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Calcium Fluoride Positive Meniscus Lenses, AR Coated: 2 - 5 µm![]()
LE5801-E (Ø1") LE5243-E (Ø1/2") ![]() Please Wait
Features
Thorlabs' Ø1/2" and Ø1" Calcium Fluoride (CaF2) Positive Meniscus Lenses are available with a broadband AR coating optimized for the 2 µm to 5 μm spectral range deposited on both surfaces. This coating greatly reduces the surface reflectivity of the substrate, yielding high transmission and minimal absorption over the entire AR coating range. See the Graphs tab for detailed information. CaF2 is commonly used for applications requiring high transmission in the infrared and ultraviolet spectral ranges. The material exhibits a low refractive index, varying from 1.35 to 1.51 within its usage range of 180 nm to 8.0 μm. Calcium fluoride is also fairly chemically inert and offers superior hardness compared to its barium fluoride, magnesium fluoride, and lithium fluoride cousins. 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. When used to focus a collimated beam, the convex side of the lens should face the source to minimze 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. 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.
2 - 5 µm AR Coating Graphs![]() Click to Enlarge Click Here for an Excel File with Plot Data Shown above is a graph of the measured percent reflectance of the enhanced AR coating as a function of wavelength. The average reflectance in the 2 - 5 μm range is <1.25%. The blue shading indicates the region for which the AR coating is optmized. Performance outside of the specified range is not guaranteed and varies from lot to lot. The excel file above provides the coating curve data over an extended wavelength range. ![]() Click to Enlarge Click Here for an Excel File with Plot Data Shown above is a graph of the measured percent transmission of the enhanced AR coating as a function of wavelength. The blue shading indicates the region for which the AR coating is optmized. Performance outside of the specified range is not guaranteed and varies from lot to lot. The excel file above provides the coating curve data over an extended wavelength range. ![]() Click to Enlarge Click Here for an Excel File with Plot Data Shown above is a graph of the measured transmission of an uncoated, 5 mm thick sample of CaF2. Total Transmission of Optic (CaF2 Substrate, Uncoated)The table below gives the approximate theoretical transmission of these uncoated optics for a few select wavelengths in the 0.18 - 8.0 μm range. To see an excel file that lists all measured transmission values for this wavelength range, please click here.
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. Wavelength-Dependent Focal Length ShiftThe paraxial focal length of a lens is wavelength dependent. The focal length listed below for a given lens corresponds to the value at the design wavelength (i.e., the focal length at 4 µm). Since CaF2 offers high transmission from 0.18 - 8.0 µm, users may wish to use these lenses at other popular wavelengths. Click on the icons below to view theoretically-calculated focal length shifts for wavelengths within the 0.18 - 8.0 µm range. The blue shading indicates the region for which the AR coating is optimized. Please see the Graphs tab for more information. Ø1/2" Positive Meniscus LensesØ1" Positive Meniscus Lenses
Mounting High-Curvature OpticsThorlabs' 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.
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