5. Reciprocal linear dispersion

A dispersive system is characterized by reciprocal linear dispersion and practical resolution. The reciprocal linear dispersion d λ /dx, is the ability of a dispersive system to spread the spectrum in the focal field. It is expressed in nm · mm ^–1 . The lower the value, the better the dispersion. The term "reciprocal" comes from the fact that we originally spoke of linear dispersion d x / d λ. Derivation of the lattice equation on the angle β leads to :

The reciprocal linear dispersion is related to the angular dispersion d λ /d β by the focal length f : $\frac{\text{d}\lambda }{\text{d}x}=\frac{1}{f}\frac{\text{d}\lambda }{\text{d}\beta }$