Paraxial Rays

paraxial rays [par′ak·sē·əl ′rāz]

(optics)

Rays which are close enough to the opical axis of a system, and thus whose directions are sufficiently close to being parallel to it, so that sines of angles between the rays and the optical axis may be replaced by the angles themselves in calculations.

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Paraxial Rays 

rays lying close to the axis of a centered optical system and forming very small angles with the axis and with the normals to the refracting and reflecting surfaces of the system.

The main relations describing the formation of optical images in centered, or axisymmetric, systems are strictly speaking valid only for paraxial rays in the limiting case of infinitesimal angles. In particular, only paraxial rays represent a point as a point, a straight line as a straight line, and a plane as a plane. Only in images produced by such rays are there no optical aberrations (apart from chromatic aberration in lens systems). In practice, however, paraxial rays are usually taken to be rays that form finite angles of up to a few degrees where the deviations from the strict relations are so small that they may be disregarded; the criteria for smallness are different for each specific case. The region about the optical axis of a system where the rays may be considered paraxial is called the paraxial, or Gauss, region.