The purpose of this guide is to identify the most common infrared transmitting materials, point out their key properties, applications, products manufactured, surface specification options, and general antireflection (AR) coating options.
AMTIR-1 (Amorphous Material Transmitting Infrared Radiation)
AMTIR-1 is glasslike amorphous material with a high homogeneity, able to transmit in the infrared. Transmission range: 0.75 to 14 μm
Index of refraction:
2.593 @ 1.064 μm
2.515 @ 4 μm
2.498 @ 10 μm
2.483 @ 14 μm
Density: 4.4 g/cm3
Hardness (Knoop): 170
Rupture modulus: 2700 psi
Thermal expansion coefficient: 12 × 10-6/°C
Special properties of AMTIR-1:
• AMTIR-1 is generally used in the MW, LW and sometimes the NIR.
• Its composition of Ge33As12Se55 makes it somewhat similar to Ge in its mechanical and optical properties.
• It is nearly as dense as Ge and has a lower index of refraction, making AMTIR-1 a good option for color correction with the use of Ge in an optical system.
• Its top use temperature is 300 °C.
• It performs especially well in the 8 to 12 μm area, where its absorption and dispersion are the lowest.
• Optical-grade AMTIR-1 is generally more expensive than Ge.
• AMTIR-1 is diamond turnable.
Typical applications: Thermal imaging, FLIR, YAG laser
Products manufactured: Lenses, aspheric lenses, binary (diffractive) lenses, windows, prisms and wedges
Surface finish: Typical specifications for surface quality in the IR are 40-20 or 60-40 scratch-dig for the 1 to 7 μm region and 60-40, 80-50 or 120-80 scratch-dig for 7 to 14 μm, depending on system requirements. Diamond-turned surface finishes of 100 Å rms or better are typical.
Surface figure: In the IR, typical required surface figure ranges from 1/4 to 1 wave @ 0.6328 μm. AR coating options: AMTIR-1 is mostly broadband-antireflection (BBAR) coated for use in the 3 to 5 or 8 to 12 μm spectral regions. Many other specialized coating bands are possible between 1 and 14 μm.

Barium Fluoride (BaF2)
Barium fluoride is a material that transmits in the UV, VIS and IR spectral regions.
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
Density: 4.89 g/cm3
Hardness (Knoop): 82
Rupture modulus: 3800 psi
Thermal expansion coefficient: 18.1 × 10-6/°C @ 20 °C ±100 °C
Special properties of BaF2:
• BaF2 is generally used in the VIS, NIR and/or MW spectral regions.
• The hardness of BaF2 is about half that of CaF2.
• BaF2 is also more susceptible to thermal shock than CaF2.
• BaF2 is somewhat more expensive than CaF2.
• Not as readily available in large sizes as CaF2.
• BaF2 is diamond turnable.
Typical applications: Thermal imaging, astronomy, 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 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 the 0.8 to 2.5, 3 to 5 or 1 to 5 μm spectral regions.

Cadmium Telluride (CdTe)
Cadmium telluride has the highest density of the common IR transmitting materials and has one of the widest transmission ranges of any infrared material.
Transmission range: 1 to 25 μm
Index of refraction:
2.693 @ 4 μm
2.676 @ 10 μm
2.640 @ 19 μm
Density: 5.85 g/cm3
Hardness (Knoop): 45
Rupture modulus: 3191 psi
Thermal expansion coefficient:
5.9 × 10-6/°C @ 20 °C
Special properties of CdTe:
• It is principally used in the 12 to 25 μm spectral region, where many other IR materials have absorption bands.
• CdTe has slightly less than half the hardness of ZnSe.
• The current available size limitation for CdTe is about 72 mm diameter.
• It is more expensive than Ge or ZnSe.
• CdTe is diamond turnable.
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)
Calcium fluoride can be used in the UV, VIS and IR spectral regions. It has a transmission above 90 percent between 0.25 and 7 μm.
Transmission range: 0.13 to 10 μm
Index of refraction:
1.428 @ 1.064 μm
1.425 @ 1.7 μm
1.4096 @ 4 μm
Density: 3.18 g/cm3
Hardness (Knoop): 158
Rupture modulus: 5295 psi
Thermal expansion coefficient: 18.85 × 10-6/°C
Special properties of CaF2:
• CaF2 is used in the UV, VIS, NIR and/or the MW spectral regions.
• It is about twice as hard as BaF2.
• CaF2 is less susceptible to thermal shock than BaF2.
• CaF2 does not degrade due to moisture under ambient atmospheric conditions.
• CaF2 is less expensive than BaF2.
• CaF2 is more readily available in large sizes than BaF2.
• CaF2 is diamond turnable.
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 IR are a 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 spectral regions. Other options are also available.

