Crown Glasses

Crown glasses are optical glasses with an Abbe number above 55 or above 50 — there is no general agreement on that limit. A high Abbe number indicates weak chromatic dispersion and typically a low refractive index, but not always lower than for flint glasses.

The term crown glass does not imply a certain chemical composition. Tentatively, crown glasses contain lower amounts of heavy metals (e.g. lead, zinc or barium) than flint glasses, and a correspondingly higher content of alkali metals like sodium and potassium. Often they are soda–lime glasses (silicate glasses) or phosphate glasses. Their mass density is comparatively low.

Crown glasses have a larger band gap energy than flint glasses, leading to a shorter-wavelength UV absorption edge. Their parasitic absorption and scattering losses can be fairly low.

The common system of names for glass types uses a combination of one or more letters with a number. The letters indicate the general type of glass, e.g. 'K' for crown (German Kron). There are various more closely specified crown glass types, see the following table:

A particularly often used crown glass is BK7, a borosilicate glass, having a refractive index 1.5168 at 587.6 nm and the Abbe number 64.17. Fused silica is also a crown glass.

Lead-free alternative glass versions with quite similar optical properties are indicated with “N-” in front. For example, N-BK7 is a lead-free version of the traditional BK7.

Many optical components such as lenses, mirror substrates, optical windows and prisms are made from crown glasses. Important aspects for the choice of such glasses can be low parasitic propagation losses (e.g. for high-power laser applications), high near-UV transmission and low losses due to Fresnel reflections (if those are not suppressed with anti-reflection coatings). Low-dispersion glasses are e.g. beneficial for imaging applications because they minimize problems with chromatic aberrations.

Combinations of crown and flint glasses are used for achromatic optics.

Note that despite the low mass density of crown glasses, light-weight optics (e.g. correction glasses) often need to be made from flint glasses, where e.g. lenses can be made with a lower thickness for a given focal length.

Window glasses are also usually crown glasses.