crown glasses (original) (raw)

Author: the photonics expert (RP)

Definition: optical glasses with low chromatic dispersion and often a low refractive index

Category: optical materials

• optical materials
  • optical glasses
    * crown glasses
    * flint glasses
    * silicate glasses
    * fluoride glasses
    * phosphate glasses

Related: optical glassesflint glassessilicate glassesphosphate glassesAbbe numberrefractive index

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DOI: 10.61835/fi8 Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

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Contents

What are 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.

Names of Crown Glasses

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.

Applications of Crown Glasses

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.

Frequently Asked Questions

What is a crown glass?

A crown glass is a type of optical glass defined by its low chromatic dispersion, which is indicated by a high Abbe number, typically above 50 or 55.

What is the main difference between crown glass and flint glass?

Crown glass has low chromatic dispersion (a high Abbe number), while flint glass has high dispersion (a low Abbe number). Crown glasses also tend to have a lower refractive index and density than flint glasses.

What are the typical applications of crown glasses?

Crown glasses are used to make various optical components such as lenses, prisms, optical windows, and mirror substrates. They are often combined with flint glasses to create achromatic lenses that correct chromatic aberrations.

What is BK7?

BK7 is a widely used borosilicate crown glass known for its excellent optical quality. It has a refractive index of approximately 1.52 and an Abbe number of 64.17.

Why do crown glasses have names like BK, SK, or LaK?

These names follow a system where letters indicate the glass type, often based on German terms. For example, 'K' is for crown (Kron), 'BK' for boron crown (Bor-Kron), and 'SK' for heavy crown (Schwerkron).

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