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glossary
Aperture : diaphragm limiting the number of light rays that are able to propagate
Angular aperture (in the image space): apparent angle of the aperture of an optical system in the image space seen from the on-axis image
Angular aperture (in the object space): apparent angle of the aperture of an optical system in the object space seen from an on-axis object
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 »
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
