Isotropic light source: light source emitting with a constant intensity whichever the direction
Aperture : diaphragm limiting the number of light rays that are able to propagate
Optical system: in geometrical optics, assembly of optical components for redirecting the light in a controlled manner
Effective focal length: algebraic distance from the primary/secondary principal plane of an optical system to its front/back focal plane
Transmission ratio: ratio between transmitted radiation power (or intensity or amplitude) and incident radiation power (or intensity or amplitude)
Irradiance ( radiometry ): optical power received by unit area
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
