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It is clear that high light transmission is the fundamental property of any glass that will be used as an optical material. Transmission is consistently high over the entire VIS wavelength range and even partially outside of this range.
The atomic bonds between the constituents of optical glass (silicon, oxygen, boron, sodium, potassium, lead, barium, lanthanum, and many others) are strong. In order to break them apart, photon energy higher than 3 eV is needed, corresponding to about 400-nm wavelength. Improved melting technology and raw materials of high purity enable a very low content of alien material that could introduce weaker bonds and thus absorption in VIS light.
Another essential property of glass is that it maintains its internal microstructure as a liquid while cooling down from melting until it becomes a rigid body. This means that no crystals or other types of micro-inhomogeneity of any size arise that could block, deflect, or scatter light and thus reduce clarity and transmission.
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