What is the difference between BK7 and K9 optical glass?

BK7 and K9 are the two most widely used optical glass materials in the global optical industry, often seen as interchangeable alternatives. Many customers ask about their differences, so let’s break down the key similarities and distinctions clearly.

BK7, also known as borosilicate crown glass, is a European standard optical glass defined by ISO and DIN specifications. It is manufactured under strict international quality control systems, ensuring consistent optical performance across all production batches. K9, on the other hand, is the Chinese equivalent of BK7, formulated to match BK7’s optical and physical properties to the highest possible degree. Both materials share nearly identical core parameters, including a visible light transmittance of over 91%, a low thermal expansion coefficient, excellent chemical resistance, and outstanding machinability.

The primary difference between BK7 and K9 lies in their regional manufacturing standards and certifications. BK7 is widely recognized and accepted in European, American and other Western markets, while K9 is the preferred choice in Asian markets. In most practical applications, such as optical lenses, prisms, optical windows and industrial optical components, the two materials are fully interchangeable, as their refractive index, dispersion, and other key optical specifications are nearly identical. For example, if you are producing a standard camera lens, microscope component or industrial optical window, K9 can serve as a cost-effective alternative to BK7 without sacrificing any performance.

Another minor difference is the production process and quality control standards. While both materials meet high-quality requirements, some manufacturers may have slight variations in their production processes, leading to minor differences in surface quality or batch consistency. However, these variations are usually negligible for most industrial and commercial applications, especially when working with a reliable supplier like Felix Glass.

Our factory provides both BK7 and K9 optical glass parts with customized processing services, including precision cutting, surface polishing, anti-reflective coating and CNC machining, to meet the specific requirements of your projects. Whether you need BK7 optical glass for international projects or K9 optical glass for cost-effective solutions, we can deliver high-quality products that meet your exact specifications.