Zinc Selenide (ZnSe)
Plano-Concave Lenses

Plano-concave lenses are negative lenses which are thicker at the edge than in the middle, when light passes through them, it diverges and the focus point is virtual. Their focal lengths are negative, as well as the radius of curvature of the curved surfaces. Given their negative spherical aberration, plano-concave lenses can be employed to balance out spherical aberrations caused by other lenses in an optical system. Plano-concave lenses are useful for diverging a collimated beam and collimating a convergent beam, they are used to expand light beams and to increase focal lengths in the existing optical systems. These negative lenses are commonly used in telescopes, cameras, lasers or glasses to help magnification systems be more compact.

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Plano-concave lenses perform well when the object and image are at absolute conjugate ratios, greater than 5:1 or less than 1:5. In this case, it is possible to reduce spherical aberration, coma, and distortion. Similarly to with plano-convex lenses, to achieve maximum efficiency the curved surface should face the largest object distance or the infinite conjugate to minimize spherical aberration (except when used with high-energy lasers where this should be reversed to eliminate the possibility of a virtual focus).

ZnSe lenses are particularly well suited for use with high-power CO or CO2 lasers. In addition, they may provide enough transmission in the visible region to allow the use of a visible alignment beam, although back reflections may be more pronounced. Paralight Optics offers Zinc Selenide (ZnSe) Plano-Concave (PCV) Lenses available with a broadband AR coating optimized for the 2 µm – 13 μm or 4.5 – 7.5 μm or 8 – 12 μm spectral range deposited on both surfaces. This coating greatly reduces the high surface reflectivity of the substrate, yielding an average transmission in excess of 92% or 97% across the entire AR coating range. Check the Graphs for your references.