The Ultimate Guide to Optical Windows

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.

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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

IR Windows

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 Firebird 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.

For more information, please visit Optical Windows supplier.

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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!

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Step Feature Specification Characteristics / Benefits Limitations 1 Specify the Quantity Quantity Required &#; The larger the quantity of pieces that can be used in an application, the less expensive each part becomes as material, labor and coating charges can be divided over the total number of parts.
&#; Advanced Optics has the ability to modify catalog/overrun optical windows (when possible) to reduce costs and lead times. Small number of prototypes may be more expensive due to lot charges for glass and coating. 2 Select the Material
Soda-lime Glass
&#; Commonly known as float glass. &#; Least expensive of all glass types. &#; Can be polished 1-3 waves/inch. &#; May be tempered making it 3 times stronger than non-tempered glass. &#; Transmission of uncoated material is ~ 89% average (dependent on thickness) from 400nm-700nm with poor performance in the NIR. Glass can be coated with an AR coating to increase transmission.
&#; Softer than borosilicate glass making it easily scribed and broken.
&#; Cannot be precision polished and is available in commercial grade only (1-3 waves/inch).
&#; Has the lowest thermal shock and chemical resistance of all glass materials used to fabricate optics.
&#; Not as scratch resistant as other materials used to fabricate optical windows. BOROFLOAT®33 &#; Borofloat®33 is a borosilicate glass with a low thermal expansion.
&#; Good all around general purpose mirror substrate that is moderately priced.
&#; Easier to polish than harder materials such as fused quartz, fused silica or Zerodur® and is much less costly. &#; Transmission of uncoated material ~ 92% average (dependent on thickness) from 400-700nm.
&#; May be polished down to λ10, but is not suitable for polishing down to λ/20.
&#; 2-3 times more costly than float glass (soda- lime glass).
&#; Not as thermally shock resistant as fused quartz or fused silica.
&#; Cannot be fully tempered like soda-lime glass.
&#; Not suitable for extreme high temperature conditions and will not hold its shape over 450° C for long periods of time. B 270® &#; Crown type soda-lime glass.
&#; Extremely clear and colorless.
&#; Good transmission in the visible into the IR with 90% average from 350nm-nm (dependent on thickness of glass). Available up to 10mm thick. D 263® T eco &#; Clear borosilicate glass.
&#; High chemical resistance.
&#; Ultra thin glass, wafers available up to 1.1mm.
&#; Excellent transmission over a large spectrum, ~93% or greater 350nm - nm (dependent on thickness of glass). Available up to 1.1mm thick. N-BK7® &#; Clear uniform color.
&#; Nearly free of bubbles and inclusions.
&#; High degree of purity.
&#; Very good refractive index homogeneity.
&#; Excellent transmission in the VIS to NIR spectrum with optimum transmission >95% from 350nm - nm (dependent on thickness of glass).
&#; Low absorption and uniform transmission in the visible spectrum. N-BK7 is not recommended for applications where thermal shock is a factor. Viosil &#; Viosil is a synthetic quartz glass substrate manufactured by ShinEtsu.
&#; The absence of bubbles and inclusions make it an excellent window substrate.
&#; Excellent transmission from the UV to the NIR, > 93% transmission from 200nm-nm.
&#; It offers excellent chemical resistance, mechanical strength and high heat resistance. Carry glass only up to .250" thick. Fused Silica &#; Made from a synthetically derived silicon dioxide that is extremely pure.
&#; It is a colorless, non-crystalline silica glass.
&#; The main difference between fused silica and fused quartz is that the former is composed of a non-crystalline silica glass while the latter is composed of a crystalline silica glass.
&#; Advantages of fused silica over fused quartz include; greater ultraviolet and infrared transmission, a wider thermal operating range, increased hardness and resistance to scratching and a lower CTE which provides resistance to thermal shock over a broad range of temperatures.
&#; As opposed to other less costly glasses, the surface figure (flatness) of optical windows made of fused silica are not at risk in applications that expose the material to coatings applied at high temperatures or applications that require the material to remain flat at high and/or varying temperatures.
&#; Fused silica is also chemically resistant and provides superior transmittance in the UV spectrum when compared to fused quartz.
&#; Fused silica comes in many grades with the most common being 2G. Please visit Corning&#;s Quality Grade Selection Chart for further information. &#; Very hard glass making it more difficult to fabricate than float or crown glasses.
&#; Raw material is more costly than float or crown glasses.
&#; The homogeneity of fused silica exceeds that of crystalline fused quartz, however standard 2G (UV grade) material has a higher OH content which has dips in transmission at 1.4µm, 2.2µm and 2.7µm. These dips can be eliminated by using a more expensive grade of IR fused silica. Quartz &#; Made from naturally occurring crystalline quartz or silica grains whereas fused silica is entirely synthetic.
&#; Fused quartz and fused silica are both extremely pure materials and have very low thermal expansion rates. However, fused quartz is more cost effective.
&#; Known for its incredible thermal shock resistance, chemical resistance and for being an excellent electrical insulator.
&#; Fused quartz has more metallic impurities and a lower OH content than standard UV grade fused silica which has dips in transmission at 1.4µm, 2.2µm and 2.7µm. These dips can be eliminated by using a more expensive grade of IR fused silica. &#; Very hard glass making it more difficult to fabricate than float or crown glasses.
&#; Raw material is more costly than float or crown glasses, but less expensive than fused silica.
&#; Fused quartz shares many of the same advantages of fused silica with the exception of metallic impurities found in the mined, natural quartz or silica sand. These impurities inhibit the materials ability to transmit well in the UV spectrum. 3 Determine the Size/Shape Round
Rectangular
Square
Custom Round provides the best opportunity for obtaining flatness/accuracy. Custom sizes and shapes available. Square, rectangular and custom shapes provide more challenges to maintaining surface flatness. 4 Refine your Mechanical Tolerances Defines the acceptable limits of both size and thickness required for an application. Specified in inches or mm and typically given a +/- value.
&#; Round: Provide tolerance for diameter.
&#; Rectangular/Square: Provide tolerance for LxW.
&#; Thickness: Provide tolerance for thickness. &#; Tighter tolerances for diameter and LxW are typically easier to hold than for thickness.
&#; Extremely tight tolerances available, but may require specialized techniques which can reduce yield leading to increased costs.
&#; Loosening your tolerances can reduce costs. 5 Establish the Correct Accuracy Commercial grade
1-3 waves/inch

