Optical glass is distinct from regular glass because it plays a crucial role in optical systems and must meet strict requirements for image quality. As a result, the evaluation of optical glass involves specific and more rigorous criteria. Key requirements include precise optical constants and consistency within the same batch. Each type of optical glass has defined refractive index values for different wavelengths, which are essential for optical designers when creating an optical system. Therefore, the actual optical properties of the glass produced by manufacturers must fall within a tight tolerance range around these standard values. If not, the final image quality may deviate from expectations, affecting the performance of the optical instrument.
Moreover, since instruments often use glass from the same batch, the refractive index tolerances for that batch are even stricter than the deviation from the standard value. This ensures uniformity and accurate calibration across all units.
Another critical factor is high transparency. The brightness of an optical system's image depends directly on the transparency of the glass. The transparency of optical glass at a given wavelength is measured by the absorption coefficient, Kλ. As light passes through multiple prisms and lenses, some energy is lost due to surface reflections, while the rest is absorbed by the glass itself. Surface reflection increases with higher refractive indices, and for high-index glass, this loss can be as high as 6%. For optical systems with many thin lenses, reducing surface reflection—such as by applying anti-reflective coatings—is the main way to improve transmittance.
For large optical components, like the objective lens of an astronomical telescope, the overall transmittance is mainly determined by the glass’s absorption coefficient, especially due to its thickness. To achieve high transmittance, the raw materials must be highly pure, and no coloring impurities should be introduced during the entire process, from mixing to melting. With such care, it is possible to produce glass with an absorption coefficient below 0.01, meaning a 1 cm thick piece of glass will transmit over 99% of the light.
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