N-BK7 Optical Windows

Name: N-BK7 Optical Windows
Brand: PhotonEdge

Key Specifications

Material	N-BK7 (equivalent to SCHOTT BK7 / K9)
Transmission Range	330 - 2,100 nm
Refractive Index	Nd = 1.5168
Diameter Range	3mm - 200mm
Thickness Range	0.5mm - 50mm
Surface Quality	20-10 / 40-20 / 60-40 (MIL-PRF-13830B)
Surface Flatness	lambda/4 to lambda/10 @ 632.8nm
Parallelism	< 1 arc-min (standard) / < 10 arc-sec (precision)
Coating	AR / HR / BBAR / Uncoated

Material Options

Select the optimal optical material based on your wavelength, power, thermal environment and mechanical requirements.

N-BK7 — Standard choice for visible-NIR windows. Excellent homogeneity, low cost, wide availability.
UV Fused Silica — Upgrade for UV transmission or high-power laser applications. Higher cost but broader transmission range.
Sapphire — For extreme scratch resistance and harsh environments. Mohs 9 hardness.

Coating Options

Coatings are engineered around your specific wavelength and operating conditions.

BBAR VIS-NIR — R < 0.5% across 400-1000nm. Maximizes throughput for multi-wavelength visible systems.
AR @ 532nm — R < 0.1% @ 532nm. For green laser beam paths.
AR @ 633nm — R < 0.1% @ 633nm. For HeNe laser systems.
Uncoated — ~4% Fresnel loss per surface. Acceptable for protective enclosures.

Manufacturing Tolerances

We optimize specifications to match your system requirements — not over-specify. Tighter tolerances increase cost without always improving system performance. Typical manufacturing capabilities:

Surface quality: from 60-40 to 10-5 scratch-dig
Surface flatness: from λ/4 to λ/20 per inch
Dimensional tolerance: ±0.01mm to ±0.05mm
Centration: <3 arc min standard, <30 arc sec precision

Inspection & Quality

Every component undergoes 100% inspection before shipment with full documentation including interferometer reports, spectral curves, surface quality images and dimensional data.

Laser interferometry: λ/100 resolution
Spectrophotometry: 190nm – 25μm
Surface inspection: MIL-PRF-13830B compliant
Dimensional metrology: ±0.01mm accuracy

Applications

Protective windows for laser enclosures
Observation ports on vacuum chambers
Beam path enclosures and light shields
Imaging system protective covers
Optical breadboard dust protection
General laboratory and industrial optics

Engineering Considerations

Cost vs. performance: N-BK7 is the most economical optical window material for visible-NIR applications. Only upgrade to fused silica or sapphire when you specifically need UV transmission, higher LIDT, or extreme hardness.

Thickness selection: For standard beam enclosures, 2-3mm thickness is sufficient. For vacuum applications or larger diameter windows (>50mm), increase thickness to prevent bowing under pressure differential.

Parallelism: For windows in laser cavities, specify wedged windows to eliminate etalon effects. For general protective use, standard parallelism (<1 arc-min) is adequate.

Is N-BK7 the Right Window Material?

N-BK7 is the most economical window material. Use this guide to confirm it matches your needs.

Wavelength

Your Need	This Material?	Note
UV needed (< 350nm)	No	Use UV fused silica
Visible-NIR (350-2000nm)	Yes ✓	N-BK7 is standard
IR beyond 2.5um	No	Use ZnSe or Ge

Power

Your Need	This Material?	Note
High-power laser (> 50W)	No	Use fused silica for higher LIDT
Low-power / non-laser	Yes ✓	N-BK7 is adequate

Environment

Your Need	This Material?	Note
Abrasive / harsh	No	Use sapphire (Mohs 9)
Indoor / cleanroom	Yes ✓	N-BK7 is sufficient

Summary

N-BK7 is the right choice for visible-NIR applications where cost matters. Upgrade to fused silica only when you need UV, high LIDT, or low thermal expansion.

Frequently Asked Questions

What is the difference between N-BK7 and K9 glass?

N-BK7 and K9 are essentially equivalent. N-BK7 is the SCHOTT designation, K9 is the Chinese optical glass standard. Both have identical optical properties (Nd=1.5168, Vd=64.2). We supply both designations depending on your documentation requirements.

Should I specify AR coating for a protective window?

If the window is in a laser beam path, yes - AR coating reduces reflection losses and prevents ghost reflections. For simple dust covers outside the beam, uncoated is usually fine.

Related Products

BK7 Windows Fused Silica Windows Sapphire Windows BK7 PCX Lenses

Related Technical Resources

→ 5 Key Specifications for Buying Optical Windows → BK7 vs UV Fused Silica Comparison