Precision polished λ/4 or λ/10


Precision polished λ/10 or λ/20











Specify requirements as surface flatness or transmitted wavefront. Commercial grade mirrors are generally made from less expensive materials such as soda-lime glass or borofloat.

Working grade windows are polished either λ/4 or λ/10 and most often made of Borofloat®33 or N-BK7.

&#; Precision grade windows are polished either λ/10 or λ/20 and are typically made from harder glass materials such as quartz or fused silica.
&#; To achieve the best accuracy, optical windows are polished in a 6:1 aspect ratio (diameter to thickness). The higher the ratio, the greater probability the glass will distort during the manufacturing process. When the glass is deblocked after polishing, windows with non-standard aspect ratios may spring as they do not have the stability to hold surface flatness.
&#; Advanced Optics manufactures precision grade windows with non-standard aspect ratios.

Surface flatness is defined as the deviation between how flat the surface of an optical window is when comparted to a perfectly flat reference such as an optical flat.

Transmitted wavefront or TWE is defined as how much the light path is distorted as it passes through an optical window and is a function of surface flatness on both sides of the window, the purity and homogeneity of the material as well as the parallelism.

For a definition of the difference between transmitted wavefront and transmission see our optical terminology page. Achievable surface accuracy is dependent on choice of substrate and thickness of material. 6 Specify the Surface Quality Provide the required
Scratch and Dig 80-50: Commercial grade mirrors, suitable for non-critical applications, easily manufactured, lowest cost.

60-40 or 40-20: Working grade windows, precision quality, suitable for low to medium power lasers systems and smaller optics, moderate increase in cost.

20-10 or 10-5: Precision grade, suitable for high power laser systems and small optics. In demanding applications, even small surface defects might result in light scattering, undesired diffraction patterns, loss of contrast and stray light which can not only degrade a systems performance, but may even damage the optical window. Extremely tight tolerances available, but may require specialized techniques which can reduce yield leading to increased costs. 7 Provide Parallelism (if required) Amount of wedge or variation in thickness allowed over the surface of a part. It is defined in arc minutes (an angular measurement that is 1/16th of a degree) or arc seconds where 60 arc seconds is equal to 1 arc minute.

Advanced Optics manufactures wedged windows as well as parallel optical windows and can hold parallelism of < 2 arc seconds. Extremely tight requirements for parallelism require specialized manufacturing techniques which may reduce yield and increase manufacturing costs. 8 Define the Clear Aperture/ Edge Bevel Requirements The clear aperture is the percentage of useable area of an optical window.

An edge bevel or safety chamfer is applied around the edge of an optical window.
Normally 90% or advise requirement.


An edge bevel or safety chamfer is applied around the edge of an optical window to eliminate sharp edges and reduce edge chips caused by cutting of the glass. Typically between .010"-.040" face width at 45 degrees depending on size of part, please advise preference and tolerance. Very small edge bevels with tight tolerances will add additional costs. 9 Choose the Proper Coating Anti-reflective coatings available for the UV-VIS-NIR regions. Choices including MgFl2, V-coats and broadband coatings as well as custom coatings. &#; Provide the wavelength(s) of interest and % reflectivity required.
&#; Provide the intended AOI (angle of incidence) for the optical window. Custom coatings for a small quantity of parts may add additional expense. 10 Customization The following attributes can be added to customize your optical window. &#; Shapes: Provide drawing of custom shape.
&#; Holes and Notches: Provide location, size with tolerances.
&#; Custom Bevels: Provide location, depth and angle.
&#; Custom Coatings: Provide expected % of reflectivity over wavelength(s) of interest and AOI (angle of incidence). Additional features may add lead time and cost.