In the world ofoctave, choosing the right glass is crucial to offering quality lenses, capable of guaranteeing optimal performance for the devices in which they are inserted. In this area, among the most used materials we find crown glass and flint glass, which have very distinct characteristics that make them suitable for different optical applications.
Composition and uses of crown and flint glass
The first difference between crown glass and flint glass is based on the composition: the former is produced from calcium-alkali silicates, while the latter contains a high percentage of lead oxide.
The second distinction is instead connected to the fields of application: crown glass is used for lenses or other optical components, such as telescopes and lenses for cameras or movie cameras; flint glass is instead suitable for optical glasses, such as prisms for refractometers (used to determine the refractive index of a substance) and achromatic lenses (with reduced chromatic aberration).
Classification issues
Crown glass is hard, resistant to atmospheric agents, clear and colorless. It has a low refractive index and low dispersion. This category includes optical glasses having a Abbe number greater than 55 (for some 50).
Flint glass is characterized, however, by a high refractive index and greater chromatic dispersion, in fact it presents a Abbe number less than 50.
What does Abbe's number mean?
We are talking about a dimensionless number that corresponds to the relationship between the optical refractive capacity and the chromatic dispersion of a transparent material at visible wavelengths. In other words, the Abbe number and his diagram allow us to classify the different types of glass (but also of other transparent materials) depending on their ability to disperse visible light, that is, to separate the different colors of a non-monochromatic light ray.
As is easy to understand from this brief comparison between crown glass and flint glass, the choice between the two will depend on the specific needs of the optical application.
The Abbe number is finally used to calculate the focal lengths and radii of curvature of the lenses in the so-called achromatic doublets. The latter are systems in which the focus is on the combination of a convex crown glass and a concave flint glass in order to minimize chromatic aberrations compared to the single lens.
Sources: wikipedia.org, treccani.it
