Introduction
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Glass manufacturers provide hundreds of different glass types with
differing optical transmissibility and mechanical strengths. Melles
Griot has simplified the task of selecting the right material for an
optical component by offering each of our standard components in a
single material, or in a small range of materials best suited to
typical applications. There are, however, two instances in which one might need to know more about optical materials: one might need to determine the performance of a catalog component in a particular application, or one might need specific information to select a material for a custom component. The information given in this chapter is intended to help those in such situations. The most important material properties to consider in regard to an optical element are as follows: |
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Transmission versus Wavelength |
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A material must be transmissive at the wavelength of interest if it
is to be used for a transmissive component. A transmission curve
allows the optical designer to estimate the attenuation of light,
at various wavelengths, caused by internal material properties.
For mirror substrates, the attenuation may be of no consequence. |
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Index of Refraction |
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The index of refraction, as well as the rate of change of index
with wavelength (dispersion), might require consideration.
High-index materials allow the designer to achieve a given power
with less surface curvature, typically resulting in lower
aberrations. On the other hand, most high-index flint glasses
have higher dispersions, resulting in more chromatic aberration in
polychromatic applications. They also typically have poorer
chemical characteristics than lower index crown glasses. |
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Thermal Characteristics |
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The thermal expansion coefficient can be particularly important in
applications in which the part is subjected to high temperatures,
such as high-intensity projection systems. This is also of concern
when components must undergo large temperature cycles, such as in
optical systems used outdoors. |
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Mechanical Characteristics |
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The mechanical characteristics of a material are significant in
many areas. They can affect how easy it is to fabricate the
material into shape, which affects product cost. Scratch
resistance is important if the component will require frequent
cleaning. Shock and vibration resistance are important for
military, aerospace, or certain industrial applications. Ability to
withstand high pressure differentials is important for windows
used in vacuum chambers. |
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Chemical Characteristics |
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The chemical characteristics of a material, such as acid or stain
resistance, can also affect fabrication and durability. As with
mechanical characteristics, chemical characteristics should be
taken into account for optics used outdoors or in harsh conditions. |
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Cost |
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Cost is almost always a factor to consider when specifying
materials. Furthermore, the cost of some materials, such as
UV-grade synthetic fused silica, increases sharply with larger
diameters because of the difficulty in obtaining large pieces of
the material. |
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Overview |
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The table below shows the general characteristics of the optical
glasses used to manufacture most Melles Griot components. Click on
the material type for more detailed information. |
| Material | Usable Transmission Range | Index of Refraction | Features |
| BK7 | 350 nm to 2.0 µm | 1.52 @ 550 nm | Excellent all-around lens material combining broad transmission with excellent mechanical characteristics. |
| LaSFN9 | 420 nm to 2.3 µm | 1.86 @ 550 nm | High-refractive-index flint glass provides more power with less curvature needed. |
| SF11 | 420 nm to 2.3 µm | 1.79 @ 550 nm | High-refractive-index flint glass provides more power with less curvature needed. |
| F2 | 350 nm to 1.9 µm | 1.62 @ 550 nm | Material represents a good compromise between higher index and acceptable mechanical characteristics. |
| BaK1 | 330 nm to 2.1 µm | 1.57 @ 550 nm | Excellent all-around lens material, but has weaker chemical characteristics than BK7. |
| Optical Quality Synthetic Fused Silica | 260 nm to 2.5 µm and 3 µm to 3.7 µm |
1.46 @ 550 nm | Material provides good ultraviolet transmission and superior mechanical characteristics.. |
| UV-Grade Synthetic Fused Silica | 180 nm to 2.5 µm and 3 µm to 3.7 µm |
1.46 @ 550 nm | Material provides excellent ultraviolet transmission and superior mechanical characteristics. |
| Optical Crown Glass | 350 nm to 2.5 µm | 1.52 @ 550 nm | This lower tolerance glass can be used as a mirror substrate or in noncritical applications. |
| Low-Expansion Borosilicate Glass (LEBG) | 375 nm to 2.3 µm | 1.48 @ 550 nm | Excellent thermal stability, low cost, and homogeneity makes LEBG useful for high-temperature windows, mirror substrates, and condenser lenses. |
| Sapphire | 180 nm to 5.0 µm | 1.77 @ 550 nm | Excellent mechanical and thermal characteristics make it a superior window material. |
| Calcium Fluoride | 180 nm to 8.0 µm | 1.40 @ 5 µm | This popular ultraviolet excimer laser material is used for
windows, lenses, and mirror substrates. |
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| Optics Guide Copyright 2002 Melles Griot Inc. |