Optical Invariant

A useful tool that allows optical designers to determine various values without having to completely ray trace a system. It is obtained by comparing two rays within a system at any axial point. It is constant for any two rays at every point in the system. In other words, if the invariant for a set of two rays is known, ray trace one of the rays and then scale that by the invariant to find the second.

See also Lagrange Invariant

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