Optical invariants are mathematical expressions representing several optical parameters. The values for each optical invariant are constant despite the fact that optical parameters change from one part or element of the optical system to another.

Optical invariants are significant during optical systems design because they make it possible to achieve quicker results while avoiding mistakes in optical paramters’ calculations.

There are 4 main invariants of optical systems:

Lagrange-Helmholtz invariant for paraxial optics Abbe sine condition for high aperture systems Straubel invariant for illuminating optical systems with light pencils transformation Etendue invariant for illuminating optical systems with extended source

Lagrange-Helmholtz invariant