Cesium Bromide (CsBr)
Optical-grade cesium bromide is a water-soluble material that transmits from the UV to the far-IR. 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
Density: 4.44 g/cm3
Hardness (Knoop): 19.5
Rupture modulus: 1218 psi
Thermal expansion coefficient: 47.9 × 10-6/°C
Special properties of CsBr:
• It is water soluble and must be protected from water moisture and humidity. This is usually done by the use of moisture-protection AR coatings, unless the part is used in a water/humidity-free environment.
• It has one of the widest transmission bands of the IR materials.
• CsBr is diamond turnable.
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 of 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 is a water-soluble material that transmits from the UV to the far-infrared.
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
Density: 4.51 g/cm3
Hardness (Knoop): 20
Rupture modulus: 809 psi
Thermal expansion coefficient: 50 × 10-6/°C
Special properties of CsI:
• It is water soluble and must be protected from water moisture and humidity. This is usually done by the use of moisture-protection AR coatings, unless the part is used in a water/humidity-free environment.
• CsI has the widest transmission band of all the readily available IR materials.
• CsI is diamond turnable.
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.

Cleartran*
Cleartran is a chemically vapor deposited (CVD) material with low absorption and scatter properties over its relatively broad transmission range. It is also known as multispectral zinc sulfide and water clear zinc sulfide.
Transmission range: 0.4 to 12 μm
Index of refraction:
2.350 @ 0.63 μm
2.289 @ 1.06 μm
2.252 @ 4 μm
2.200 @ 10 μm
Density: 4.09 g/cm3
Hardness (Knoop): 160
Rupture modulus: 8704 psi
Thermal expansion coefficient:
6.3 × 10-6/K @ 273 K
7.0 × 10-6/K @ 373 K
7.5 × 10-6/K @ 473 K
Special properties of Cleartran:
• Due to its good transmission in the VIS and IR, Cleartran is an ideal choice for systems with a visible camera, and various IR detectors or IR cameras.
• Cleartan is about one-third harder than ZnSe.
• Cleartran is about two-thirds the hardness of ZnS regular.
• The relative price of Cleartran is about one-third more than ZnS regular.
• The relative price of Cleartran is about the same as ZnSe.
• The relative price of Cleartran is generally more than Ge.
• Cleartran is diamond turnable.
Typical applications: Visible imaging, thermal imaging, FLIR, astronomy
Products manufactured: Lenses, aspheric lenses, binary (diffractive) lenses, windows, beamsplitters and optical filters and prisms
Surface finish: Typical specifications for surface quality are 40-20 scratch-dig in the 0.4 to 3 μm spectral region and 60-40 or 80-50 scratch-dig in the 3 to 12 μm spectral region. Diamond-turned surface roughness of 100 Å rms or better is typical.
Surface figure: In the VIS and NIR spectral regions, specified surface figure ranges from 1/10 to 1/2 wave @ 0.6328 μm. In the IR, typical required surface figure ranges from 1/2 to 2 waves @ 0.6328 μm.
AR coating options: Typical available coatings include BBAR for the 0.8 to 2.5, 3 to 5 and 8 to 12 μm bands. Other specialized bands are possible from 0.4 to 12 μm.
* Trade name of Rohm & Haas Advanced Materials.

Fused Silica, IR Grade
Infrared-grade fused silica is used in near-IR systems usually along with other materials such as CaF2.
Transmission range: 0.25 to 3.5 μm
Index of refraction:
1.4505 @ 1 μm
1.4382 @ 2 μm
Density: 2.203 g/cm3
Hardness (Knoop): 461
Rupture modulus: 7100 psi
Thermal expansion coefficient: 0.58 × 10-6/°C @ 0 to 200 °C
Special properties of fused silica:
• It has high homogeneity and good transmission in the VIS and NIR spectral regions.
• Due to the material’s inherently hard SiO2 amorphous structure, fused silica is not diamond turnable, making it much more difficult to fabricate finished aspheric surfaces.
Typical applications: Visible and thermal imaging, astronomy, laser
Products manufactured: Lenses, windows, wedges, beamsplitters, optical filters and prisms
Surface finish: Typical specifications for surface quality in the NIR regions are a 40-20 scratch-dig.
Surface figure: In the VIS and NIR, typical surface figure ranges from 1/10 to 1 wave @ 0.6328 μm.
AR coating options: Typical available NIR coatings are BBAR for 0.8 to 2.5 μm and AR for 1.064 μm. Other coatings for UV, VIS and NIR applications are also available.

