Fused Silica, Quartz, Fused Quartz, Quartz Glass, Silica, Silica Glass, all these names are confusing and often not very well defined by their users. See below definitions about these terms so you will understand them better:

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Click on the button below to download Fused Silica MSDS, don’t hesitate to come back to us if any other information is needed.

Please note that normal glass is also mainly made of SiO2 but is not pure SiO2, other chemical elements are also added like NaO2 and other elements.

If the medium is not air, as is common for some microscope objectives, the approximation above can be multiplied by the index of refraction of the medium as shown below.

Now that we have briefly explained what numerical aperture is, we can equate it to f/#. As explained here, f/# is also a measure of how much light can get through a lens. f/# of a simple lens is defined by the following equation, where f is the focal length of the lens and D is the diameter (or more specifically the entrance pupil diameter for more complex lens systems).

Fused silica

So what if the customer needs a numerical aperture of 0.25? To get this, at least one of the other specifications need to change. To do this, lets start with the initial specification for NA=0.25 and find what the f/# would be using this spec.

Quartz / Fused silica is a very interesting material due to the combination of many interesting properties. It is fully transparent with very good transmission from UV to IR, have a high resistance to water and chemical elements, and having a very low Coefficient of Thermal Extension can resist even better than borosilicate to thermal shock.

This is an exact equation relating the NA to the f/#, but it is often convenient to have an approximation for this. When n = 1 (medium is air) and if we use a small angle approximation (sin α ≈ tan α) then:

Key questions in the selection of your fused silica optics supplier is the complexity of the parts and optical specifications tightness, ability to provide an optical coating on the optics can be critical too.

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To determine whether lens specifications are compatible, we need to find the resulting numerical aperture from the other three specifications. To do this we will first need to use the equation below to relate the image height h, focal length f and the half field of view Θ. This equation can be derived using simple geometry using the relationships shown in the red triangle in figure 3.2.

Numerical aperture (NA) refers to the cone of light that is made from a focusing lens and describes the light gathering capability of the lens (similar to f/# ). NA is defined by the following equation, where n is the index of refraction of the medium (often n=1 for air), and α is the half angle of the cone of light exiting the lens pupil.

Fused SilicaWafer

Now we can find the focal length of the lens by assuming that the customer wants entrance pupil diameter to stay at the specified 20mm.

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In order to equate NA and f/#, we can use simple geometric relationships. Figure 3.1 shows a simple lens focusing light rays (blue lines) from infinity to a point. This creates a cone of light that can be described by numerical aperture using the previous equation. The half angle, α, can now be defined by the following equation:

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Below are three sets of lens specifications that would result in the desired system NA. There is an infinite number of specifications that will give the desired NA if one is allowed to change more than one spec.

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Most of precision optics suppliers are providing optics made of fused silica. The material is rather easy to work with and therefore contrary to Sapphire doesn’t need specific equipment to be cut and polished.

Often times when starting the design process one can inadvertently request conflicting specifications. This example will show how easy this is to do and how to avoid it when specifying a lens.

Key advantages are that the raw material is rather affordable and easy to source, relatively easy to be machined, and highly resistant to chemicals and temperatures (can be used up to 1000°C).