The optical materials selected for an optical system depend upon the application, the required system performance and the environment in which the system is to perform; thus the materials’ optical, mechanical, thermal and thermo-optic properties must be taken into account. There are three major bands that are worked with when optical systems are used within the Earth’s atmosphere. They are the short-wave infrared (SWIR) or near-infrared (NIR), which covers from 0.75 to 3 μm; the mid-wave (MW) infrared, which covers from 3 to 5 μm; and the long-wave infrared, which covers 8 to 14 μm. The areas in between these bands cannot be used due to absorption by various molecules in the Earth’s atmosphere. However, these in-between bands are usable if the system is at high altitude or in space. You may also like... Molecular Coating Outperforms Traditional Material to Improve Solar Cell PerformanceFoldable Display Material Protects Smartphones, Increases FlexibilityFrom Contradictory Polymers, Scientists Develop Nanocoatings with Optimal Properties for OpticsMetamaterial Tiles Perform in Vast Temperature Range, Support Highly Sensitive MeasurementsMaterials added to this updated guide include chalcogenide glasses; although available for many years, they have come of age in the optics industry due to their unique material properties. Other materials added to this guide are sapphire, aluminum, copper, electroless-nickel plated metal and copper nickel alloy. Optical coatings for infrared materials have evolved over time with the availability of advanced technologies, automation, and processes using plasma-enhanced chemical vapor deposition, ion assist deposition with electron beam sputtering and resistance sources. With the development of new chalcogenide materials and increased demands for lens systems that perform over multispectral bands from the visible (VIS) to the LWIR, the demand for new coating designs and processes has increased for both commercial and defense applications. Examples of current and new coatings for IR materials are mentioned in the updated guide, along with their environmental durability tests. There is a glossary at the end of the article in case a definition is required. Barium Fluoride (BaF2) • Good transmission in the UV, VIS, NIR and MW spectral regions • Hardness about half that of CaF2 • Is about 70% the mechanical strength of CaF2 • More susceptible to thermal shock than CaF2 • Somewhat more expensive than CaF2 • Not as readily available in large sizes as CaF2 • Diamond turnable • Magnetorheological finishable Transmission range Transmission is above 90 percent between 0.25 and 9.5 μm Index of refraction 1.466 @ 1.7 μm 1.455 @ 4 μm dn/dT −15.2 × 10−6/K Density: 4.89 g/cm3 Hardness (Knoop): 82 kg/mm2 Rupture modulus 3800 psi Thermal expansion coefficient 18.1 × 10−6/°C @ 20 °C ±100 °C Typical applications Thermal imaging, astronomy, lasers Products manufactured Lenses, aspheric lenses, windows, beamsplitters, optical filters, wedges and prisms Surface finish Polishes of 20-10 scratch-dig are mostly specified for UV and VIS applications. Typical specifications for surface quality in the IR are a 40-20 scratch-dig in the NIR spectral region and 60-40 scratch-dig for the MW area. Surface figure Surface figure of 1/10 to 1/4 wave @ 0.6328 μm are specified mostly on lenses for UV and VIS use. In the IR, typical surface figure ranges from 1/2 to 2 waves @ 0.6328 μm. AR coating options Typical available coatings for BaF2 include BBAR for 0.8 μm to 2.5 μm, 3 μm to 5 μm, or 1 μm to 5 μm, and dual-band AR for the 3.5- to 5.1-μm and 7.5- to 10.5-μm spectral regions. Cadmium Telluride (CdTe) • Extra handling and safety precautions are required when machining this material due to its toxicity; thus, few companies will process it • Has the highest density of the common infrared-transmitting materials • One of the widest transmission ranges of any infrared material • Principally used in the 12- to 25-μm spectral region, where many other infrared materials have absorption bands • Slightly less than half the hardness of ZnSe • Significantly more expensive than Ge and ZnSe • Diamond turnable Transmission range 1 to 25 μm Index of refraction 2.693 @ 4 μm 2.676 @ 10 μm 2.640 @ 19 μm dn/dT 5.0 × 10−5/K Density 5.85 g/cm3 Hardness (Knoop) 45 kg/mm2 Rupture modulus 3191 psi Thermal expansion coefficient 5.9 × 10−6/°C @ 20 °C Typical applications Thermal imaging, low-power CO2 laser systems, detectors Products manufactured Lenses, aspheric lenses, windows, detector windows, beamsplitters, optical filters, wedges and prisms Surface finish Typical specifications for surface quality in the IR are a 40-20 scratch-dig in the NIR spectral region and 60-40 scratch-dig for the MW spectral region, and 60-40, 80-50 or 120-80 scratch-dig above 7 μm. Surface figure Ranges from 1/2 to 2 waves @ 0.6328 μm AR coating options CdTe can be AR-coated for various wavelengths or wavelength ranges between 1 and 25 μm. Calcium Fluoride (CaF2) • Good transmission in the UV, VIS, NIR and MW spectral regions • It has a transmission above 90 