Invariant

In optics the Lagrange invariant is a measure of the light propagating through an optical system. It is defined by H = n u ¯ y − n u y ¯ {\displaystyle H=n{\overline {u}}y-nu{\overline {y}}} , where y and u are the marginal ray height and angle respectively, and ȳ and ū are the chief ray height and angle. via Wikipedia ↗