Excellent Optical Homogeneity
Uniform refractive index supports precision imaging and repeatable optical performance.
BK7 Optical Glass
BK7 Optical Glass
Reliable Optical Performance for Visible and Near-Infrared Systems.
A widely used borosilicate crown glass offering excellent optical homogeneity, good mechanical durability, and economical performance for precision windows, lenses, prisms, and general imaging optics.
Why BK7
A Proven Standard for Precision Visible Optics
BK7 is one of the most commonly specified optical glasses because it combines consistent optical quality, broad visible transmission, good polishability, and practical cost. It is well suited to general-purpose imaging, laser, inspection, laboratory, and industrial optical systems.
Broad Visible Transmission
High transmission across the visible spectrum and useful performance into the near infrared.
Good Manufacturability
Machines and polishes efficiently into windows, lenses, prisms, filters, and custom shapes.
Cost-Effective Performance
A practical material choice when sapphire-level hardness or deep-UV performance is not required.
Engineering Properties
Optical, Mechanical, and Thermal Characteristics
Representative values for optical-grade BK7-type borosilicate crown glass. Final values vary by supplier, melt, grade, wavelength, and test method.
Physical & Mechanical Properties
| Glass Type | Borosilicate crown optical glass |
|---|---|
| Density | Approximately 2.51 g/cm³ |
| Knoop Hardness | Approximately 610 |
| Young’s Modulus | Approximately 82 GPa |
| Poisson’s Ratio | Approximately 0.21 |
| Thermal Expansion | Approximately 7.1 × 10⁻⁶/K |
| Transformation Temperature | Approximately 557°C |
Optical Properties
| Transmission Range | Approximately 0.35–2.5 µm, thickness-dependent |
|---|---|
| Refractive Index nd | Approximately 1.5168 |
| Abbe Number Vd | Approximately 64.17 |
| Dispersion | Low-to-moderate crown-glass dispersion |
| Birefringence | Isotropic glass; stress birefringence depends on processing |
| Coating Compatibility | AR, reflective, beamsplitter, filter, and custom coatings |
| Available Grades | Commercial, optical, precision, and high-homogeneity grades |
Optical Transmission
High Transmission Across the Visible Spectrum
BK7 provides strong transmission throughout the visible range and useful near-infrared performance. Actual transmission depends on thickness, surface finish, absorption, and optical coating.
Representative only. Final performance varies with glass grade, melt data, thickness, surface finish, and coating design.
Material Comparison
When BK7 Is the Practical Choice
| Property | Sapphire | BK7 | Fused Silica | CaF₂ |
|---|---|---|---|---|
| Visible Imaging | Very Good | Excellent | Excellent | Very Good |
| UV Performance | Very Good | Limited | Excellent | Excellent |
| Scratch Resistance | Excellent | Moderate | Good | Low |
| Thermal Stability | Excellent | Moderate | Excellent | Moderate |
| Relative Cost | Higher | Low | Moderate | Higher |
Manufacturing Capabilities
From BK7 Blank to Finished Optical Component
COE Sapphire supports prototype, low-volume, and repeat production of precision BK7 optical components with integrated fabrication, coating, and inspection.
Precision Machining
Cutting, edging, coring, beveling, centering, and custom geometry fabrication.
Optical Polishing
Precision flat, spherical, and custom polishing for demanding surface requirements.
Optical Coating
AR, reflective, beamsplitter, filter, and application-specific multilayer coatings.
Quality Assurance
Dimensional inspection, interferometry, surface inspection, and documentation.
Design Guide
Specify the Right BK7 Optical Component
Provide operating wavelength, dimensions, tolerances, surface quality, flatness, wedge or parallelism, clear aperture, edge treatment, coating, quantity, and environmental requirements.
Discuss Your SpecificationSurface QualityCommercial through precision optical grades
FlatnessApplication-specific interferometric control
Wedge / ParallelismPrecision parallel, wedged, or custom geometry
Edge TreatmentChamfer, bevel, radius, and blackened edges
CoatingsSingle-band, broadband, reflective, and filter coatings
DocumentationInspection reports and traceability available
Related Optical Components
BK7 Components Manufactured to Your Specification
Frequently Asked Questions
BK7 Optical Glass FAQ
What is BK7 optical glass commonly used for?
BK7 is widely used for visible-light windows, lenses, prisms, filters, mirrors, beam splitters, and general imaging components.
What wavelength range can BK7 transmit?
BK7 typically offers useful transmission from approximately 350 nm into the near infrared, with the practical range depending on thickness and coating.
How does BK7 compare with fused silica?
BK7 is generally more economical and easy to manufacture, while fused silica offers better deep-UV transmission, lower thermal expansion, and stronger thermal-shock performance.
How does BK7 compare with sapphire?
BK7 is more cost-effective for many visible applications, while sapphire provides much greater hardness, abrasion resistance, and high-temperature capability.
Can BK7 be optically coated?
Yes. BK7 is compatible with anti-reflection, reflective, beamsplitter, filter, and custom multilayer coatings.
Can COE Sapphire manufacture custom BK7 shapes and tolerances?
Yes. COE Sapphire can manufacture custom dimensions, wedges, bevels, radii, apertures, holes, polished surfaces, and application-specific tolerances.
What information is needed for a BK7 quotation?
Please provide dimensions, tolerances, surface quality, flatness, wedge or parallelism, clear aperture, coating requirements, quantity, and any inspection documentation needed.
Custom BK7 Optical Components
Request a QuoteReady to Discuss Your BK7 Project?
Send us your drawing, specification, or application requirements.