Definition of focal surface in US English:

focal surface

noun

  • 1Geometry a surface formed by the centres of curvature of a surface.

  • 2Optics an imaginary curved (usually spherical) surface on which initially parallel rays come to a focus after passing through a given lens or other optical system, and on which (in the case of visible light) the images of distant objects would be formed.

Origin

Early 19th century; earliest use found in Transactions of the Royal Irish Academy.