H-k9l Datasheet

[Favio Cassidy]

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N-BK7(H-K9L) is a Schott designation for the most common Borosilicate Crown glass used for a wide variety of applications, especially for precision optics in the visible spectrum. Generally, the families of optical crown glass feature low dispersion( Abbe number) and low refractive index. N-BK7 developed by SCHOTT is the most iconic one among them, its strong chemical and scratch resistance making it competent at producing the smooth and durable surface required for optics.

These two optical glasses are selected as representative of typical Crown Glass formulations in common use. There are many other glasses including higher density and higher dispersion glasses. Click the reference links below to gain access to the datasheets of more types of Optical glass.

Selecting a glass material is important since different glass types have different characteristics. Edmund Optics offers a wide variety of glass types which can be selected on the basis of the following characteristics.

The index of refraction and Abbe number of a glass are typically used by designers as degrees of freedom when designing systems. The index of refraction refers to the ratio of the speed of light in a vacuum to the speed of light through a given material at a given wavelength, while the Abbe number of a material quantifies the amount of dispersion (variations in index) for a specific spectral range. For instance, a higher index of refraction generally bends light more efficiently so there is less of a need of curvature in the lens. Spherical aberration is less present in lenses with higher indices of refraction, while light travels faster through materials with lower indices of refraction. A high Abbe number generally gives less color dispersion and reduces color aberration. Also, certain glass types have different transmission wavelength regions.

The density of a glass helps determine the weight of the optical assembly and, along with lens diameter, becomes critical for weight sensitive applications. When dealing with applications involving extreme temperatures and quick temperature differentials, a glass' coefficient of expansion also becomes a key factor. Opto-mechanical designers need to keep this in mind when designing optical assemblies.

Today, the quality and integrity of optical glass is a fundamental assumption made by optical designers. Nearly 125 years ago, Otto Schott began a revolution by systematically researching and developing glass compositions. His development work on composition and the production process took glass manufacturing from the realm of trial and error to its state today as a truly technical material. Now optical glass properties are predictable, reproducible and homogeneous - the essential prerequisites of a technical material. The fundamental properties that characterize optical glass are refractive index, dispersion, and transmission.

Refractive Index is the ratio of the speed of light in a vacuum to the speed of light in the specified material - a description of how light slows down as it passes through an optical material. The refractive index for optical glasses, $ \smalln_d $, is specified at a wavelength of 587.6nm (Helium d-line). Materials with a low index of refraction are commonly referred to as "crowns" whereas materials with a high index of refraction are referred to as "flints."

When designing an optic that will be used in an extreme environment it is important to realize that each optical glass will have slightly different chemical, thermal, and mechanical properties. These properties can be found on the glass datasheet.

Optical systems have to be optimized for a total set of functional characteristics. Geometrical and color-induced aberrations can be compensated only by the use of more than one glass type. In most cases three or more glass types are used. The requirements on optical systems for different applications cover a range so wide that they cannot be met with just a small set of glass types. So a wide range of glass types has been developed. Traditionally they are shown in the refractive index versus dispersion diagram - the Abbe diagram.

In the Abbe diagram, glass materials are divided into type denominations like BK, SK, F, SF, etc. These "glass families" correspond to the regions in the Abbe diagram defined by the blue lines. There is a major line that separates crown glass types (last letter "K" from German "Kron" for crown) from flint glass types (last letter "F"). This line starts upwards from the bottom at Abbe number 55, steps aside at refractive index 1.60 to Abbe number 50, and continues upwards to the top.

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