Integrating these vectors over any collection of lights, or over the entire sphere of directions, produces a single scalar value—the "total irradiance" at that point, and a resultant direction. The figure at right, reproduced from Gershun's paper, shows this calculation for the case of two light sources. In computer graphics, this vector-valued function of 3D space is called the "vector irradiance field" (Arvo, 1994). The vector direction at each point in the field can be interpreted as the orientation one would face a flat surface placed at that point to most brightly illuminate it.