Infrared Lenses

Knight Optical can offer a variety of custom infrared lenses made from different IR optical materials. We offer cylindrical, planoconvex/concave and meniscus lenses, as well as achromatic doublets and aspheric options to reduce aberrations within the system.

The following infrared materials are suitable options for IR lenses:

Custom fluoride lenses – Knight Optical offers a variety of custom fluoride lenses including Calcium Fluoride (CaF₂), Barium Fluoride (BaF₂) and Magnesium Fluoride (MgF₂).

• Custom Calcium Fluoride lenses are used for IR applications due to the materials large transmission band of 0.13µm to 10µm and high refractive index homogeneity. It is also cost effective as it can be grown naturally. Due to their transmission performance and stability, they are ideal in FLIR systems.
• Custom Barium Fluoride lenses offer similar transmission to CaF₂, though extends into the IR to around 12µ However, it is not found naturally and therefore is a more expensive option. It is also a softer material.
• Custom Magnesium Fluoride lenses offer a slightly lower transmission range, covering 0.12µm to 7µm, but it is a more durable IR material compared to calcium fluoride and barium fluoride.
• Lithium fluoride can also be considered, it transmits up to 6µm but is more commonly used in UV applications due to its high transmission in the UV-Vis region. However, it is very soft.

Custom Germanium (Ge) lenses are used due to their high refractive index and high IR transmission.

• Germanium can transmit from 1.8µm to 23µm and fully blocks the visible region.
• Uncoated germanium offers around 45% transmission, so they are often coated with AR/AR or AR/DLC coatings to increase the transmission and durability of the lens.
  • The AR coating is generally a broadband antireflective (BBAR) coating, for regions such as 7-14 micron, as to increase the transmission in the LWIR range.
  • The DLC (diamond-like carbon) coating displays similar properties to diamond, and therefore can offer increased hardness and corrosion resistance to the surface it is applied to.
• They fully transmit over the LWIR region, making them ideal for us in thermal imaging and for night vision applications, such as weapon sights.
• In thermal imaging cameras spherical aberration can be an issue, a solution is diamond-turned aspheric germanium lenses where one single lens can collimate the light where before a system of lenses may have been needed.

Custom Silicon lenses are ideal for us in MWIR laser systems because of silicon’s optical properties.

• Silicon transmits from 1.2µm to 15µm, but it performs the best in the MWIR region – 3 to 5µm.
• It is usually doped (5 to 40 ohm cm) to prevent absorption within these transmission bands.
• Silicon is lightweight but also extremely hard and therefore is a very durable IR material within a lens system.

Custom Zinc Selenide (ZnSe)/Zinc Sulphide (ZnS) – Zinc Selenide is generally the chosen material for CO₂ laser systems due to its low absorbance in the IR range, specifically 10.6µm. It also has a broad transmission range from 0.6µm up to 21µm, with an average transmission of 70%, this can be further increased if an AR coating is applied. Under normal atmosphere, it is not recommended to use ZnSe above 250°C.

AMTIR (Amorphous Material Transmitting Infrared Radiation, Ge₃₃As₁₂Se₅₅) is a glass-like, chalcogenide material with a high refractive index homogeneity and transmits over a broad IR range. It has similar optical and mechanical properties to germanium.

• It can transmit from 0.7-14µm, but performs particularly well between 8-12µ It works well for the Nd:YAG laser wavelength of 1.064µm.
• It is a soft material, so DLC (diamond-like carbon) coating can be applied to increase its durability.
• A BBAR coating can also be applied to increase its transmission in areas such as the 3-5µm and 8-12µm wavebands.
• When paired with germanium, in an IR achromatic doublet lens, it can correct chromatic aberration due to their differing refractive indices – AMTIR has a lower refractive index compared to germanium.
• AMTIR lenses also offer low thermal change in refractive index, and therefore are beneficial in optical systems which need to avoid thermal defocusing.

Other IR materials to consider are:

• Sodium chloride – a crystalline IR material, which transmits over 0.2-16µm range. Care needs to be taken over the environment in which these lenses are used in as they are highly sensitive to moisture.
• Potassium chloride/bromide – these materials can be used in the MWIR and LWIR regions, with KBr showing an average transmission of above 90% between 0.3-20µ
• GaAs – this is an extremely durable material so can be used in harsher environments compared to a lot of other IR materials. It transmits over 40% between 2-15µm, a broadband AR coating would increase this performance.
• Caesium iodide – this material offers the widest transmission band of any widely available IR materials, it transmits above 80% between 0.4-40µ It is very resistant to shock and repaid changes to temperature. Knight Optical does recommend a protective coating, such as a moisture resistant antireflective coating, as they do degrade under certain atmospheric conditions.

All our lenses can also be coated with specific AR coatings targeting different wavelengths including NIR, SWIR, MRIW and LWIR regions to optimise your transmission output.

Knight Optical also offer stock IR lenses in different materials including sapphire and zinc selenide/zinc sulphide. Our stock and custom infrared lenses come quality assured. Contact our multilingual technical sales team and discover how Knight Optical’s high quality IR lenses and service can improve your instrumentation and supply chain experience.