N-BK7 HT (High-transmittance melt)
Heat-treated state
GB H-K9L · also Optical Borosilicate
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.
Values for the HT condition
Other N-BK7 tempers and conditions
N-BK7 is listed in 3 conditions; see the grade overview at N-BK7.
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 |
HTThis page 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 HT?
N-BK7 HT is N-BK7 heat treated to HT — 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. N-BK7 HT 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). N-BK7 HT has a density of 2.51 g/cm³ (0.0907 lb/in³), lighter than 70% of glass grades. Annealed Glass is the default condition FabDigit quotes; HT is available on request or by drawing note.
Advantages
- Low embodied carbon — 2 kg CO₂/kg, better than 81% of glass grades
N-BK7 HT properties
Typical room-temperature values for N-BK7 HT; 16 are specific to this condition. Other listed values are N-BK7 reference data and may not represent this condition. Bars are the position among glass grades.
Physical4
N-BK7 HT has a density of 2.51 g/cm³ (0.0907 lb/in³), lighter than 70% of glass grades.
Mechanical13
N-BK7 HT 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 HT 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 HT 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 HT 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 HT's machinability is poor (18/100, better than 52% of glass grades).
Values are nominal handbook figures for design screening. Confirm any required mill or lot documentation when quoting.
Working with N-BK7 HT
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.
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.
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) | — |
N-BK7 HT — frequently asked
What is N-BK7 HT used for?
N-BK7 HT 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 HT?
N-BK7 HT 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 HT 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 HT 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 HT?
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 HT 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 HT?
N-BK7 HT 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.
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. Nothing on this page is a certification — request mill certs, CoC or material test reports with your order.