spherical sector

Spherical sector, portion of a sphere enclosed by a cone of radii from the center of the sphere.

In geometry, a spherical sector is a portion of a sphere defined by a conical boundary with apex at the center of the sphere.

At least some Diamondtron CRT faceplates are spherical sectors; reflections of such objects as ceiling light fixtures are reduced in size and do not change shape regardless of where they appear.