Optical Lens Terminology

All optical lenses obey Snell's law of refraction. Therefore, it is the geometric shape (i.e., surface profile) of the lens that determines how light behaves as it propagates through the optical element.

Abbr./Symbol	Full Term	Definition
D, Dia.	Diameter	The physical size of the lens.
R, R1, R2	Radius of Curvature	The directed distance from the vertex of a curved surface to its center of curvature.
EFL	Effective Focal Length	The optical measurement of the distance from the principal plane of a lens to its image plane.
BFL	Back Focal Length	The mechanical measurement of the distance from the last surface of the lens to the image plane.
P, P'	Principal Plane	A hypothetical plane where incident rays can be considered to bend due to refraction; EFL is measured from this plane.
CT, CT1, CT2	Center Thickness	The distance from the principal plane position to the end of the optical element.
ET	Edge Thickness	A value calculated based on the lens radius, diameter, and center thickness.
db	Input Beam Diameter	The diameter of collimated light entering an axicon.
dr	Output Beam Diameter	The diameter of the ring-shaped light exiting an axicon.
L	Length	The physical distance from end to end of a cylindrical element (e.g., cylindrical lens) or from the apex of an axicon to the workpiece.