percent between 0.25 and 7 μm • Is about twice as hard as BaF2 • Less susceptible to thermal shock than BaF2 • Does not degrade due to moisture under ambient atmospheric conditions • Less expensive than BaF2 • More readily available than BaF2 in large sizes • Diamond turnable • Magnetorheological finishable Transmission range 0.13 to 10 μm Index of refraction 1.428 @ 1.064 μm 1.425 @ 1.7 μm 1.4096 @ 4 μm dn/dT −11.0 × 10−6/K Density 3.18 g/cm3 Hardness (Knoop) 158 kg/mm2 Rupture modulus 5295 psi Thermal expansion coefficient 18.85 × 10−6/°C Typical applications Imaging, thermal imaging, astronomy, microlithography, laser Products manufactured Lenses, aspheric lenses, windows, beamsplitters, optical filters, wedges and prisms Surface finish Polishes of 20-10 scratch-dig are mostly specified for use in UV and VIS applications. Typical specifications in the infrared are 40-20 scratch-dig for the NIR spectral region and 60-40 scratch-dig for the MW spectral region. Surface figure In the UV and VIS spectral regions, specified surface figure ranges from 1/10 to 1/4 wave @ 0.6328 μm. In the IR, typical required surface figure ranges from 1/4 to 2 waves @ 0.6328 μm. AR coating options Available coatings include BBAR for the 0.8 to 2.5, 3 to 5, or 1 to 5 μm, dual-band AR for MWIR and LWIR and triple-band for NIR and MWIR spectral regions. Other options are also available. Cesium Bromide (CsBr) • Optical-grade cesium bromide transmits from the UV to the far-IR; one of the widest transmission bands of the IR materials. • It is water-soluble, requiring protection from water moisture and humidity: accomplished by using moisture-protection AR coatings or by ensuring uncoated part is in a water-/humidity-free environment. • Diamond turnable • Extremely fragile Transmission range Transmission is above 80 percent from 0.35 to 32 μm. Index of refraction 1.668 @ 4 μm 1.663 @ 10 μm 1.629 @ 25 μm dn/dT +79 × 10−6/°C Density 4.44 g/cm3 Hardness (Knoop) 19.5 kg/mm2 Rupture modulus 1218 psi Thermal expansion coefficient 47.9 × 10−6/°C Typical applications FTIR spectroscopy, laser systems, lens protectors for CO2 laser systems, imaging systems, analytical instruments Products manufactured Windows, lenses, laser lens protectors, aspheric lenses, wedges and prisms Surface finish Typical specifications for surface quality in the IR are 60-40, 80-50 or 120-80 scratch-dig. Surface figure In the IR, the typical surface figure specified ranges from 1/10 to 1/40 wave @ 10.6 μm. AR coating options Moisture-protection AR and BBAR coatings are available for various wavelengths or wavelength ranges within CsBr’s transmission range. Cesium Iodide (CsI) • Optical-grade cesium iodide transmits from the UV to the far-infrared; it has the widest transmission band of all the readily available IR materials. • It is water-soluble, requiring protection from water moisture and humidity: accomplished by using moisture-protection AR coatings or by ensuring uncoated part is in a water-/humidity-free environment. • Diamond turnable • Extremely fragile Transmission range Transmission is above 80 percent from 0.42 to 40 μm. Index of refraction 1.743 @ 4 μm 1.739 @ 10 μm 1.708 @ 30 μm dn/dT −99 × 10−6/°C @ 0.6 μm Density 4.51 g/cm3 Hardness (Knoop) 20 kg/mm2 Rupture modulus 809 psi Thermal expansion coefficient 50 × 10−6/°C Typical applications FTIR spectroscopy, laser systems, lens protectors for CO2 laser systems, imaging systems, analytical instruments Products manufactured Windows, lenses, laser lens protector windows, aspheric lenses, prisms and wedges Surface finish Typical specifications for surface quality in the IR are 60-40, 80-50 or 120-80 scratch-dig. Surface figure In the IR, the typical surface figure specified ranges from 1/10 to 1/40 wave @ 10.6 μm. AR coating options Moisture-protection AR and BBAR coatings are available for various wavelengths or wavelength ranges within CsI’s transmission range. Chalcogenide Glass • The IR optical materials known as the chalcogenides, of which the various AMTIR glasses are a part, are optimized for pairing with other IR materials in optical designs. • The low dn/dT of the chalcogenides makes athermalization of a lens system much simpler by the removing required mechanical compensation complexity for the athermalization of optics with higher dn/dT’s. • The chalcogenide series of glasses can be processed by generating, polishing, diamond turning, magnetorheological finishing or molding. • Generally used in the MW, LW and sometimes the NIR. • Nearly as dense as Ge and has a lower index of refraction, making it a good option for color correction with the use of Ge in an optical system. • Top use temperature is 300 °C. • Performs especially well in the 8- to 12-μm area, where its absorption and dispersion are the lowest. • Generally more expensive than Ge • Diamond turnable • Magnetorheological finishable Note: Transmission values are typical for reference and are based on samples 6 to 6.4 mm in thickness. Source: Amorphous Materials Inc., SCHOTT North America, Inc. – Advanced Optics and Vitron Spezialwerkstoffe GmbH.