Optical windows from Advanced Optics. Source: Advanced Optics

N-BK7 is a Schott borosilicate material used in optical applications from the visible to the NIR. Optical windows made of N-BK7 offer advantages such as a clear, uniform color free of bubbles and inclusions; a high degree of purity and homogeneity; excellent chemical and stain resistance; superior transmission in the visible and near-infrared spectrums; and low absorption and uniform transmission in the visible spectrum.

Due to the above properties, N-BK7 can be used to fabricate both optical windows and optical mirrors. It is most often selected due to its relatively low cost, high refraction index (1.51680 at 587 nm, yellow helium line) and high transmission between 350 nm and 2.5 µm. For further technical information, visit Advanced Optics' website.

While there are plenty of good reasons to choose N-BK7, it is temperature-sensitive and therefore not suitable for applications where it will be exposed to thermal shock. A good but more expensive alternative is fused silica, which offers a lower coefficient of thermal expansion, a much greater resistance to thermal shock and a broader range of optimal transmission when compared to N-BK7. A solid understanding of your application will help you decide if N-BK7 is the right choice.

Advanced Optics carries a wide range of N-BK7 in-house and also has the ability to:

• fabricate custom shapes;
• achieve parallelism of greater than 2 arc seconds;
• drill holes, notch glass and provide custom bevels;
• polish glass down to 3 mm in diameter and 0.5 mm thick;
• modify and resurface customer supplied materials;
• precision polish optics with non-standard aspect ratios; and
• provide broadband AR coatings for the visible spectrum as well as custom coatings.