Hauptstrahl

Hauptstrahl, literally meaning "principal ray" in German, is a term used in optics to denote the chief or principal ray of an imaging system. In English-language optics, the corresponding term is often chief ray, while Hauptstrahl appears in German texts and in some technical discussions. The Hauptstrahl is defined as the ray from a given object point that passes through the centre of the aperture stop and is traced through the optical system in ray-tracing analyses. This ray is central to determining how the image point is formed, particularly the image height for off-axis object points and the system’s field of view.

In imaging systems, several rays are considered, including the chief (Hauptstrahl) ray, the marginal or edge ray, and the focal or principal ray. The chief ray is frequently used to represent how a specific field point maps to the image plane, indicating where on the sensor the image will appear, especially at the edge of the field. The other rays are used to assess aberrations and distortion, providing complementary information about performance across the field.

In design practice, identifying the Hauptstrahl helps engineers visualize and optimize field coverage, verify the placement of apertures and stops, and evaluate field curvature and distortion. The concept is essential in ray-tracing analyses and paraxial approximations, where linear relations between object and image heights are assumed for small angles.

Related concepts include the chief ray, marginal ray, paraxial ray, and aperture stop.