ω=n(F)−n(C)n(D)−1. Here C, D and F refer to the wavelengths of the C, D and F Fraunhofer lines in the solar spectrum, which are respectively, Hα (656.3 nm), Na I (589.3 nm), Hβ (486.1 nm), and which may be loosely referred to as “red”, “yellow” and “blue”.

Doublets improve the optical quality of a lens reducing both chromatic dispersion and spherical aberration. Their were first discovered in the XVIII century in England for telescopes applications. Currently many imaging applications in the visible are using doublets : Astronomy.

Achromatic doublet: With additional degrees of freedom, optical designers have more latitude to correct optical aberrations more thoroughly meaning that they have superior performance in comparison to singlet lenses. In addition, they typically offer superior off-axis performance.

Description: A lens which is specially designed to control the effects of chromatic distortion or aberration (a defect of optical lenses to bring the focus of all colours to the same convergence point) is called an achromatic lens. It is widely known as an 'achromat'.