Technical Note:
Optical Materials

Newport offers a wide variety of spherical and aspherical lenses made from BK 7, UV grade fused silica, Infrared grade Calcium Fluoride (CaF₂), Magnesium Fluoride (MgF₂), AMTR and Zinc Selenide materials. For applications in the visible and infrared up to about 2.1 µm, BK 7 offers excellent performance at a good value. In the ultraviolet down to 195 nm, UV fused silica is a good choice. UV fused silica also has excellent transmission in the visible and infrared up to about 2.1 µm, better homogeneity, and a lower coefficient of thermal expansion than BK 7. CaF₂ and MgF₂ are excellent choices for deep UV or infrared applications—for custom applications, Newport can provide quotes on a build-to-print basis.

For common optical materials, this technical note offers specifications for important basic parameters such as index of refraction, transmission across various wavelength ranges, reflectance, Abbe Number, coefficient of thermal expansion, conductivity, heat capacity, density, Knoop hardness, and Young's modulus.

Also see Optical Material Properties, for additional information.

BK 7 is one of the most common borosilicate crown glasses used for visible and near infrared optics. Its high homogeneity, low bubble and inclusion content, and straightforward manufacturability make it a good choice for transmissive optics. BK7 is also relatively hard and shows good scratch resistance. The transmission range for BK 7 is 380–2100 nm. It is not recommended for temperature sensitive applications, such as precision mirrors.

UV Grade Fused Silica is synthetic amorphous silicon dioxide of extremely high purity. This non-crystalline, colorless silica glass combines a very low thermal expansion coefficient with good optical qualities, and excellent transmittance in the ultraviolet region down to 195 nm. Transmission and homogeneity exceed those of crystalline quartz without the problems of orientation and temperature instability inherent in the crystalline form. Fused silica is ideally used with high-energy lasers due to its high energy damage threshold. UV fused silica also has excellent transmission in the visible and infrared region up to approximately 2.1 µm, and a low content of inclusions with high refractive index homogeneity. Fused silica is used for both transmissive and reflective optics, especially where high laser damage threshold is required.

Calcium Fluoride is a cubic single crystal material grown using the vacuum Stockbarger Technique with good vacuum UV to infrared transmission. CaF₂ has a wide spectral range and is an excellent choice for deep UV to infrared applications because of its non-birefringent properties. CaF₂ has a transmission above 90% between 0.25 and 7µm, and is commonly used for excimer laser optics due to its low absorption and high damage threshold. Material for IR use is grown using naturally mined fluorite, at much lower cost. CaF₂ has very poor thermal properties with its high coefficient of thermal expansion, and should be avoided in high operating temperature environment. Calcium fluoride can be used without an Anti-Reflection coating due to its low index of refraction.

Note that this material is mildly hydroscopic. If used under normal operating conditions, no pitting is expected. However, it is not recommended for water immersion applications.

Magnesium Fluoride is a positive birefringent crystal grown using the vacuum Stockbarger Technique with good vacuum UV to infrared transmission. It is typically oriented with the c axis parallel to the optical axis to reduce birefringent effects. MgF₂ is another excellent choice for deep UV to infrared regions, with good transmission range from 0.15 µm to 6.5 µm, and its proven use in fluorine environments make it ideal for lenses, windows, and polarizers for Excimer lasers. MgF₂ is resistant to the thermal and mechanical shock, and has higher energy damage threshold. MgF₂ is one of the lowest index infrared materials, which usually doesn’t require anti-reflection coating. MgF₂ is extremely durable compared to other materials which are transparent from the UV to the IR spectrum. MgF₂ is an ideal choice for many biological and military imaging applications that require wide broadband laser pulses.

Note that this material is mildly hydroscopic. If used under normal operating conditions, no pitting is expected. However, it is not recommended for water immersion applications.

Crystal Quartz is a positive uniaxial birefringent single crystal grown using a hydrothermal process. It has good transmission from the vacuum UV to the near infrared. Due to its birefringent nature, crystal quartz is commonly used for wave plates.

Borofloat® is a borosilicate glass with a low coefficient of thermal expansion. It is mainly used for non-transmissive optics, such as mirrors, due to its low homogeneity and high bubble content.

Zerodur® is a glass ceramic material that has a coefficient of thermal expansion approaching zero, as well as excellent homogeneity of this coefficient throughout the entire piece. This makes Zerodur ideal for mirror substrates where extreme thermal stability is required. Zerodur should not be used for transmissive optics due to inclusions in the material.

Zinc selenide is a chemically vapor deposited material commonly used in thermal imaging and medical systems. ZnSe is an excellent choice for IR lens with its broad wavelength range (600nm- 16 µm). ZnSe has a high index of refraction which normally requires an anti-reflection coating to achieve high transmission. ZnSe is relatively soft with low scratch resistance thus not recommended for use in harsh environment. Extra caution is required during cleaning, handling, and mounting. ZnSe is the best choice for optics used in high power CO₂ laser systems due to its low absorption coefficient and high resistance to thermal shock

These graphs compare the transmission of standard optical materials. The transmission values listed here are equivalent to "external transmittance" and takes into consideration the reflectances you get from uncoated optical elements.

All metal reflectors deteriorate slowly in polluted atmosphere. Cumulative exposure to intense ultraviolet radiation also affects performance; overheating of the reflective surface will destroy the reflector.

The AlMgF₂ coating has been optimized for performance in the near UV.