BK7 and N-BK7

BK7 and N-BK7 are high purity, low cost borosilicates with excellent optical properties, making them a frequent choice for optics, prisms, and windows. 


Relative Cost of Material


Machining Degree of Difficulty


A frequent choice for high quality optics and prisms, N-BK7 (and BK7 before it) is a relatively low-cost, high-purity borosilicate with an extremely small amount of bubbles or inclusions that makes it ideal for optical applications in the visible spectrum. Made by Schott, this glass has good homogeneity in terms of refractive index throughout its surface, and as a block glass it comes in a variety of forms. N-BK7 is a softer glass that is significantly more sensitive to thermal variations than either fused quartz or low CTE materials such as Zerodur so special considerations have to be made in manufacturing to adjust for these properties. At Mindrum Precision, we have decades of experience machining, grinding, polishing, and acid etching this material, and we work in it on a routine basis. Click here for our contact information or to request a quote, and we will show you why more engineers are trusting us with their mission-critical needs.


As mentioned above, N-BK7 is primarily used in high-quality optics, but its thermal sensitivity in terms of both CTE and thermal shock make it a better choice for more controlled environments, as opposed to space-based applications. Outside of optical windows, prisms, and lenses, it is also used in many machined components, ranging from medical applications to semiconductor and research designs, and is a versatile material with many benefits.


Material PropertyFused Quartz
Density (g/cm^3)2.2
Hardness (Knoop)570
Tensile Strength (MPa)48
Modulus of Elasticity (GPa)72
Flexural Strength (MPa)1138
Compressive Strength (MPa)1103
Poisson's Ratio.17
Dielectric Strength (ac-kV/mm)50
Dielectric Constant (@1MHz)3.8
Volume Resistivity (ohm-cm @25°C)7.0E9
Coefficient of Thermal Expansion (1 x 10^-6/°C)0.6
Thermal Conductivity (W/m K)1.4
Specific Heat (J/kg*K)669
Maximum Working Temperature (°C)600 - 1000
Index of Refraction1.46
*Note: Properties vary by manufacturer. The above information should be used for general reference purposes only.

related Materials

  • Fused Silica – A synthetic, higher-purity version of Fused Quartz
  • Borosilicate – A cost-effective, generic optical medium
  • ULE – A near-zero CTE glass that is clear in color