Fig. 4

Lens design constraints for the central element after preceding surface S. The orthogonal optical axes are denoted by the dot-dashed lines, and they also represent the dummy surfaces D (vertical axis) and D’ (horizontal axis). The marginal ray leaves the leading surface S at point q, at the marginal angle β. The closest the orthogonal surface can get to the intersection of the two orthogonal axes is limited by line A which is the tangent of surface S′, defined by the marginal angle β at point p (grazing incidence for the marginal ray). Line B is perpendicular to line A and is defined by points o and p