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N-BK7 is a widely used optical glass, appreciated for its superior optical performance and cost-effectiveness. This material, mainly composed of silica (SiO2) and boron oxide (B2O3), stands out for its applications in the visible to near-infrared range, making it a common choice in the field of precision optics.

Composition and Properties

N-BK7 is a borosilicate crown glass with a primary composition of silica and boron oxide, along with minor amounts of sodium oxide (Na2O), potassium oxide (K2O), and barium oxide (BaO).

Optical Performance

  • Refractive Index (nd): 1.51680 (at 587.6 nm)
  • Abbe Number (vd): 64.17
  • Transmission Range: 350 nm to 2.5 μm

Physical Properties

  • Density: 2.51 g/cm³
  • Knoop Hardness: 610 kg/mm²
  • Thermal Expansion Coefficient: 7.1 x 10⁻⁶ K⁻¹
  • Transformation Temperature: 557°C


N-BK7 is extensively used in manufacturing lenses, prisms, and optical windows, particularly suitable for visible and near-infrared applications. It is a common choice for precision optics and industrial applications.

Advantages and Limitations

N-BK7 is known for its high purity, good chemical stability, and scratch resistance, making it a cost-effective choice in the visible spectrum. However, compared to fused silica, it has weaker resistance to thermal shock, lower UV transmission, and is more susceptible to surface damage.

Manufacturing and Variants

This optical glass is available in various forms and sizes, typically as glass blocks. It requires grinding, polishing, and shaping to be fabricated into optical components. N-BK7 is an environmentally friendly version of the original BK7, free of lead and arsenic, maintaining all the advantages of the original material.

N-BK7 remains one of the most commonly used optical glasses due to its versatility, good optical properties, and relative low cost. It is particularly well-suited for applications in the visible spectrum where high precision is required but extreme environmental conditions are not a concern. When selecting the appropriate optical material, it is essential to consider both application requirements and material characteristics to make the best choice.


H-K9L is a borosilicate crown glass known for its high optical quality and durability. It is composed mainly of silica, boron oxide, sodium oxide, potassium oxide, barium oxide, and a minuscule amount of arsenic oxide. This glass is recognized for being environmentally friendly, adhering to RoHS 2.0 standards by excluding harmful substances like lead, cadmium, and arsenic.

Key Optical and Physical Properties

The optical properties of H-K9L are notable, with a refractive index of 1.51680 at the wavelength of 587.56 nm and an Abbe number of 64.20. It demonstrates excellent transmission rates across the visible to near-infrared spectrum, ranging from 350 nm to 2.0 μm. Physically, H-K9L has a density ranging from 2.51 to 2.52 g/cm³ and exhibits significant hardness with a Knoop hardness of 610 kg/mm². Its thermal expansion coefficient stands at 7.1 x 10^-6 K^-1 for temperatures between 20°C and 300°C.

Thermal Characteristics and Applications

Regarding thermal properties, H-K9L softens at 719°C, has an annealing point between 557°C to 657°C, and a strain point at 624°C. These characteristics make H-K9L an ideal choice for manufacturing optical components such as lenses, prisms, and windows, along with its utility in laser applications, mirror substrates, and optical coatings, especially where high precision in the visible spectrum is crucial.

Advantages and Manufacturer Information

H-K9L stands out for its high optical quality, minimal bubble and inclusion content, and robust mechanical properties that offer excellent scratch resistance. It also exhibits chemical stability, simplifying the grinding and polishing processes. Notably, H-K9L is manufactured by CDGM (ChengDuGuangMing), a leading Chinese optical glass manufacturer, providing a cost-effective alternative to Western equivalents like Schott N-BK7. This affordability, coupled with its comparable optical and physical properties, renders H-K9L a preferred choice for a broad spectrum of optical applications.

In summary, H-K9L’s superior optical and physical properties, coupled with its environmental compliance and cost-effectiveness, make it a versatile and popular choice in the optics industry. Its broad application range and the backing of a reputable manufacturer like CDGM only add to its appeal, solidifying its position as a competitive alternative to similar Western-made optical glasses.


This article presents a comprehensive comparison between two types of optical glass: N-BK7 and H-K9L. While both glasses are borosilicate crown types, primarily composed of silica (SiO2) and boron oxide (B2O3), H-K9L distinguishes itself by containing a minor proportion of arsenic oxide (0.36% As2O3), unlike the arsenic-free N-BK7.

In terms of optical properties, both glasses share identical refractive indices (nd) of 1.51680, with closely matched Abbe numbers (vd) — N-BK7 at 64.17 and H-K9L slightly higher at 64.20. The transmission range for N-BK7 extends from 350 nm to 2.5 μm, whereas for H-K9L, it spans from 350 nm to 2.0 μm.

Physically, the two glasses are nearly identical, each possessing a density of approximately 2.51 g/cm3 (with H-K9L ranging up to 2.52 g/cm3) and a Knoop hardness of 610 kg/mm2. Both materials exhibit a thermal expansion coefficient of 7.1 x 10^-6 K^-1 across a temperature range of 20-300°C. The thermal properties reveal a transformation temperature of 557°C for N-BK7; H-K9L has a broader range, extending from 557 to 657°C, marking its annealing point.

Both N-BK7 and H-K9L find extensive use in the manufacture of optical components such as lenses, prisms, and windows, applicable in visible and near-infrared spectrums. They are prized for their superior optical quality, minimal bubble and inclusion content, commendable mechanical properties, and scratch resistance, alongside chemical stability which simplifies grinding and polishing processes.

SCHOTT, a German company, produces N-BK7, positioning it as an environmentally friendly option devoid of lead and arsenic. Conversely, H-K9L is manufactured by CDGM (ChengDuGuangMing) in China and contains a trace amount of arsenic oxide. While N-BK7 is marketed as an eco-friendlier alternative, H-K9L offers a more competitive price point.

Ultimately, the choice between N-BK7 and H-K9L hinges on factors such as availability, cost, and specific environmental or regulatory requirements, with both glasses being largely interchangeable for most applications.
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