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UV Fused Silica Positive Meniscus Lenses, Uncoated
Thorlabs' UV Grade Fused Silica Positive Meniscus Lenses are available either uncoated or with a UV antireflection coating for the 290 - 370 nm range deposited on both surfaces. Compared to N-BK7, UV-grade fused silica offers high transmission deeper into the UV (down to 185 nm), better homogeneity, and a lower coefficient of thermal expansion. In addition, UV fused silica exhibits virtually no laser-induced fluorescence (as measured at 193 nm), making it an ideal choice for applications from the UV to the near IR.
Positive meniscus (convex-concave) lenses, which are thicker in the middle than at the edges and cause light rays to converge, are designed to minimize third-order spherical aberration. They are often used in conjunction with other lenses to decrease the focal length, and therefore increase the numerical aperture (NA), of an optical assembly. When used to focus a collimated beam, as shown in the diagram above, the convex side of the lens should face the source to minimze spherical aberration. Since a positive meniscus lens has a greater radius of curvature on the concave side of the lens than on the convex side, real images can be formed.
Thorlabs offers their negative meniscus lenses in Ø1 and Ø2" versions. Each size is compatible with a multitude of Thorlabs lens mounts. Please see the Mounting Options tab for details.
Using Positive Meniscus Lenses
Positive meniscus lenses are designed to minimize spherical aberration. They have one convex and one concave surface. When used in combination with another lens, a positive meniscus lens will shorten the focal length and increase the NA of the system. Figure 1c shows a meniscus lens being used to shorten the focal length of a 100 mm focal length plano-convex lens. In addition, the transverse and lateral aberrations are greatly reduced. The convex surface of both lenses should be facing away from the image.
Figure 1. These figures illustrate the performance gains that can be achieved by using multi-element imaging systems. The combination of a meniscus lens and a plano-convex lens yields a 21 µm focused spot versus a 240 µm spot from the single plano-convex lens.
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.