Infrared Bandpass Filter Coating Specifications

Topic: Thin-film multilayer design, CWL, bandwidth, blocking OD, angle shift & substrate selection

Overview

Infrared bandpass filters rely on dielectric multilayer thin-film coatings to transmit a narrow IR waveband while blocking out-of-band stray light. They are core components for thermal imaging, machine vision, gas detection and multi-spectral sensing. Felix Glass customizes bandpass coatings for NIR, MWIR and LWIR wavebands on sapphire, silicon, germanium, ZnS and ZnSe substrates.

Core Optical Parameters

• CWL (Central Wavelength): The peak transmission wavelength; tolerance typically ±1~±3 nm for narrowband IR filters. Temperature rise causes wavelength shift toward shorter wavelengths.
• Bandwidth (FWHM): Full width at half maximum transmission; available from 1% up to 50% of CWL. Narrow bandwidth improves signal-to-noise ratio for precision detection.
• Peak Transmittance: ≥80% for standard IR bandpass filters; high-grade coatings reach above 90% with minimal ripple inside the passband.
• Out-of-Band Blocking (OD): Optical Density; OD3~OD6 deep blocking suppresses background noise. Blocking range covers UV, visible and adjacent infrared bands.
• Angle Sensitivity: Oblique incidence shifts CWL bluewards. Hard-sputtered coatings minimize wavelength drift under large field-of-view angles.

Coating Technology & Substrate Matching

• Ion-Assisted E-Beam Evaporation: Low cost for prototypes, suitable for medium-temperature indoor equipment.
• Magnetron Sputtering: Dense, low-moisture hard coatings; excellent thermal stability and adhesion for airborne, outdoor and high-temperature applications.
• Substrate Selection
  • Silicon / Germanium: MWIR & LWIR (3–14 μm) thermal band filters
  • Sapphire: UV–MWIR, high hardness for rugged outdoor use
  • ZnS/ZnSe: Long-wave infrared thermal imaging filters, often paired with DLC protective overcoat

Common Design Pitfall

Without thermal compensation, narrowband IR filters will suffer obvious CWL blue shift when working above 70°C, resulting in signal attenuation. Always specify spectral testing at actual operating temperature for high-temperature systems.

Spectral Filter Environmental Durability Test

Topic: ISO & MIL qualification, thermal cycling, damp heat, adhesion, abrasion & spectral stability

Overview

Thin-film filter coatings easily delaminate, develop haze or suffer wavelength drift under high temperature, high humidity and thermal shock. Environmental qualification verifies long-term spectral stability and coating adhesion before mass production, following ISO 9211 and MIL-STD-810G standards.

Standard Test Items & Acceptance Criteria

1. Adhesion Tape Test (Cross-Hatch): 3M high-strength tape applied 10 times vertically; no peeling, flaking or film detachment after test.
2. Damp Heat Test: 60°C, 95% RH for 48 hours; no coating whitening, delamination; CWL shift ≤2 nm, transmittance change <3%.
3. Thermal Cycling Test: -55°C ↔ +125°C, 30 cycles with 30 min dwell time; no spectral drift beyond specified tolerance, no coating cracking.
4. Abrasion Resistance: 50 strokes with cheesecloth under 5N load; no visible scratches on the clear aperture.
5. Solvent Immersion: Soak in ethanol and acetone for 5 minutes; no film damage or spectral degradation after cleaning.

Long-Term Thermal Stability Control

High-temperature operation causes thin-film layer expansion and moisture infiltration, leading to wavelength drift. Two reliable solutions: 1. Use magnetron-sputtered dense coatings to reduce water absorption; 2. Add a protective top coat (SiO₂ or DLC) to seal the multilayer stack, locking spectral performance over years of field use.

RFQ Specification Tip

When submitting filter drawings, clearly separate optical specs (CWL, bandwidth, blocking OD) and environmental test requirements. List full test conditions, cycle counts and maximum allowable wavelength shift to avoid performance inconsistency after delivery.