What types of optical filters are used in military imaging systems?
Military imaging systems rely on specialized optical filters to ensure sensors perform at their best—no matter the mission or environment. Here is a practical, system-designer guide to the main types of optical filters used in defense applications, grouped by what they do in the field.
1. Spectral Selection: Define the Sensor's 'Eyes'
Bandpass filters (UV, VIS, NIR, SWIR, MWIR, LWIR):
These let through only specific wavelength bands, matching the atmospheric windows used for military sensing (e.g., 0.9–1.7 µm SWIR, 3–5 µm MWIR, 8–12 µm LWIR). Ultra-narrow bandpass filters (like 532, 1064, or 1550 nm) are used for laser designators, rangefinders, and LIDAR.
Shortpass/longpass (edge) filters: Trim unwanted UV or IR backgrounds, or enforce a spectral cutoff for night imaging.
Notch (line-blocking) filters:
These block specific hostile laser wavelengths (532, 1064, 1540–1550 nm) while keeping neighboring bands open for imaging. Used in EO/IR turrets and for laser eye protection gear, they shield both sensors and human operators from dazzling or damaging laser threats.
Linear/circular polarizers:
Reduce glare from water, glass, or painted surfaces, and improve target contrast. Wire-grid IR polarizers are used where polymer types can't survive. Paired with polarimetric analyzers, they help detect camouflaged threats invisible to the naked eye.
Neutral density (ND) filters:
These lower the light intensity, preventing detector saturation or serving as neutral laser safety attenuation in front of sensitive detectors—critical under bright sun or across a range of daylight conditions.
Dichroic beamsplitters & multispectral filter arrays (MSFA):
These route different wavelength bands (VIS/SWIR/MWIR/LWIR) to different detectors or pixels, enabling advanced multi-band imaging and target identification while keeping size, weight, and power (SWaP) low.
LCTF, AOTF, Fabry–Pérot (MEMS) filters:
Let sensors actively scan across wavelengths to ID chemical, terrain, or paint signatures. These are core in UAV pods and man-portable sensors where adaptability is critical.
Solar-rejection/solar-blind filters:
Suppress the solar background, or pass only the 'solar-blind' UV (about 240–280 nm) for missile-plume warning sensors. In IR seekers, stacks of bandpass and blocking filters cut unwanted out-of-band radiation.
NVIS (MIL-STD-3009) minus-blue filters:
Used on cockpit lights/displays to prevent NVGs from washing out, while keeping human-readable colors intact (Class A: ~625 nm cutoff, Class B: ~665 nm).
AR + DLC coatings:
Anti-reflection (AR) and diamond-like carbon (DLC) on Ge, Si, or sapphire windows maintain good transmission and resist sand, salt, or rain, making them standard on forward optical windows.
EMI-shielded & heated (ITO) windows: Conductive coatings such as ITO provide EMI shielding and can be powered as heaters to prevent fog/ice on sensor windows or cockpit glass.
Sapphire windows are often used for ultimate abrasion resistance.
- Ultra-narrow laser bandpass: SAL seekers, spot trackers, LIDAR, rangefinders (532/1064/1550 nm)
- SWIR/MWIR/LWIR bandpass: Thermal sights, missile seekers, ISR systems
- Notch/laser-protection: EO/IR sensor protection, pilot and gunner safety
- NVIS filters: Cockpit displays/lighting, soldier systems
- Polarizers: Glare reduction, camouflage detection
- Dichroics/MSFA: Multi-band imaging (compact sensors)
- Tunable filters: Hyperspectral/fieldable sensing
- Solar-blind UV: Missile approach/UV-plume warning
- DLC/AR, ITO windows: Survival in harsh environments, EMI/heating
- Match each filter's bandpass to the right target and atmospheric window.
- Check for AOI (angle of incidence) and stray-light handling early in design.
- Select rugged coatings (DLC, sapphire) for abrasive environments; ITO for EMI/heating needs.
- For multi-band & fused-imaging, use dichroics or MSFA for SWaP savings.
- For spectral discovery, add LCTF, AOTF, or MEMS-tunable stages.
Key takeaway: Military optical filters are vital to mission success, from basic wavelength sorting to advanced fused or tunable stacks. The right filter design—tuned to the mission and environment—means better detection, durability, and operational edge.
