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N-BK7

Optical borosilicate-crown glass

Special Properties

  • Excellent optical quality
  • Uniform transmission in the visible
  • Virtually free of bubbles and inclusions
  • Clear, colorless appearance
  • Very high refractive index homogeneity
  • High purity of raw materials

Typical Applications

  • Demanding optical applications
  • Base material for precision optics
  • Windows, lenses, and prisms
  • Substrates for mirror and filter coatings
  • Measurement and sensor technology use
  • Lenses and dispersion prisms

Related glass types

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48h-Express Service Available in 48-hour express service*

N-BK7 is a very pure universal optical glass made by SCHOTT. It is one of the most common standard glass types used to produce high-quality optics, mainly for the visible spectrum. Polished window optics made of SCHOTT BK 7 glass The relatively hard borosilicate crown material consists of the purest raw ingredients. It contains very few inclusions and is almost free of bubbles.

The high material quality is a primary technical reason why N-BK7 glass is often the number one choice for optical windows, lenses, and prisms. Its smooth transmission and low absorption in the entire visible wavelength range are crucial features for numerous optical applications.

Unpolished N-BK7 raw materials are available with a maximum length of about 350 mm and a thickness of around 70 mm. SCHOTT BK7 glass blanksLarger glass dimensions are sometimes possible on request. N-BK7 belongs to the optical block glasses and does not come as smooth glass sheets. Hence, sawing, grinding, polishing, and shaping are always necessary production steps to fabricate optics from BK7. Due to the material's stable chemical properties and high stain resistance, special treatment of the glass blanks is unnecessary before grinding and polishing.

If you want to request optical windows, substrates, wafers, or other components made of N-BK 7 material with individual dimensions, please select a thickness from the table in the specifications or click on the following button.

 

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Note: If you are searching for optical glass materials with similar purity that can transmit ultraviolet light, please visit our fused silica and quartz glass web page.

 

Material Specifications


Transmission spectrum of N-BK7
for 10 mm of glass thickness

N-BK 7 glass optical transmission


Glass Thicknesses

BK7 raw material comes in glass blocks. Hence, it is not available as glass sheets with standard thicknesses. Therefore, the glass thickness always needs to be specified individually.

To offer our customers the best possible service and high availability, we continually polish the glass thicknesses shown in the following table. We can manufacture these thicknesses quickly and, if required, even as part of our express service for dimensions up to approximately 200 mm by 200 mm.
Please click on glass thickness to request a quote:

Thickness (mm)

Tolerance

48h-Express Service
0,50 ±0,025
1,00 ±0,025
1,00 ±0,100
2,00 ±0,025
2,00 ±0,100
2,00 ±0,200
3,00 ±0,025
3,00 ±0,100
3,00 ±0,200
Custom thicknesses are available on request.
The thicknesses marked with "" are also available within our 48-hour express service.

Optical Properties

Refractive index data

Refractive Index name

Wavelength (nm)

Refractive Index

ni
365,0
1.53627
nh
404,7
1,53024
ng
435,8
1,52668
nF'
480,0
1,52283
nF
486,1
1,52238
ne
546,1
1,51872
nd
587,6
1,51680
nD
589,3
1,51673
n632,8
632,8
1,51509
nC
643,8
1,51472
nc
656,3
1,51432
nr
706,5
1,51289
ns
852,1
1,50980
nt
1014,0
1,50731
  • Abbe number:
    • ve = 63,96
    • vd = 64,17

Mechanical Properties

  • Density: 2,51 g/cm3
  • Young's modulus E: 82 × 103 N/mm2
  • Poisson's ratio μ: 0,206
  • Knoop hardness HK0,1/20: 610

Thermal Properties

  • Coefficient of expansion:
    • 7,1 × 10-6/K (-30 °C to +70 °C)
    • 8,3 × 10-6/K (+20 °C to +300 °C)
  • Viscosities:
    • Softening Point (107,6 dPa): 719 °C
    • Annealing Point (1013 dPa): 557 °C
  • Transformation temperature Tg: 557 °C
  • Thermal conductivity: 1,114 W/(m × K)
  • Specific heat capacity: 0,858 J/(g × K)
    (20 °C ; 100 °C)

Chemical Resistance

  • Climate Resistance Class 2
    The resistance of N-BK 7 glass against humidity is expressed by CR-Classes 1 (high) to 4 (low).
  • Stain Resistance Class 0
    The resistance of N-BK 7 glass against staining is expressed by FR-Classes 0 (high) to 5 (low).
  • Acid Resistance Class 1
    The resistance of N-BK 7 glass against acid solutions is expressed by SR-Classes 0 (high) to 4 (low).
  • Alkali Resistance Class 2
    The resistance of N-BK 7 glass against alkali solutions is expressed by AR-Classes 0 (high) to 4 (low).
  • Phosphate Resistance Class 2,3
    The resistance of N-BK 7 glass against an alkali phosphate solution is expressed by PR-Classes 1 (high) to 4 (low).

Additional detailed optical specifications about the SCHOTT N-BK7 glass material are available on request.

 

All given details and specifications are mean reference values and are not guaranteed. Also, please consider our "Notes on technical specifications".

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