Optical Properties

• Refractive index (n_d): 1.51680 at 587.6 nm
• Abbe number (V_d): 64.17
• Transmission range: 350 nm ~ 2.5 μm (UV edge cut-off around 300 nm; poor transmission below 350 nm)
• Internal transmittance τ: >99% in visible band (400–700 nm)
• Dispersion: Low dispersion crown glass, suitable for achromatic lens groups

Mechanical & Thermal Properties

• Knoop hardness: 610 Kg/mm² (softer than sapphire; not suitable for high-abrasion or deep-sea high-pressure standalone use)
• Density: 2.51 g/cm³
• Coefficient of thermal expansion: 7.1 × 10^-6 /K
• Thermal conductivity: 1.11 W/(m·K)
• Water resistance: Class 1; Acid resistance: Class 2; Not recommended for long-term direct seawater immersion without protective coating

Fabrication Limits & Machining Notes

N-BK7 is easy to grind, polish and chamfer, but designers need to understand its inherent limitations:

• Not UV-deep compatible: Avoid applications below 350 nm; fused silica is the alternative for deep UV systems
• Pressure limit: Not a high-strength optical crystal. For deep subsea pressure housings, sapphire is the preferred substrate
• Geometry: Supports planar windows, hemispherical domes, spherical lenses, prisms, blanks and custom step flanges
• Surface quality: Can reach 40/20, 60/40 scratch/dig per drawing requirement

Compatible Optical Coatings

N-BK7 is a friendly substrate for most visible-band anti-reflective coatings. Common coating options:

• Single-layer / Multi-layer AR coating for visible 400–700 nm (PTZ surveillance, machine vision)
• Broadband AR coating for near-infrared up to 1064 nm
• Anti-fog coating (optional, for camera dome covers)

Note: N-BK7 is not ideal for high-power CO₂ laser infrared applications; ZnSe / ZnS should be selected instead.

Typical Application

Visible-band surveillance camera domes, PTZ optical covers, machine vision windows, achromatic lens assemblies, optical prisms and laboratory optical blanks.