Hauptebene

Hauptebene, in optics commonly referred to as the principal plane, is a theoretical plane in a thick lens or an optical system from which paraxial light rays can be considered to diverge or converge after refraction. The concept helps describe the imaging behavior of complex lenses by reducing them to a thinner equivalent system.

In a thick lens or multi-element system, there are typically two principal planes: the front principal plane (H1) and the rear principal plane (H2). The positions of these planes depend on the curvatures and arrangement of the lens surfaces and the surrounding medium. The focal length of the system, called the effective focal length (EFL), is defined with respect to these principal planes. For a thin lens, the principal planes coincide with the lens plane, and the focal length is measured from this single plane.

The Hauptebene is widely used in the design and analysis of optical instruments such as cameras, microscopes, telescopes, and other imaging systems. It allows a simplified description of how an object at a given distance is mapped to an image, independent of the exact path of rays through the individual surfaces. In practice, engineers use the principal planes together with focal length, nodal points, and principal rays to characterize and optimize system performance.

Limitations exist: the concept relies on the paraxial approximation, so it describes idealized, small-angle rays. For wide-angle or highly aberrated rays, the actual light paths deviate from those predicted by the principal planes, and the description becomes less exact. Nevertheless, the Hauptebene remains a fundamental tool in geometric optics for understanding and designing optical systems.