The vergence (typical symbol: L) of a light bundle in optics is the reciprocal of the distance between the point of focus and a reference plane. It is measured in dioptres (1 D = 1 m−1). Vergence can also be understood as a measure of the curvature of the optical wavefront. However, it is only fully valid in geometrical optics, not in Gaussian beam optics or in wave optics where the wavefront at the focus is wavelength dependent and the curvature is not proportional to the distance from the focus.
Convergence describes the situation when the light rays are approaching each other and its values are positive. Divergence is measured as negative dioptres and means that the beams are originating from a point.