Smith–Helmholtz invariant

In optics the Smith–Helmholtz invariant is an invariant quantity for paraxial beams propagating through an optical system. Given an object at height y ¯ {\displaystyle {\bar {y}}} and an axial ray passing through the same axial position as the object with angle u {\displaystyle u} , the invariant is defined by H = n y ¯ u {\displaystyle H=n{\bar {y}}u} , where n {\displaystyle n} is the refractive index.

Source: Wikipedia — Smith–Helmholtz invariant (CC BY-SA 4.0)