N-BK7
GB H-K9L · also Optical Borosilicate
N-BK7 (also designated H-K9L) is a borosilicate crown optical glass and the most widely used substrate for visible-spectrum optics, with good transmission from 350 to 2000 nm at low cost.
Reference datasheet — N-BK7 is not in the instant-quote catalog. Upload your part and an engineer will quote it in this grade or suggest the closest stocked equivalent.
Values for the Annealed Glass condition
N-BK7 tempers and conditions
N-BK7 is supplied in 3 conditions, soft to hard. Annealed Glass is the default FabDigit quotes; the values on this page are for it.
Compare all 3 conditionsHide
| Condition · open for datasheet | Best for | Yield MPa | Tensile MPa | Elong. % | Hardness | Conductivity |
|---|---|---|---|---|---|---|
Annealed GlassDefault standard (coarse/normal) annealed optical blank | Default choice for general lenses, windows and prisms where standard index tolerance (±0.001) and normal stress birefringence (≤10 nm/cm) are acceptable. | 50 | 0 | 610 HV | 1 W/m·K | |
High-transmittance melt | Choose for thick elements, illumination and UV-blue applications where internal transmittance at 350–400 nm and low solarization matter; external transmission is still Fresnel-limited unless AR coated. | 50 | 0 | 610 HV | 1 W/m·K | |
Fine/precision annealed | Pick for interferometer optics, polarization-sensitive systems and high-precision imaging where stress birefringence ≤4 nm/cm and index tolerance ±0.0002 (H-grades) are required. | 55 | 0 | 610 HV | 1 W/m·K |
What is N-BK7?
N-BK7 (also designated H-K9L) is a borosilicate crown optical glass and the most widely used substrate for visible-spectrum optics, with good transmission from 350 to 2000 nm at low cost. It is supplied as annealed, fine-annealed (FA) and heat-treated (HT) blanks, plates and pressings for lenses, windows and prisms where standard refractive index and wide availability matter more than special dispersion. It is brittle (about 50 MPa tensile), limited to roughly 500 °C, machinable only by diamond and abrasive processes (score 18), and its high Tg of 557 °C makes precision glass molding impractical. Fused silica is the alternative for UV transmission or low thermal expansion. N-BK7 has a density of 2.51 g/cm³ (0.0907 lb/in³), lighter than 70% of glass grades. N-BK7 has a tensile strength of 50 MPa (7.25 ksi), stronger than 59% of glass grades. Elongation at break is 0% and the elastic modulus is 82 GPa (11.9 Msi).
What is N-BK7 used for?
- Camera and imaging lenses
- Telescope optics
- Laser and instrument windows
- Prisms and beamsplitter substrates
- Achromat doublet crown elements
- Optical filter and mirror substrates
- Sight and inspection windows
Advantages
- Low embodied carbon — 2 kg CO₂/kg, better than 81% of glass grades
N-BK7 properties
Typical room-temperature values for N-BK7-Annealed Glass; bars are the position among glass grades.
Physical4
N-BK7 has a density of 2.51 g/cm³ (0.0907 lb/in³), lighter than 70% of glass grades.
Mechanical13
N-BK7 has a tensile strength of 50 MPa (7.25 ksi), stronger than 59% of glass grades. Elongation at break is 0% and the elastic modulus is 82 GPa (11.9 Msi).
Thermal5
N-BK7 is rated for continuous service to 500°C (932 °F). It conducts heat at 1.1 W/m·K (0.644 BTU/hr·ft·°F), worse than 64% of glass grades. Thermal expansion is 7.1 µm/m·K (3.94 µin/in·°F).
Electrical4
N-BK7 is an electrical insulator with a dielectric strength of 15 kV/mm (381 V/mil) and a resistivity of 1×10¹² Ω·m.
Chemical & Environmental2
Corrosion resistance is very good (78/100), better than 53% of glass grades.
Sustainability4
Producing a kilogram of N-BK7 takes about 30 MJ of energy and emits 2 kg of CO₂ — less than 79% of glass grades. Typical recycled content is 5%.
Manufacturability2
N-BK7's machinability is poor (18/100, better than 52% of glass grades).
Common Calculations4
Values are nominal handbook figures for design screening. Certified mill or lot data ships with every FabDigit order on request.
Strength versus weight in the glass family
Yield strength against density for every glass grade FabDigit runs. N-BK7 is highlighted; hover a dot for its name, or open the interactive family chart.
Working with N-BK7
Is N-BK7 easy to machine?
