off Range: 400-2800nm, OD >4
- Substrate Material: Calcium Fluoride/ Chalcogenide Glass
- Critical Design: High sensitivity, narrow bandwidth
4. Considerations
- Environmental Adaptation:
Prioritize silicon/germanium substrates for high-temperature scenarios; use chalcogenide glass or protective coatings in high-humidity environments.
Ensure filter spectral response matches detectors (e.g., PbSe, thermopile). A 4500nm filter should pair with mid-infrared-sensitive detectors.
Regularly clean filter surfaces to prevent dust contamination; monitor film aging in high-temperature environments.
By following these configurations, 4500nm filters effectively address interference issues in industrial temperature monitoring and sensitivity limitations in gas detection, ensuring reliable performance in complex environments.
