Central beam

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A central ray , in geometrical optics also the main ray, is in an imaging system the ray of a bundle of rays that represents the bundle of rays when the aperture of the system is reduced to almost zero.


In radiology, a central ray is understood to mean those x-rays that run straight from the radiation source (focus) through the center of the radiation exit window. The region of the body that the central ray hits is shown with the least distortion in the X-ray image. X-rays propagate in a straight line from a point source of radiation and, through divergence, form a cone of rays after exiting the tube window, within which a distinction is made between vertical, axial and central rays . By definition, these three rays emanate from the center of the focus, with the axial ray passing through the center of the aperture , the central ray passing through the center of the ray exit window and the vertical ray striking the image plane at a right angle. With symmetrical insertion and additional right-angled projection, as is usual with cephalometric radiographs, these three rays are identical.

Geometric optics

In the case of a single thin lens , the lens also forms the aperture stop . In this case, the main ray passes through the geometric center of the lens and maintains its direction unchanged when passing through the lens and in the image space.

In the case of more complex systems, the main ray generally runs from the object point to the geometric center of the entrance pupil , the image of the aperture diaphragm on the image side, enters the optical system there, passes through the geometric center of the aperture diaphragm, and exits again in the geometric center of the exit pupil the system and passes through the image point of the object point. The course of the main ray in front of, behind and between these fixed points depends on the construction of the optical system.

With the help of the main ray and the optical axis , the meridional plane and the sagittal plane can be constructed. These are two auxiliary levels that are helpful in calculating and assessing the optical properties of the imaging system.


  • Eugene Hecht: Optics . 4th ed. Addison-Wesley, an Francisco 2002, ISBN 978-0-321-18878-6 (EA Reading, Mass. 1982).

Individual evidence

  1. a b Theodor Laubenberger, Jörg Laubenberger: Technology of medical radiology: diagnostics, radiation therapy, radiation protection; for doctors, medical students and MTRA; [with 71 tables] . Deutscher Ärzteverlag, 1999, ISBN 978-3-7691-1132-3 , p. 110 ff.
  2. Z. Ševkušić: Differentiation of the different imaging geometries of analog and digital cephalometric images. Dissertation, 2008 ( PDF )