Gallium Arsenide (GaAs)
Optical-grade gallium arsenide is IR transmitting and semi-insulating.
Transmission range: 2 to 15 μm
Index of refraction:
3.307 @ 4 μm
3.278 @ 10 μm
3.251 @ 14 μm
Density: 5.31 g/cm3
Hardness (Knoop): 750
Rupture modulus: 10,436 psi
Thermal expansion coefficient: 6 × 10-6/K
Special properties of GaAs:
• It is nearly as hard, strong and dense as Ge.
• It is commonly used in applications where toughness and durability are of great importance.
• It has a low absorption coefficient of 0.01 cm-1 from 2.5 to 12 μm.
• Its optical-grade material is generally more expensive than Ge and ZnSe.
• GaAs is diamond turnable.
Typical applications: Thermal imaging, CO2 laser systems, FLIR
Products manufactured: Lenses, aspheric lenses, windows, wedges and prisms
Surface finish: Typical specifications for surface quality in the IR are 40-20 or 60-40 scratch-dig in the 2 to 7 μm region and 60-40, 80-50 or 120-80 scratch-dig for 7 to 15 μm.
Surface figure: In the IR, typical surface figure ranges from 1/2 to 2 waves @ 0.6328 μm.
AR coating options: Typical available coatings include BBAR in the 3 to 5 and 8 to 12 μm regions. Other specialized bands are also possible within the 2 to 15 μm region.

Germanium (Ge)
Germanium has the highest index of refraction of any commonly used IR-transmitting material. It is very popular for systems operating in the 3 to 5 or 8 to 12 μm spectral regions.
Transmission range: 2 to 14 μm up to about 45 °C
Index of refraction:
4.025 @ 4 μm
4.003 @ 10 μm
Density: 5.323 g/cm3
Hardness (Knoop): 780
Rupture modulus: 10,500 psi
Thermal expansion coefficient:
2.3 × 10-6/K @ 100 K
5.0 × 10-6/K @ 200 K
6.0 × 10-6/K @ 300 K
Special properties of Ge:
• It blocks UV and visible light, as well as IR up to about 2 μm.
• Its high index is desirable for the design of lenses that might not otherwise be possible.
• It has nearly the highest density of the IR-transmitting materials, a consideration when designing for weight-restricted systems.
• It is subject to thermal runaway; the hotter it gets, the more the absorption increases. Pronounced transmission degradation starts at about 100 °C and begins rapidly degrading between 200 °C and 300 °C, resulting in possible catastrophic failure of the optic.
• Ge is generally less expensive than ZnSe and Cleartran.
• Ge is diamond turnable.
Typical applications: Thermal imaging, FLIR, FTIR, analytical instruments
Products manufactured: Lenses, aspheric lenses, binary (diffractive) lenses, windows, beamsplitters, optical filters, wedges and prisms
Surface finish: Typical specifications for surface quality in the IR are 40-20 or 60-40 scratch-dig in the 2 to 7 μm spectral region and 60-40, 80-50 or 120-80 scratch-dig for 7 to 14 μm. Diamond-turned surface finishes of 50 Å rms or better are typical.
Surface figure: In the IR, the typical specified surface figure ranges from 1/4 to 2 waves @ 0.6328 μm.
AR coating options: Typical available coatings include BBAR for the 3 to 5, 8 to 14 and 3 to 14 μm spectral regions. Other application-specialized bands are also possible between 2 and 14 μm. Ge can also be diamondlike carbon coated in the 3 to 5 or 8 to 12 μm regions.

Lithium Fluoride (LiF)
Lithium fluoride has the lowest index of refraction of all the common IR materials.
Transmission range: 0.12 to 8.5 μm
Index of refraction:
1.373 @ 2.5 μm
1.349 @ 4.0 μm
Density: 2.639 g/cm3
Hardness (Knoop): 102
Rupture modulus: 1566 psi
Thermal expansion coefficient: 37 × 10-6/°C
Special properties of LiF:
• It is slightly plastic and has a relatively high thermal expansion coefficient.
• It is the most expensive of the fluoride series of crystals.
• LiF is diamond turnable.
Typical applications: Visible and thermal imaging, astronomy, laser
Products manufactured: Lenses, aspheric lenses, windows, wedges, prisms
Surface finish: Typical specifications for surface quality in the IR are 40-20 scratch-dig in the NIR and 60-40 or 80-50 scratch-dig for the MW area.
Surface figure: In the IR, typical required surface figure ranges from 1/2 to 2 waves @ 0.6328 μm.
AR coating options: LiF can be AR coated for use in the IR, but often without much improvement in transmission due to its low index of refraction and already high transmission.

