The Biconcave lens is similar to the plano-concave lens. The focal length is negative, but the parallel incident light diverges outward. The two side surfaces of the double-concave lens have the same radius of curvature, and are generally used for beam expansion and projection.
Example with BK7 material: These N-BK7 biconvex lenses are available uncoated or with one of five antireflection coatings (-A, -AB, -B, -C, or -D), which reduce the amount of light reflected from each surface of the lens (see the Selection Guide table below for coating option). Since approximately 4% of the incident light is reflected at each surface of an uncoated substrate, the application of an AR coating improves transmission, which is important in low-light applications, and prevents the undesirable effects (e.g., ghost images) associated with multiple reflections. Having optics that are AR coated on both surfaces is particularly desirable for applications utilizing multiple optical elements. Please see the Graphs tab for additional coating information.
| N-BK7 Biconvex Lens Selection Guide | ||
| Coating name | wavelength band |  |
| Uncoated | Uncoated | |
| A Coating | 350nm-700nm | |
| AB Coating | 400nm-1100nm | |
| B Coating | 650nm-1050nm | |
| C Coating | 1050-1700nm | |
| D Coating | 1650nm-3000nm | |
