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Ø1" N-BK7 Best Form Spherical Lenses, Uncoated
Thorlabs' N-BK7 Best Form lenses are designed to minimize spherical aberration while still using spherical surfaces to form the lens. They are typically employed at infinite conjugates in high-power applications where doublets are not an option. The lenses presented here are uncoated. Versions of our best form lenses are also available with one of three antireflection coatings deposited on both surfaces to reduce the light reflected from each surface of the lens (see table to the right).
For spherical lenses, a given focal length can be produced 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 (see the Graphs tab). The radius of curvature for each surface of these "best form" lenses has been chosen 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. Since the lenses are optimized for minimum spot size, they can theoretically reach diffraction-limited performance for small input beam diameters (see the table below for details).
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. Lens kits are also available. Please click here for more information.
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The transmission curve for N-BK7, a RoHS-compliant form of BK7, is shown above. Total transmission is given for a 10 mm thick, uncoated sample and includes surface reflections.
Lens Shape vs.
The plot to the right shows coma and spherical aberration plotted as a function of the curvature (c) of the front face of the lens for an N-BK7 lens with focal length of 100 mm, the stop set at the front surface of the lens, and a ±20 field. The curvature (c) is related to the more familiar radius of curvature (r) by c = 1/r. The minimum of the spherical abberation almost coincides with the zero coma location. The lens shape where the spherical aberration is at a minimum provides the basis for a "best form" design.
Mounting High-Curvature Optics
Thorlabs' retaining rings are used to secure unmounted optics within lens tubes or optic mounts. These rings are secured in position using a compatible spanner wrench. For flat or low-curvature optics, standard retaining rings manufactured from anodized aluminum are available from Ø5 mm to Ø4". For high-curvature optics, extra-thick retaining rings are available in Ø1/2", Ø1", and Ø2" sizes.
Extra-thick retaining rings offer several features that aid in mounting high-curvature optics such as aspheric lenses, short-focal-length plano-convex lenses, and condenser lenses. As shown in the animation to the right, the guide flange of the spanner wrench will collide with the surface of high-curvature lenses when using a standard retaining ring, potentially scratching the optic. This contact also creates a gap between the spanner wrench and retaining ring, preventing the ring from tightening correctly. Extra-thick retaining rings provide the necessary clearance for the spanner wrench to secure the lens without coming into contact with the optic surface.