Types of Optical Windows
Oct. 24, 2022
Ultraviolet (UV) Windows
When it comes to the use of laser instruments in diverse medical, defense or industrial applications, UV windows are often the weapon of choice.
The standard material is high-purity fused silica, which transmits high percentages of light deep into the UV range below 200nm. UV Fused Silica has high transmission from 180-2,500nm. In addition, you can expect high levels of parallelism, low scattering, low distortion and laser-grade surface quality.
Additional UV coatings or Anti-Reflective (AR) coatings can be used to maximize the transmission properties in this range and can be applied to one or both surfaces.
Visible Windows (VIS)
VIS are a more cost-effective, specific option for use in the small range of visible light from 400-700nm. A UV window would work in similar applications but Optical Glass is a more specific, economical choice. N-BK7 is the material most commonly referred to by the name Optical Glass and features a transmission range of 350-2,000nm.
A VIS window is a common tool for use in imaging/display systems as well as a standard base substrate for use with mirror and filter coatings. Featuring a high index of refraction, high transmission and a high standard of material purity, these windows are often a crucial component in various optical systems. Additionally, N-BK7 has a high degree of stain resistance
The extended family of IR windows encompasses the largest and most frequently used assortment of optical windows. You can visit the full family of optical windows on the CLZ Precision Optics website.
Each particular window has its own unique property and transmission profile, which are needed for specialized applications. We will break down why you might be interested in each of these types of windows. Each material features a link which delves into more of the material specifics:
Barium Fluoride (BaF2)- features transmission from deep in the UV from 200nm-12μm, BaF2 can be used in multiple setups in the UV, VIS and IR range. Its main properties include resistance to high-energy radiation and its low index of refraction. AR coatings are often not needed.
Calcium Fluoride (CaF2)- is very similar to BaF2 in terms of its high damage threshold, low index of refraction and low absorption coefficient. CaF2's main standout is its outstanding transmission range of 130nm-9.5μm, which dips even deeper into the UV range and farther out into the IR range than UV Fused Silica. Like BaF2 it is mostly used in laser and cryogenic applications.
Germanium (Ge)- Germanium's standout property is its low dispersion, making it top choice for low power CO2 laser applications where a focused beam with minimal scattering is a must. Additionally, with its 2-16μm range, no unwanted radiation from the UV, VIS or even most of the NIR range can interfere with measurements. Germanium also has remarkable chemical properties and is inert to air, water, alkalis and many acids.
Potassium Bromide (KBr)- a mainstay in FTIR spectroscopy, KBr is sought after for its gigantic transmission range of 250nm all the way out to 26μm. KBr will withstand high temperatures up to 300ºC and mechanical shocks but care must be taken to avoid moist environments, which degrade the material.
Potassium Chloride (KCl)- often used interchangeably with KBr due to its similar transmission properties (210-20μm), KCl might be chosen over KBr due to its high damage thresholds and low index of refraction. Similar to Germanium, KCl is ideal for low-power CO2 laser applications but unlike Germanium, it can be used in the UV, VIS and NIR range.
Sapphire (Al2O3)- with a large transmission range of 150nm-4.5µm sapphire is a good generalist but where sapphire truly shines is its material robustness. You can use sapphire in almost any harsh environment and it will take the punishment. From extreme resistance to thermal conductivity, a high dielectric constant and chemical resistance, sapphire will take almost anything thrown at it and ask for seconds. Only behind diamond in terms of material hardness but unlike diamond can be made extremely thin, which further improves transmission.
Sodium Chloride (NaCl)- NaCl is the closest to a disposable option you'll find the IR window family. Since the window is essentially table salt, as you may imagine, it is sensitive to water and thermal shocks. With a wavelength range of 250-20μm, its main feature is that it is a cost-effective, wide range FTIR generalist.
Zinc Selenide (ZnSe)- The main reason to use a ZnSe window is in a high power Co2 laser system. High resistance to thermal shock, low absorption coefficient and low dispersion properties make it so you can concentrate high energy radiation and bring it to a focused, minimally scattered point through this window. Care must be taken as the material is soft and susceptible to scratches. ZnSe is not recommended for use in harsh environments. Get yourself a sapphire window instead!
|Material||Wavelength range||Refractive index (nd)||Density (g/cm3)||Abbe Number (vd)||Coefficient of Thermal Expansion (x10-6/°C)||Knoop Hardness (kg/mm2)|
|Barium Fluoride (BaF2)||200nm-12μm||1.48||3.18||81.61||18.1||82|
|Calcium Fluoride (CaF2)||130nm-9.5μm||1.43||3.18||95.1||18.85||158.3|
|Potassium Bromide (KBr)||250nm-26μm||1.527||2.75||33.6||43||7|
|Potassium Chloride (KCl)||210-20µm||1.49||1.98||44.39||36.6||7.2|
|Sodium Chloride (NaCl)||250nm-20μm||1.491||2.17||42.9||44||18.2|
|Zinc Selenide (ZnSe)||1-14μm||2.403||5.27||N/A||7.1||120|