NIRCam has two complete optical systems for redundancy.[1] The two sides can operate at the same time, and view two separate patches of sky; the two sides are called side A and side B.[3] The lenses used in the internal optics are triplet refractors.[3] The lens materials are lithium fluoride (LiF), a barium fluoride (BaF2) and zinc selenide (ZnSe).[3] The triplet lenses are collimating optics.[12] The biggest lens has 90 mm of clear aperture.[12]