Magnesium Fluoride (MgF2)
Magnesium fluoride is one of the lowest index IR materials, second only to LiF.
Transmission range: 0.11 to 7.5 μm
Index of refraction:
1.376 @ 0.7 μm
1.370 @ 1.7 μm
1.349 @ 4.0 μm
Density: 3.18 g/cm3
Hardness (Knoop): 415
Rupture modulus: 7108 psi
Thermal expansion coefficient:
13.7 × 10-6/°C parallel to C-axis.
8.48 × 10-6/°C perpendicular to C-axis
Special properties of MgF2:
• Its birefringence should be taken into consideration before selection of this material in an optical design.
• It is resistant to thermal and mechanical shock.
• It is twice as hard as CaF2 but only half as hard as Ge.
• It is significantly more expensive than CaF2 and BaF2, but usually not more expensive than LiF.
• MgF2 is diamond turnable.
Typical applications: Visible and thermal imaging, astronomy, excimer laser
Products manufactured: Lenses, aspheric lenses, windows, beamsplitters, optical filters, wedges and prisms
Surface finish: Polishes of 10-5 or 20-10 scratch-dig are achieved at extra costs respectively mainly for UV applications. Typical specifications for surface quality in the VIS and NIR regions are 40-20 and 60-40 scratch-dig in the MW range.
Surface figure: In the UV and VIS, surface figure ranges from 1/10 to 1/2 wave @ 0.6328 μm. In the IR, typical required surface figure ranges from 1/2 to 2 waves @ 0.6328 μm.
AR coating options: MgF2 can be AR coated for use in the IR but often without much improvement in transmission due to its low index of refraction and already high transmission.

Potassium Bromide (KBr)
Optical-grade KBr is water soluble and transmits from the UV to the far-IR.
Transmission range: Transmission is above 80 percent from 0.26 to 23 μm.
Index of refraction:
1.535 @ 4 μm
1.525 @ 10 μm
1.490 @ 20 μm
Density: 2.754 g/cm3
Hardness (Knoop): 5.9
Rupture modulus: 159 psi
Thermal Expansion Coefficient: 43 × 10-6/°C
Special properties of KBr:
• It is water soluble and must be protected from water moisture and humidity. This is usually done by the use of moisture-protection AR coatings, unless the part is used in a water/humidity-free environment.
• It has one of the widest transmission bands of the IR materials.
• It does not have as great a damage threshold as KCl or NaCl.
• It is generally more expensive than KCl and quite a bit more than NaCl.
• KBr is diamond turnable.
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, windows, 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 KBr’s transmission range.

Potassium Chloride (KCl)
Optical-grade KCl is water-soluble and transmits from the UV to the far-IR.
Transmission range: Transmission is above 80 percent from 0.3 to 21 μm.
Index of refraction:
1.472 @ 4 μm
1.456 @ 10 μm
1.426 @ 16 μm
Density: 1.989 g/cm3
Hardness (Knoop): 7.2
Rupture modulus: 330 psi
Thermal expansion coefficient: 36 × 10-6/°C
Special properties of KCl:
• It is water soluble and must be protected from water moisture and humidity. This is usually done by the use of moisture-protection AR coatings, unless the part is used in a water/humidity-free environment.
• It has one of the widest transmission bands of the IR materials.
• It has a higher damage threshold than KBr, but not NaCl.
• It is generally less expensive than KBr and more expensive than NaCl.
• KCl is diamond turnable.
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, wedges and prisms
Surface finish: Typical specifications for surface quality in the IR region 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 KCl’s transmission range.

