Optical windows are used to allow optical radiation to pass from one environment to another without allowing other elements of these environments to mix. And it does not change the wavelength distribution of the beam, the transmitted wave front, nor does it scatter any of the light out of the beam.
BK7 Windows: Optical Properties and Applications
A BK7 Window is an optical window made of high quality Schott glass. These windows have excellent optical quality, and feature uniform transmission in the visible as well as into the near infrared. They have a very high refractive index homogeneity and a clear, almost colorless appearance.
Our BK7 windows are manufactured from Schott Optical borosilicate-crown glass. This glass has virtually no bubbles or inclusions, and is produced from very pure raw materials.
Shanghai Optics BK7 windows are suitable for demanding optical applications and can be manufactured with laser-grade precision. BK7 glass has a density of 2.51 g/cm3 and a Knoop hardness of 610. It has coefficient of expansion of about 7.1 x 10-6 K from -30 °C to +70 °C, and about 8.3 x10-6/K from +20 °C to +300 °C.
If good transmission further into the UV or a lower coefficient of thermal expansion are required, we recommend our UV fused silica windows instead.
BK7 windows are highly resistant to staining. They are also resistant to acid; where SR-classes are rated from 0 (high) to 4 (low), it may be graded as 1. It has a slightly lower resistance to alkali reagents; where AR-classes are rated from 0 (high) to 4 (low) it is graded as class 2.
Our factory standard for BK7 windows calls for a clear aperture of greater than 85 percent the diameter of the aperture. Diameter tolerance is +0.0, -0.05mm, and thickness tolerance +/- 0.1mm.
Surface Quality Specifications for BK7 Windows
Surface quality of our optical windows is given by a two part scratch dig number, as specified in U.S. Military Performance Specification MIL-PRF-13830B. Here the first number refers to the brightness of scratches, and can be 10,20,40,60, or 80, where the brightness of the scratches increases from 10 to 80. It is calibrated by comparison with reference scratches.
A dig, which is a pit or small crater on the surface of the glass, is defined by its diameter. The dig number is the actual size of the largest allowable dig, given in hundredths of a millimeter. If a dig is irregularly shaped, the diameter is calculated as half of the length + width.
Standard quality for most applications is considered 40-20, while laser requires a surface quality of at least 20-10. The factory standard for our BK7 windows is 10-5 scratch dig; which is considered precision laser quality. This means our windows are suitable for even high power or UV lasers. The high surface quality also results in minimum wavefront distortion or light scatter when optical radiation is passed through our windows. The maximum wavefront distortion, per our factory standards, is l/10 per 25mm.
Custom Options
Our BK7 windows are available with or without AR coatings. A protective bevel is also optional. We carry in stock circular windows from 3.5 to 25.4 mm diameter, and 1-3 mm thickness. Square BK7 windows with similar dimensions are also possible.
Our design team is happy to assist at any step of the way as you design your optical system. Please contact us for a free consult, and to discuss what custom needs you might have. Our state of the art machinery and metrology equipment allow us to produce precise, high quality optics for every application.
Factory Standard
- Material: BK7
- Irregularity (fringe): l/10
- Diameter Tolerance: +0.0, -0.05mm
- Thickness Tolerance: +/- 0.1mm
- Clear Aperture: >85%
- Parallelism: 1 arc min
- Surface Quality: 10-5 scratch and dig
- Wavefront Distortion: l/10 per 25mm
Contact us for manufacturing limit or custom specifications.
