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glossary

Refraction index: for a transparent medium, ratio between the light speed in the air and the light speed in the medium ; for a reflective surface in the air or vacuum, the refraction index is « - 1 »
Center thickness: for an optical component or system centered on an optical axis, distance between the vertexes of the first and the last optical surface
Vertex: top of a surface
Primary principal plane: plane of the object space of an optical system whose conjugation by the said optical system occurs with a magnification of +1 and whose conjugate plane is called the secondary principal plane
Secondary principal plane: plane conjugate by an optical system of an object plane ( the primary principal plane ) with a transverse magnification of +1
Object: element (point, line, plane, or whatever,…) of the space linked to another element called image in a way that rays coming from the object and transmitted by an optical system focus on the image ; object and image are said to be conjugate by the optical system
Image: conjugate of an object by an optical system (see object)
Transverse magnification: size ratio of an image and an object, the image being conjugated to the object by an optical system; for centered optical systems, the transverse magnification is defined with objects in a plane perpendicular to the optical axis - transverse magnification is then positive if object and image have the same orientation and negative otherwise
Aperture : diaphragm limiting the number of light rays that are able to propagate
Clear aperture: aperture diameter of an optical system
Numerical aperture ( optical system ): dimensionless number characterizing the aperture in the object space or in the image space and defined as the product of the refraction index by the sine of half the aperture angle