Silicon (Si),Transmitting Grade
Silicon is a semiconductor material commonly used in IR optical systems, operating in the NIR and MW spectral bands.
Transmission range: 1.2 to 7.0 μm (also from 25 out to beyond 300 μm)
Index of Refraction:
3.4289 @ 4 μm
Density: 2.329 g/cm3
Hardness (Knoop): 1150
Rupture modulus: 18,000 psi
Thermal expansion coefficient:
2.55 × 10-6/°C @ 25 °C
Special properties of Si:
• With one of the lowest densities of the common IR materials, it is ideal for systems with weight constraints.
• It is harder than Ge and not as brittle.
• It is the lowest material cost option of all the IR materials.
• Si is diamond turnable.
Typical applications: Thermal imaging, FLIR
Products manufactured: Lenses, aspheric lenses, binary (diffractive) lenses, windows, beamsplitters, optical filters, wedges and prisms.
Surface finish: Typical specifications for surface quality in the IR are 40-20 scratch-dig in the 1.2 to 3 μm region and 60-40 for 3 to 7 μm. Diamond-turned surface finishes of 50 Å rms or better are typical.
Surface figure: In the IR, typical required surface figure ranges from 1/4 to 2 waves @ 0.6328 μm.
AR coating options: The most common AR coating for Si is BBAR for 3 to 5 μm. Many other specialized wavelength bands are possible for the 1.2 to 7.0 μm range. Si can also be hard carbon or diamondlike carbon coated for 3 to 5 μm.

Sodium Chloride (NaCl)
Optical-grade NaCl is water soluble and transmits from the UV to the far-IR.
Transmission range: Transmission is above 80 percent from 0.23 to 12 μm.
Index of refraction:
1.522 @ 4 μm
1.495 @ 10 μm
Density: 2.165 g/cm3
Hardness (Knoop): 15.2
Rupture modulus: 345 psi
Thermal expansion coefficient: 44 × 10-6/°C
Special properties of NaCl:
• It is water soluble and must be protected from water moisture and humidity. This is usually done by the use of moisture-protection AR coatings, unless the part is used in a water/humidity-free environment.
• It has a higher damage threshold than KBr or KCl.
• It is the lowest cost option of the water-soluble materials.
• NaCl is diamond turnable.
Typical applications: FTIR spectroscopy, laser systems, lens protectors for CO2 laser systems, imaging systems and analytical instruments
Products manufactured: Windows, lenses, laser lens protectors, aspheric lenses, wedges and prisms
Surface finish: Typical specifications of 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 NaCl’s transmission range.

Thallium Bromoiodide (KRS-5)
Optical-grade KRS-5 is relatively insoluble in water and transmits from the visible to the far-IR. Transmission range: Transmission is above 70 percent @ 0.7 to 32 μm.
Index of refraction:
2.382 @ 4 μm
2.371 @ 10 μm
2.318 @ 25 μm
Density: 7.371 g/cm3
Hardness (Knoop): 40
Rupture modulus: 3772 psi
Thermal expansion coefficient: 58 × 10-6/°C
Special properties of KRS-5:
• It cold flows when subjected to pressure.
• Tight surface figures are difficult to hold due to the nature of KRS-5.
• It is much harder than NaCl, KBr and KCl and only about one-third as hard as ZnSe.
• KRS-5 is diamond turnable.
Typical applications: FTIR spectroscopy, laser systems, imaging systems and analytical instruments
Products manufactured: Windows, lenses, laser lens protectors, 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: AR and BBAR coatings are available for various wavelengths or wavelength ranges within KRS-5’s transmission range.

Zinc Selenide (ZnSe)
A CVD material, ZnSe is the material of choice for optics used in high-power CO2 laser systems due to its low absorption at 10.6 μm. ZnSe is also a popular choice in systems operating at various bands within its wide transmission range.
Transmission range: 0.6 to 16 μm
Index of refraction:
2.4332 @ 4.0 μm
2.4065 @ 10.0 μm
2.4028 @ 10.6 μm
Density: 5.27 g/cm3
Hardness (Knoop): 110
Rupture modulus: 7979 psi
Thermal expansion coefficient:
7.1 x 10-6/K @ 273 K
7.8 x 10-6/K @ 373 K
8.3 x 10-6/K @ 473 K
Special properties of ZnSe:
 • Its high resistance to thermal shock makes it the prime material for high-power CO2 laser systems.
 • ZnSe is only about two-thirds the hardness of ZnS multispectral grade, but the harder antireflectance coatings serve to protect ZnSe.
• ZnSe’s cost is about the same as ZnS multispectral grade.
• Generally more expensive than Ge.
• ZnSe is diamond turnable.
Typical applications: CO2 laser systems, thermal imaging, FLIR, astronomy, medical
Products manufactured: Lenses, aspheric lenses, binary (diffractive) lenses, windows, beamsplitters, optical filters and prisms
Surface finish: Typical specifications for surface quality in the IR are 40-20 or 60-40 scratch-dig in the NIR and MW, and 60-40, 80-50 or 120-80 scratch-dig in the LW region. Diamond-turned surface finishes of 100 Å rms or better are typical.
Surface figure: In the NIR and IR, typical required surface figures range from 1/4 to 2 waves @ 0.6328 μm.
AR coating options: Typical available coatings include BBAR for the 0.8 to 2.5, 3 to 5, 1 to 5, 8 to 12 and 3 to 12 μm spectral regions; and single-wavelength coating AR at 10.6 μm. Many other specialized wavelength bands are possible from 0.6 to 16 μm.

