What is an optical window?

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An optical window is a transparent, optically flat substrate that is used to separate two different environments while still allowing light to pass through. They may be either parallel (used to minimize the distortion of a transmitted beam of light of a specific wavelength) or wedged (used to direct transmitted light at a specific angle and reduce back reflection) and come either uncoated or with an anti-reflective coating used to reduce reflectance and improve transmission.

Optical windows are designed to work in all areas of the UV-VIS-IR spectrum and are therefore made of a variety of substrates. Choice of substrate is dependent on multiple factors such as a materials transmission, index of refraction, hardness, thermal expansion and chemical resistance. Advanced Optics focuses on optical windows made of glass substrates such as N-BK7®, fused silica, BOROFLOAT®33, B270 and Viosil as well as a wide range of high index glasses.

Like other flat optics, optical windows are used in many industries including aerospace, military and commercial avionics, scientific and medical instrumentation, academics and research, as well as industrial applications. Optical windows come in various sizes and shapes including round, square and rectangular and range in flatness from commercial grade to precision polished. Advanced Optics offers a broad range of in-stock optical windows and provides free engineering consultation services and custom fabrication from prototype to production or anywhere in between.

Factors to consider when selecting an optical window.

There are many factors to consider when designing an optical window. Careful consideration of each of the factors below will determine how effectively an optical window will perform in a given application as well as the impact to cost and delivery.

• Select the Correct Material: Since optical windows can be made of various materials, specifying the correct material is essential. Factors to consider include the amount of transmission required in a particular area of the spectrum, the index of refraction, the Abbe number, the coefficient of thermal expansion, Knoop hardness, and the density of the material. For further information on each of these, click here.


• Determine the Shape and Size: As with optical flats and mirrors, optical windows can be made into almost any shape to fit a specific application. However, if surface accuracy is critical, a round shape will yield a higher degree of surface flatness than a square or rectangular shape. Aspect ratio also plays a role. The closer the aspect ratio is to 6:1 (size to thickness) the easier it will be to achieve a higher degree of surface flatness and the less the part may flex or bend, causing a change in figure. However, if a 6:1 aspect ratio does not meet your requirements, Advanced Optics manufactures optical windows with non-standard aspect ratios.


• Establish the Correct Accuracy for your Application: Optical windows come in a wide range of accuracies from commercial grade to precision polished. Optical windows used as view ports, for example, do not require a high degree of flatness as they are typically viewed using with the naked eye. However, precision applications that require an optical window to pass light through with little to no distortion require both surfaces to be precision polished to a high degree of flatness. Advanced Optics fabricates optical windows with surface flatnesses of λ/4, λ/10 and λ/20 measured peak to valley at 632.8nm and can maintain parallelism ≤ 2 arc seconds.


• Refine Your Tolerances: As with all optics, optical windows may require tight tolerances. The tighter the tolerance in diameter, length and width or thickness, the more advanced techniques and longer manufacturing times are required to produce the window which both dictate an increase in price. The most difficult tolerance to hold is thickness. Tighter thickness tolerances require special blocking techniques and have the potential to increase the scrap rate as any scratch may be unable to be removed to stay within tolerance. Properly defining your tolerances can save you manufacturing time and reduce your costs.


• Specify the Correct Scratch and Dig: Scratch and dig specifications are typically expressed using MIL-PRF-13830B and are used to classify any cosmetic flaws in an optical surface or coating.  A scratch is defined as any marking or tearing of the surface; a dig is defined as any rough spot or pit on the surface of an optic.  Scratch and dig is expressed as two numbers separated by dash.  The first number indicates the allowable size and width of a scratch and the second number indicates the allowable size and number of pits over a defined area of an optic.  For further information on how Scratch and Dig Specifications are defined, click here.
  
  As with accuracy, imperfections in the surface of an optical window can distort the light traveling through the window. Commercial grade windows typically have a high scratch and dig number such as 80-50 whereas precision grade windows have a lower scratch and dig number such as 40-20. Extremely high precision laser optics require a very low scratch and dig such as 10-5. Understanding your application well enough to choose the correct scratch and dig number can save you time and money.


• Define the Wedge: Optical windows can be specified by how much wedge, or variation in thickness, is allowed over the surface of a part.  It is defined in arc minutes (an angular measurement that is 1/60th of a degree) or arc seconds where 60 arc seconds is equal to 1 arc minute.
  
  Wedge can be induced into an optical window to reduce the interference of reflections between the front and back surfaces or to separate and direct transmitted light at a particular angle. 
  
  Conversely, parallel optical windows are manufactured using special blocking techniques to ensure very little variation in thickness between the front and back of the window across its entire surface.   This ensures that a beam of light traveling through the window is straight and not distorted due to angular deviation of the optical surfaces.  A high degree of parallelism makes an optical window more expensive due to the amount of labor and scrap rate in the fabrication process.  


• Choose the Proper Coating: Optical windows may come either uncoated or with an anti-reflective coating. Standard anti-reflective coatings include MgF2 or a BBAR (broad band anti-reflective) coating for the visible spectrum. Anti-reflective coatings help to improve transmission and eliminate ghost images.
  
  If the requirement is to have a low reflectivity at a single wavelength, V-coats are the superior choice. V-coats can be designed to provide low reflectivity at several specific wavelengths as opposed to higher reflectivity over a broad wavelength range.
  
  Optical windows for use with lasers require durable coatings (dielectric coatings) designed for use at specific wavelengths. When choosing an anti-reflective coating it is important to specify the percentage of reflectivity you require at a specific wavelength(s) as well as the AOI (angle of incidence) you will be using the optical window. Advanced Optics can provide MgF2, BBAR, V-coats, dielectric coatings as well as custom coating designed to meet your specific requirements.

Making the final decision.

A good understanding of your application will help you choose the correct optical window.  Determining the best optical window for your project will require you to:

• Consider the environment in which the optical window will be used. Environmental factors may influence the type of material, cosmetics and coating selection best suited to your application.


• Define your requirements and expectations for critical specifications such as accuracy, mechanical tolerances, scratch and dig and wedge.


• Select the correct optical coating (if required) to maximize the performance of your window. Identify at what angle of incidence you will be using your optical window, as well as what percentage of reflectivity you require at specific wavelength(s). If using the optical window with a laser, provide information as to the power of your laser, the spot size and whether or not the laser will be pulsed or continuous over a specified period of time.

Carefully defining your requirements can save manufacturing time which translates to reduced costs. Should you have any questions or need help refining your requirements, Advanced Optics offers free engineering services.