N-BK7 Achromatic Doublet Lenses

Name: N-BK7 Achromatic Doublet Lenses
Brand: PhotonEdge

Key Specifications

Material	N-BK7 (positive element) + SF11 (negative element)
Transmission Range	380 - 2,100 nm (optimized for VIS)
Chromatic Correction	Corrected for 2 wavelengths (typically 486nm & 656nm)
Diameter Range	5mm - 100mm
Focal Length Range	10mm - 1,000mm
Surface Quality	20-10 / 40-20 (MIL-PRF-13830B)
Centration	< 3 arc min (standard) / < 1 arc min (precision)
Wavefront Error	< lambda/4 @ 632.8nm
Coating	BBAR VIS / Custom

Material Options

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

BK7/SF11 (Standard) — Best value for visible imaging. Corrected across 400-700nm. Industry standard achromat configuration.
N-BAK1/NF2 (Alternative) — Similar performance with slightly different dispersion properties. Available when BK7/SF11 geometry is not optimal for your f-number.
N-BK7/CaF2 (Fluorite) — Superior chromatic correction (semi-apochromatic). For demanding imaging applications requiring <50nm residual color error.

Coating Options

Coatings are engineered around your specific wavelength and operating conditions.

BBAR VIS — R < 0.5% across 400-700nm. Maximizes throughput for color imaging systems.
BBAR VIS-NIR — R < 0.5% across 400-1000nm. For systems spanning visible to NIR.
AR @ specific wavelength — For monochromatic imaging systems where maximum throughput at one wavelength is needed.

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

Machine vision and industrial inspection
Microscopy relay and objective lenses
Photographic and video imaging systems
Color measurement and spectroscopy optics
Medical endoscope and diagnostic imaging
Projection and collimation systems

Engineering Considerations

Chromatic correction limits: Standard achromats correct for two wavelengths (typically 486nm and 656nm). Residual color error (secondary spectrum) is typically 1-2% of the focal length. For tighter color correction, consider apochromatic triplets or fluorite elements.

Cemented vs air-spaced: Cemented doublets are standard for most applications. Air-spaced doublets offer slightly better correction and higher LIDT for laser applications but cost more and are more sensitive to alignment.

Temperature range: Standard optical cement operates from -40 degrees C to +80 degrees C. For extended temperature ranges, specify high-temperature cement or consider air-spaced designs.

Frequently Asked Questions

What is the advantage of an achromatic doublet over a singlet lens?

An achromatic doublet corrects chromatic aberration that a singlet cannot. For broadband (white light) imaging, singlet lenses produce colored fringes and blurred focus. Achromatic doublets bring two wavelengths to the same focal point, producing sharp, color-accurate images.

Can achromatic doublets be used for laser applications?

Yes, but for monochromatic laser use, the chromatic correction is unnecessary. A singlet lens is more cost-effective for single-wavelength laser focusing. Achromatic doublets are valuable when you need diffraction-limited performance across a broad wavelength range.

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Related Technical Resources

→ How to Choose the Right Optical Lens → Precision Lens Supplier Guide