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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.

The basic data here is given for N-BK7. Full optical design data on N-BK7 and other glasses can be found by following the weblinks at the bottom of this page.

Figure 1. The transmittance of BK7 compared to B270 at full wavelength range

Figure 1. The transmittance of BK7 compared to B270 at 200-1100nm


Physical and optical properties  

Transmission Range : 350nm to 2.5 μm
Refractive Index : 1.51680 @ 587.5618 nm (Yellow Helium Line)
Reflection Loss : 8.1% at 587.5618 nm (2 surfaces)
Absorption Coefficient : --
Reststrahlen Peak : --
dn/dT : See link of SCHOTT N-BK7 datasheet below
dn/dμ = 0 : --
Density : 2.51
Melting Point : 557°C (Transformation Temperature)
Thermal Conductivity : 1.114 W m-1 K-1
Thermal Expansion : 7.1 x 10-6 K-1
Hardness : Knoop 610
Specific Heat Capacity : 858 J Kg-1 K-1
Dielectric Constant : n/a
Youngs Modulus (E) : 82 GPa
Shear Modulus (G) : n/a
Bulk Modulus (K) : 34 GPa
Elastic Coefficients : n/a
Apparent Elastic Limit : 63.5MPa (9206psi)
Poisson Ratio : 0.206
Solubility : Insoluble in water
Molecular Weight : n/a
Class/Structure : Amorphous glass


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. 


  1. Full Datasheet of SCHOTT N-BK7
  2. Datasheet of SCHOTT B270
  3. SCHOTT Optical Glass Data and Catalog

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