10.5μm Filter Selection Guide for Specific Applications

I. SF6 Gas Leak Detection Systems

Application Context

In the power industry, SF6 gas is widely used as an insulating medium in high-voltage equipment. However, leaks can lead to equipment failure and environmental risks. Detection systems based on Non-Dispersive Infrared (NDIR) technology require precise capture of SF6's characteristic absorption peak at 10.56μm.

Filter Configuration Requirements

1. Central Wavelength (CWL): 10.56μm ± 0.05μm

• Must strictly match the absorption peak of SF6 molecules in the mid-infrared band (10.56μm) to ensure signal specificity. Wavelength deviations exceeding ±0.1μm may cause misdetection of other gases (e.g., CO₂ absorption at 10.6μm).

1. Bandwidth (FWHM): 150–200nm

• Narrow-band design eliminates interference from adjacent wavelength bands (e.g., water vapor absorption in 9–11μm) while maintaining sufficient light flux for improved signal-to-noise ratio. Excessively narrow bandwidth reduces signal strength, while overly wide bandwidth decreases detection accuracy.

1. Peak Transmittance (Tavg): ≥80%

• High transmittance ensures the detector receives adequate infrared energy, especially for long-distance detection (e.g., leaks 5 meters away), reducing reliance on high-power light sources. Transmittance below 70% may require increased light intensity or extended integration time, leading to response delays.

1. Cutoff Range: 2–18μm (outside passband)

• Full-band cutoff design suppresses interference from visible light (0.4–0.76μm) and other infrared bands (e.g., 3–5μm thermal radiation), preventing background noise from masking weak SF6 absorption signals. For example, high-temperature industrial equipment emitting strong 3–5μm radiation must be completely blocked by the filter's cutoff coating.

1. Substrate Material: Single-Crystal Silicon (Si)

• Silicon offers high transmittance (>90%) in 8–14μm and excellent mechanical strength, suitable for industrial environments with vibrations and temperature fluctuations. Compared to germanium (Ge) substrates, silicon is more cost-effective and requires no additional anti-reflective coatings (uncoated Ge has ~36% reflectance at 10.5μm, needing ZnS/MgF₂ composite coatings for improvement).

Key Advantages

• Precision Identification: Narrow-band filters achieve specific SF6 detection through spectral matching, solving the problem of humidity and cross-gas interference plaguing traditional electrochemical sensors. In mixed-gas environments (e.g., containing CO₂ and H₂O), this filter reduces false alarm rates to below 0.1%.
• Environmental Durability: Silicon substrates with hard coatings (e.g., diamond-like carbon) withstand temperature fluctuations from -40°C to 85°C and high humidity (RH>95%), making them suitable for harsh outdoor substation environments.

II. High-Temperature Object Surface Temperature Monitoring Systems

Application Context

Industrial heating equipment (e.g., furnaces, heat treatment ovens) requires real-time surface temperature monitoring to optimize process parameters. High-temperature objects (>200°C) emit most of their radiation in the 8–14μm band, with a typical peak around 10.5μm.

Filter Configuration Requirements

1. Spectral Range: 8–14μm Long-Pass (LP)

• Covers the primary energy range of blackbody radiation (8–14μm accounts for over 60% of total radiation from high-temperature objects) while excluding mid-wave infrared (3–5μm) interference. For example, furnace flames emitting strong 3–5μm radiation are filtered out by the long-pass design.

1. Cutoff Steepness: Transition region <5% (7.5–8μm)

• A sharp cutoff edge prevents mid-wave infrared signals from mixing in, improving temperature measurement accuracy. Wide transition regions (>10%) may cause overestimated temperature readings due to mixed 3–5μm radiation.

1. Peak Transmittance: ≥85% (8–14μm)

• High transmittance ensures the detector captures sufficient radiation energy, especially at lower temperatures (e.g., 10.5μm radiation intensity is only 30% of the peak at 200°C). Transmittance below 80% may lead to missed detection in low-temperature ranges.

1. Substrate Material: Germanium (Ge) or Zinc Sulfide (ZnS)

• Germanium: Offers >90% transmittance in 8–14μm but requires anti-reflective coatings (e.g., ZnS/MgF₂) to reduce surface reflectance (36% for uncoated Ge). Suitable for high-precision measurements but higher cost.
• Zinc Sulfide: Provides ~85% transmittance with better temperature resistance (up to 400°C) and no need for coatings. Ideal for high-temperature environments (e.g., furnace surfaces), though slight chromatic aberration may occur due to its refractive index (2.2) at 10.5μm.

Key Advantages

• Wide Dynamic Response: Long-pass filters cover 200–1000°C temperature ranges. According to Wien's displacement law (λmax=2898/T), 10.5μm corresponds to a peak temperature of ~276°C, ensuring high sensitivity across the full 量程. For example, 10.5μm radiation intensity increases ~12x as temperature rises from 300°C to 800°C, requiring linear response from the filter.
• Interference Resistance: By blocking mid-wave infrared, these filters effectively exclude environmental light (e.g., workshop lighting) and self-luminescence from high-temperature metals, solving the failure problem of traditional visible-light thermometers under strong light.

III. Key Configuration Comparisons and Selection Logic

Core Objectives

• SF6 Gas Detection: Precise capture of specific gas absorption peaks
• Temperature Monitoring: Broad-band coverage of blackbody radiation energy

Spectral Design

• SF6 Detection: Narrow-band (10.56μm ±0.05μm, 150–200nm bandwidth)
• Temperature Monitoring: Long-pass (8–14μm, transition region <5%)

Material Selection

• SF6 Detection: Silicon substrates (cost-effective + high mechanical strength)
• Temperature Monitoring: Germanium or zinc sulfide (high transmittance + high-temperature resistance)

Performance Priorities

• SF6 Detection: Specificity > Light Flux > Environmental Tolerance
• Temperature Monitoring: Light Flux > Thermal Stability > Interference Resistance

Typical Challenges

• SF6 Detection: Excluding adjacent gas absorption interference
• Temperature Monitoring: Suppressing mid-wave infrared and visible light background noise

Selection Decision

1. For molecular-specific detection (e.g., gas composition analysis), prioritize narrow-band filters with central wavelengths strictly matching the target gas absorption peak (e.g., 10.56μm for SF6).
2. For wide-range temperature monitoring, choose long-pass filters. Select materials based on environmental temperature (ZnS for >300°C, Ge for <300°C) and cost requirements.
3. In mixed scenarios (e.g., gas leak detection in high-temperature environments), use dual-band filter designs: a 10.56μm narrow band for gas detection and an 8–14μm long-pass for temperature compensation. Algorithm fusion eliminates temperature effects on gas absorption.