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  • N-BK7-Best-Form-lens

Best Form Spherical Lenses

For spherical lenses, a given focal length can be defined by more than one combination of the front and back radii of curvature. Each combination of surface curvatures will result in a different amount of aberration caused by the lens. The radius of curvature for each surface of best form lenses has been designed to minimize the spherical aberration and coma produced by the lens, optimizing it for use at infinite conjugates. This process makes these lenses more expensive than plano-convex or bi-convex lenses, but still significantly less expensive than our premium line of CNC-polished aspheric lenses or achromats.

Since the lenses are optimized for minimum spot size, they can theoretically reach diffraction-limited performance for small input beam diameters. For best performance in focusing applications, place the surface with the shorter radius of curvature (i.e., the more steeply curved surface) towards the collimated source.

Paralight Optics offers N-BK7 (CDGM H-K9L) Best Form Spherical lenses which are designed to minimize spherical aberration while still using spherical surfaces to form the lens. They are typically used at infinite conjugates in high-power applications where doublets are not an option. The lenses are available either uncoated or our antireflection (AR) coatings deposited on both surfaces to reduce the light reflected from each surface of the lens to reduce the amount of light reflected from each surface of the lens. These AR coatings are optimized for the spectral range of 350 – 700 nm (VIS), 650 – 1050 nm (NIR), 1050 – 1700 nm (IR). This coating greatly reduces the high surface reflectivity of the substrate less than 0.5% per surface, yielding a high average transmission across the entire AR coating range. Check the following Graphs for your references.




CDGM H-K9L or customs


Best Possible Performance from a Spherical Singlet, Diffraction-Limited Performance at Small input Diameters


Optimized for Infinite Conjugates

Coating Options:

Available Uncoated with AR Coatings Optimized for the wavelength Range of 350 - 700 nm (VIS), 650 - 1050 nm (NIR), 1050 - 1700 nm (IR)

Focal Lengths:

Available from 4 to 2500 mm


Ideal for High-Power Applications


Common Specifications:


Reference Drawing for

Best Form Spherical Lens

f: Focal Length
fb: Back Focal Length
R: Radius of Curvature
tc: Center Thickness
te: Edge Thickness
H”: Back Principal Plane

Note: The focal length is determined from the back principal plane, which does not necessarily line up with the edge thickness.



Ranges & Tolerances

  • Substrate Material

    N-BK7 (CDGM H-K9L)

  • Type

    Best Form Spherical Lens

  • Index of Refraction (nd)

    1.5168 at designed wavelength

  • Abbe Number (Vd)


  • Thermal Expansion Coefficient (CTE)

    7.1X10-6 /K

  • Diameter Tolerance

    Precison: +0.00/-0.10mm | High Precision: +0.00/-0.02mm

  • Center Thickness Tolerance

    Precison: +/-0.10 mm | High Precision: +/-0.02 mm

  • Focal Length Tolerance

    +/- 1%

  • Surface Quality (Scratch-Dig)

    Precison: 60-40 | High Precision: 40-20

  • Spherical Surface Power (Convex Side)

    3 λ/4

  • Surface Irregularity (Peak to Valley)


  • Centration

    Precison: < 3 arcmin | High Precision: < 30 arcsec

  • Clear Aperture

    ≥ 90% of Diameter

  • AR Coating Range

    See the above description

  • Transmission over Coating Range (@ 0° AOI)

    Tavg > 92% / 97% / 97%

  • Reflectance over Coating Range (@ 0° AOI)

    Ravg < 0.25%

  • Design Wavelength

    587.6 nm

  • Laser Damage Threshold (Pulsed)

    7.5 J/cm2 (10ns,10Hz,@532nm)



This theoretical graph shows the percent reflectance of the AR coating as a function of wavelength (optimized for 400 - 700 nm) for references.


Reflectance Curve of Broadband AR-coated (350 - 700 nm) NBK-7


Reflectance Curve of Broadband AR-coated (650 - 1050 nm) NBK-7


Reflectance Curve of Broadband AR-coated (1050 - 1700 nm) NBK-7

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