Zinc Sulfide, Multispectral (See Cleartran)

Zinc Sulfide (ZnS), Regular Grade
A CVD material, ZnS regular has good imaging quality from 8 to 12 μm. It also transmits in the 3 to 5 μm band, but with higher absorption and scatter.
Transmission range: 3 to 12 μm
Index of refraction:
2.252 @ 4 μm
2.200 @ 10 μm
Density: 4.09 g/cm3
Hardness (Knoop): 200
Rupture modulus: 14,943 psi
Thermal expansion coefficient:
6.6 × 10-6/K @ 273 K
7.3 × 10-6/K @ 373 K
7.7 × 10-6/K @ 473 K
Special properties of ZnS regular:
• It exhibits high strength and hardness, and good resistance to hostile environments.
• It is about one-third harder than Cleartran and about twice as hard as ZnSe.
• It does not transmit well in the visible spectral region.
• It has the relatively low cost of about two-thirds the price of Cleartran or ZnSe.
• ZnS regular is diamond turnable.
Typical applications: Thermal imaging, FLIR
Products manufactured: Lenses, aspheric lenses, windows, domes, wedges and prisms
Surface finish: Typical specifications for surface quality in the 3 to 12 μm spectral region are 60-40, 80-50 or 120-80 scratch-dig.
Surface figure: In the IR, typical surface figure is specified from 1/4 to 2 waves @ 0.6328 μm.
AR coating options: The most typical available coating specified for ZnS regular is BBAR for 8 to 12 μm regions. It can also be hard carbon or diamondlike carbon coated.

Notes
• With the employment of magnetorheological finishing (MRF) on many of the materials listed in this guide, tight surface figure such as 1/20 or 1/10 wave @ 0.6328 μm can be achieved reliably on spherical and aspheric surfaces, where it might not have been otherwise possible in the past with conventional manufacturing methods.
• The achievable surface figure will depend largely on the material, aspect ratio of the optic, the ability to test the surface figure and the holding of the optic without distortion while testing.

References
Handbook of Optics, Second Edition, McGraw Hill, Inc. 1995.
The Infrared Handbook, Revised Edition, William L. Wolfe & George J. Zissis, ERIM 1985.
Commercially available data.


Glossary

UV: For the purposes of this discussion, wavelengths from 0.1 to 0.4 μm. (The ultraviolet region actually goes down to 0.001 μm).
VIS: The visible region of the spectrum band from 0.4 to 0.75 μm.
NIR: Near-infrared region of the spectrum band from 0.75 to 3 μm.
MW: Mid-wave region of the spectrum band from 3 to 7 μm.
LW: Long-wave region of the spectrum band from 7 μm and up.
Scratch-dig: A surface finish specification applying to the polishing process, where the ‘scratch’ refers to allowable scratch width on the polished surface in microns. A ‘dig’ is a defect in the polished surface resulting from the polishing process and refers to the maximum allowable diameter of the imperfection in microns. The dig specification is also commonly applied to inclusions or impurities within a visibly transparent material, and coating imperfections. Diamond-turned surface finishes, however, are expressed as surface roughness in angstroms root mean squared (Å rms) rather than scratch-dig, because the single-point diamond-turning process does not produce the random scratches and digs that result from the polishing process.
Surface figure: Commonly referred to as irregularity of the surface and usually expressed in fringes or waves at the test wavelength. In the optical design, careful consideration should be given when specifying the surface figure as it relates to the materials, wavelength of use, diameter, the aspect ratio and the geometry of the lens.
Aspect ratio: Diameter-to-thickness ratio. An optic with a high aspect ratio makes it difficult to achieve tight surface figure specifications and is more expensive.