The angle Lambda is defined classically as the angle between the axis of the pupillary (from the center of the pupil to a point on the surface of the cornea, such that the tangent to the cornea in this point is perpendicular to the pupillary defined axis) and the line of sight (or line of sight), which connects the centre of the pupil at the attachment point. There is also a definition using the Visual axis according to some authors. In practice, this angle is close to the angle Kappa, whereby it can be likened.
Lambda terms, as any angle corresponds to the angular area defined by two directions (axis pupillary/line of sight). However, the common use of the angle Lambda (or Kappa) often means the distance (in mm) between vertex and geometric center of the pupil. This design is a simplification. Some topographers as the Orbscan, provide an estimate of the angle Lambda, but which is displayed as an estimate of the angle Kappa.