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ABCD Matrix

Infrared Lens

An infrared (IR) lens is an optic designed to transmit, focus, collimate, or image light in the infrared (roughly 0.7 µm to tens of µm) rather than the visible band. Ordinary optical glass absorbs strongly beyond the near-IR, so IR lenses use specialized substrates, coatings, and often aspheric or diamond-turned surfaces.


In lasers and photonics they handle laser beams (fiber, diode, Nd:YAG, CO₂, quantum-cascade) and passive thermal radiation.


Spectral bands used in practice:


  • NIR (~0.75–1.4 µm): fiber comms, many diode and solid-state lasers, some night-vision.


  • SWIR (~1.4–3 µm): InGaAs cameras, 1.55 µm lasers/LIDAR, material inspection.


  • MWIR (~3–5 µm): atmospheric window; cooled detectors, hot-target tracking, some spectroscopy.


  • LWIR (~8–14 µm): uncooled thermal imaging of ambient-temperature objects; CO₂ lasers at 10.6 µm.


Atmospheric “windows” in MWIR and LWIR are why those bands dominate long-range imaging and many high-power laser systems.


Materials and technical points:


Common substrates (approximate transmission ranges):


(Material: Typical Range: n (approx.): Typical Use)


  • ZnSe: 0.6–18+ µm: ~2.4 @ 10.6 µm: CO₂ laser focusing, broadband


  • Germanium: 2–14 µm: ~4.0L: WIR thermal cameras


  • Silicon: ~1.2–7 µm: ~3.4: MWIR, cost/weight-sensitive


  • ZnS: Visible–~14 µm: ~2.2: Multispectral, harder than ZnSe


  • CaF₂, sapphire, chalcogenides: Varies: Lower n: Spectroscopy, molded LWIR, harsh environments


Key engineering notes:


  • High-index materials (especially Ge) reflect a large fraction of light uncoated; broadband AR coatings are standard.


  • Low absorption and high laser-induced damage threshold matter for high-power beams; ZnSe is the workhorse for 10.6 µm CO₂ cutting/welding because of low absorption and thermal-shock resistance.


  • Aspheres and hybrid (refractive + diffractive) surfaces are common to control aberrations with fewer elements.


  • Thermal effects (dn/dT, expansion, Ge thermal runaway at elevated temperature) must be designed for.


Applications:


  • High-power IR lasers: focusing and beam-delivery optics for CO₂, fiber, and diode lasers used in cutting, welding, marking, and additive manufacturing.


  • Thermal imaging / FLIR: LWIR (and some MWIR) lenses on uncooled or cooled cameras for industrial inspection, security, firefighting, and medical thermography.


  • Beam shaping and coupling: collimators and focusing lenses for NIR/SWIR laser diodes, fiber lasers, and quantum-cascade lasers.


  • Spectroscopy: FTIR and other IR instruments (CaF₂, ZnSe, Ge windows and focusing elements).


  • Defense and sensing: targeting, missile seekers, LIDAR, optical gas imaging.


  • Medical and scientific: selected IR surgical lasers, OCT-related NIR work, and lab sources.


Design choice is driven by wavelength, power, required field of view / f-number, environment (temperature, humidity, abrasion), and whether the system must also pass visible light for alignment.


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