Doublets and Triplets
An achromatic doublet is the combination of positive and negative elements with different refractive indices. They are computer-optimized to correct for on-and off-axis spherical and chromatic aberrations. When these doublets are used to focus or collimate, the most steeply curved (shortest radius) surface should face the collimated light.
These achromatic triplets consist of a low-index, symmetrical convex crown glass elements which are cemented between two identical high-index meniscus flint glass elements. These lenses are reversible because they are symmetrical.