ORTHOPTIC SURFACE

 From the Greek orthos "right" and optikos "relating to sight".

The orthoptic surface of a surface is the locus of the points through which pass 3 planes tangent to the surface and perpendicular 2 by 2.

Examples:
- the orthoptic surface of a sphere with radius R is a concentric sphere with radius .

- more generally, the orthoptic surface of a centered quadric is a sphere called orthoptic sphere or Monge sphere.

- The orthoptic surface of the elliptic paraboloid is a plane called directrix plane????

A similar notion, bearing the same name, is that of orthoptic surface of a subset of the space: locus of the vertices of the right trihedra that circumscribe X (i.e. containing X, and the three faces of which meet X).

Example: the orthoptic of a circle with radius R is a sphere with the same center and radius .

But for some authors, the orthoptic sphere of a conic is the locus of the vertices of the cones of revolution with right angle at the vertex in which the conic is included.