Optical axis

The optical axis is a term used in geometrical optics. The straight line which coincides with the symmetry axis of a reflecting or refracting optical element is referred to as an optical axis. Wherein a lens system, the optical axis is the line formed by the optical axis of the individual elements. In a simple observation one can consider the straight line connecting the two circles of curvature as the optical axis. In special cases, for example, an offset parabolic reflector, the optical axis of the system does not necessarily coincide with the axis of symmetry.