No conventional cutting — shape by diamond sawing, curve generating, loose-abrasive lapping and CNC grinding with copious coolant; core-drill from both sides to avoid exit chipping.
Can N-BK7 be welded?
Not weldable; join by optical contacting, UV-cure or epoxy cementing, or frit bonding — CTE-match partners to ~7×10⁻⁶/K to avoid stress birefringence.
Can N-BK7 be formed, bent or molded?
Hot slumping/pressing above Tg (557 °C) is possible for blanks but requires re-annealing; precision glass molding is uncommon for N-BK7 (high Tg wears molds) — low-Tg molding glasses are preferred.
N-BK7 chemical composition (wt %)
Limits by weight percent from the governing specification, written the way the spec states them — a single maximum for impurities, a range for alloying elements, and the base element as balance. Nominal is the typical mid-range value.
| Element | Spec limit (wt %) | Nominal |
|---|---|---|
| B2O3 | 9 – 12 | 10.5 |
| Na2O | 8 – 11 | 9.5 |
| K2O | 5 – 8 | 6.5 |
| BaO | 2 – 4 | 3 |
| Sb2O3fining agent; N-glass is arsenic- and lead-free | 0 – 0.6 | 0.3 |
| SiO2network former | ≥ 68 (balance) |
What surface finishes work on N-BK7?
Pitch or synthetic-pad polishing with ceria gives λ/10 surfaces and 20-10 to 40-20 scratch-dig; edges must be beveled, and AR / metallic / dielectric coatings adhere well after standard clean.
Equivalent designations
Standards and trade names that resolve to N-BK7 in FabDigit's catalog. Equivalence is nominal — check the exact specification when certification matters.
- GB (China)
- H-K9L
- Trade names
- Optical Borosilicate
- Also called
- BK7
How much does N-BK7 cost?
N-BK7 is a standard-cost glass ($$), cheaper than about 60% of the materials FabDigit quotes. Part price depends far more on geometry, tolerance and quantity than on the raw stock — upload a CAD file for a live quote.
N-BK7 — frequently asked
What is N-BK7 used for?
N-BK7 is typically used for camera and imaging lenses, telescope optics, laser and instrument windows, prisms and beamsplitter substrates, achromat doublet crown elements and optical filter and mirror substrates. In short: premium optical glass.
What is the tensile strength of N-BK7?
N-BK7 has a typical tensile strength of 50 MPa (7.25 ksi) — stronger than 59% of glass grades. Strength varies by condition: see the 3 listed tempers.
Is N-BK7 easy to machine?
Not especially — machinability is rated poor (18/100, better than 52% of glass grades). No conventional cutting — shape by diamond sawing, curve generating, loose-abrasive lapping and CNC grinding with copious coolant; core-drill from both sides to avoid exit chipping.
Can N-BK7 be welded?
Not weldable; join by optical contacting, UV-cure or epoxy cementing, or frit bonding — CTE-match partners to ~7×10⁻⁶/K to avoid stress birefringence.
What surface finishes work on N-BK7?
Pitch or synthetic-pad polishing with ceria gives λ/10 surfaces and 20-10 to 40-20 scratch-dig; edges must be beveled, and AR / metallic / dielectric coatings adhere well after standard clean.
Can N-BK7 be formed, bent or molded?
Hot slumping/pressing above Tg (557 °C) is possible for blanks but requires re-annealing; precision glass molding is uncommon for N-BK7 (high Tg wears molds) — low-Tg molding glasses are preferred.
What is the maximum service temperature of N-BK7?
N-BK7 is rated for continuous use to about 500°C (932 °F). Strength falls off well before that limit — check the elevated-temperature data for load-bearing parts.
How much does N-BK7 cost?
N-BK7 is a standard-cost glass ($$), cheaper than about 60% of the materials FabDigit quotes. Part price depends far more on geometry, tolerance and quantity than on the raw stock — upload a CAD file for a live quote.
Sources
- N-BK7 Optical Glass Data Sheet — SCHOTT AG (2023)
- Optical Glass Catalogue / TIE-19, TIE-27, TIE-33 technical information — SCHOTT AG (2022)
- Engineered Materials Handbook Vol. 4: Ceramics and Glasses — ASM International (1991)
- GB/T 7962 / ISO 12123 optical glass — SAC / ISO (2010)
Property data is compiled from published supplier and standards handbooks and normalised for comparison; hardness values on non-Brinell scales are converted approximately for charting only. Manufacturability limits are FabDigit quote-engine defaults and may be relaxed by engineering review. Nothing on this page is a certification — request mill certs, CoC or material test reports with your order.