N-BK7, UV Fused Silica Biconcave Lenses

Biconcave Lenses or Double Concave Lenses are optical lenses with two inward-bent spherical surfaces of identical radii of curvature. A double concave lens has a negative focal length and diverges a collimated light beam to the virtual focal point (that is the point at which the extension lines of the diverging light path intersect at the object side of the concave lens) and increases the focal lengths of a lens group. The usages of Biconcave lenses are diversified, encompassing divergence of collimated or focused light beams, and beam diameter modulation (e.g. Galilean beam expanders), and because of their negative focal lengths, bi-concave lenses could also be applied in the correction of spherical aberration of optical assemblies. Due to its symmetric structure, a double concave lens works best when the conjugate ratio (object distance: image distance) is close or equal to 1:1. In such situations, the distortion, spherical/chromatic aberration, and coma could be offset as a result of the equilibrium of the lenses. Whilst, when the intended magnification ratio is <1/5 or >5, a plano concave lens will be a better alternative.

This page features Shalom EO’s Off-the-shelf N-BK7 Double Concave Lenses and UV Fused Silica Double Concave Lenses. 

N-BK7 or its equivalent known as H-K9L is a ROHS-compliant borosilicate crown glass with a high transmission rate in the spectral range from visible to near-infrared (350-2200nm) and is probably the most frequently-chosen optical glass to manufacture professional optical components. The most attractive advantage of N-BK7 is its high optical homogeneity, which means its refractive indexes will only vary within a small amplitude. Besides, the hardness, chemical, and aberration resistance of N-BK7 are also very remarkable, it contains low levels of inclusions and bubbles, therefore N-BK7 is excellent for making high-precision optical lenses.

UV Fused Silica, equivalently known as JGS1, is an optical glass material with incomparably superior transparency to the UV wavelengths, meanwhile also being transparent to visible and NIR radiations (transmission range 200-2200nm). In fact, UV Grade Fused Silica is one of the best ultraviolet-transmitting media. The thermal properties of UVFS are prominent, too, including high-temperature durability and low thermal expansion. Other benefits of UV Fused Silica lenses encompass few bubbles/striae, high homogeneity, and advantages of birefringent properties. Therefore UVFS lens is the primary option for building a stable optical system under harsh working conditions.

Shalom EO’s stock N-BK7 Bi-concave Lenses have four standard coating options: uncoated, 350-650nm, 650-1100nm, and 1050-1580nm broadband anti-reflection (BBAR) coatings. The stock UV Fused Silica Bi-concave Lenses only have uncoated versions so far. While custom V-coatings that exhibit optimized transmission and low-cost MgF2 AR coatings with various transmission wavelengths could be furnished onto the N-BK7 and UVFS biconcave lenses upon request as well. Besides the stocked N-BK7 and UV Fused Silica Biconcave lenses, Shalom EO also provides custom biconcave lenses made from BaF2, CaF2, MgF2, Ge, ZnSe, Sapphire, and other optical materials. 

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