BK7

BK7 is a widely used type of optical glass known as borosilicate crown glass. It is used in a variety of optics such as lenses, prisms, and windows due to its balance of good transmission, mechanical stability, and relatively low cost.

The designation BK7 originates from manufacturers’ naming conventions, with BK standing for borosilicate crown. It is produced by several glass makers, including Schott, and is commonly labeled as BK7 or N-BK7 in supplier catalogs. The glass is not a pure compound but a borosilicate crown formulation optimized for optical performance.

Optical properties typically cited for BK7 include a refractive index around n_d of 1.516 to 1.517 at the sodium D line (587.6 nm) and an Abbe number near V_d of about 64. It provides good transmission across the visible spectrum and well into the near-infrared, with a UV cut-off around roughly 350 nm. It also features relatively low dispersion compared with some other glasses and is straightforward to fabricate into precision components.

Applications for BK7 include camera and photographic lenses, eyepieces, binoculars, prisms, and optical windows. Its combination of optical quality, machinability, chemical stability, and affordability has made it a standard reference glass in many laboratories and commercial optical systems.