• Material: BK7 or SF11
• Wavelegnth Ranges - BK7: 350 nm - 2.0 μm, SF11: 420 nm - 2.3 μm

Thorlabs BK7 and SF11 Bi-Concave lenses are popular for many applications. Bi-Concave lenses have a negative focal length and are best used to diverge a converging beam. Similar to a Plano-Concave lens, a Bi-Concave lens can diverge a collimated beam to a virtual focus as used in a Galilean type beam expander (see Drawing tab for reference).

BK7 has an Abbe Number of 64.17, while SF11 has an Abbe Number of 25.76. A lower Abbe Number means the lens is made from material with a higher dispersion. SF11 lenses have a higher dispersion than BK7 lenses.

An Antireflection Coating reduces the amount of light reflected from each surface of the lens. Thorlabs BK7 and SF11 Lenses are available with our standard -A, -B, -C Broadband Antireflection Coating (please see Graphs tab for coating details).

Lens kits with bi-concave lenses made from BK7 are available.