When designing an optical window or mirror, there are a number of things you have to keep in mind. In particular, you should understand your application and know how efficiently the optic will perform. Other things you should know include the costs and delivery of the optic.
How to Choose the Right Optical Mirror or Window
Here are six things to keep in mind when looking for an optical mirror or window:
- Check The Material
Optical mirrors and windows can be made of different materials. Therefore, it is important to specify the material that you would like to be used. Here, you want to check the kind of application you have. For example, optical windows and mirrors that are polished to a higher degree of flatness have to be made from harder materials like quartz or silica. These materials are quite expensive. On the other hand, optical windows and materials with a lower surface flatness can be manufactured using less expensive substrates like borosilicate glass.
To determine the right optical mirrors and windows to buy, consider the density of the material required for your application, the Knoop hardness, and the coefficient of thermal expansion. For an optical window, check the Abbe Number, the index or refraction, and the amount of transmission required in a specific area.
- Shape And Size
The shape and size of the optical mirror or window should also be considered. These materials can be made into almost any shape to fit any application. However, the accuracy of the surface is critical. For example, a round shape will have a higher degree of surface flatness than a rectangular or square shape.
The aspect ratio (size to thickness) is another important factor to keep in mind. The closes the aspect ratio is 6:1, the easier it will be to achieve a higher degree of surface flatness, and the less the part may bend or flex, which can make the figure to change.
If the ^:1 aspect ratio is not perfect for your project, you can go for optical windows and mirrors with non-standard aspect ratios.
- Accuracy Of Your Application
Optical windows and mirrors are manufactured in different accuracies. For example, there are both commercial grade and precision polished windows. You should establish the correct accuracy for your application.
Commercial grade optics are not as expensive as their precision polished counterparts. A good example of these optics are windows used as a viewport with the naked eye.
When the quality of the image produced by an optical mirror is important or when the mirror is required to pass light with as little distortion as possible, you should go for polished optics. The cost of the mirrors or windows will depend on the degree of flatness required. The more the degree of flatness required, the most expensive the optical window or mirror would be.
- Determine The Level Of Tolerance
Like is the case with all optics, optical windows and mirrors may require higher tolerances. As the tolerance in width (thickness), length, and diameter of an optical mirror or window increases, so does the manufacturing time and price.
Thickness is the most difficult tolerance to hold. To achieve tighter thickness tolerance, the optical mirror or window has to be subjected to blocking techniques, which can increase the scrap rate.
You can save time and manufacturing costs by properly defining the required tolerance.
- Scratch And Dig
It is also important to specify the correct scratch and dig. The specification of these attributes is usually used to classify cosmetic flaws in an optical coating or surface.
- Scratch. A scratch refers to any tearing or marking on the surface of an optic.
- Dig. A dig is any pit or rough spot on the surface
Scratch and dig are expressed as two numbers separated by a dash.
- Select The Proper Coating
Various reflective and anti-reflective coatings can be used to coat optical windows and mirrors. When choosing a coating, consider the angle of incidence as well as the wavelength range of your application. After this, determine the percentage of reflection that would be required at the angle of incidence and wavelengths(s) that you want.
Optical mirrors can be coated with different metallic coatings, which may have or lack a protective overcoat. While bare coatings produce a higher reflectivity, they are softer and more likely to get damaged than optical mirrors with a more robust protective overcoat.
The most commonly used optical coatings include protected Ag, enhanced AI, bare and protected Ag, and bare and protected AI. Some manufacturers such as Evaporated Coatings Inc also offer dielectric coatings as well as custom coatings that will meet your specifications.
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