Lagrange invariant

In optics the Lagrange invariant is a measure of the light propagating through an optical system. It is defined by

$H = n\overline{u}y - nu\overline{y}$,

where y and u are the marginal ray height and angle respectively, and ȳ and ū are the chief ray height and angle. n is the ambient refractive index. In order to reduce confusion with other quantities, the symbol Ж may be used in place of H.^[1] Ж² is proportional to the throughput of the optical system (related to étendue).^[1] For a given optical system, the Lagrange invariant is a constant throughout all space, that is, it is invariant upon refraction and transfer.

The optical invariant is a generalization of the Lagrange invariant which is formed using the ray heights and angles of any two rays. For these rays, the optical invariant is a constant throughout all space.^[2]

References

1. Greivenkamp, John E. (2004). Field Guide to Geometrical Optics. SPIE Field Guides vol. FG01. SPIE. p. 28. ISBN 0-8194-5294-7.
2. Optics Fundamentals, Newport Corporation, retrieved 9/8/2011