Equivalent Sphere Illumination. The level of sphere illumination which would produce task visibility equivalent to that produced by a specific lighting environment. Suppose a task at a given location and direction of view within a specific lightings system has l00 fc of illumination. Suppose this same task is now viewed under sphere lighting and the sphere lighting level is adjusted so that the task visibility is the same under the sphere lighting as it was under the lighting system. Suppose the lighting level at the task from the sphere lighting is 50 fc for equal visibility. Then the Equivalent Sphere Illumination of the task under the lighting system would be 50 ESI fc.