The light vector determines the magnitude and direction of transfer of that part of the energy of electromagnetic radiation that can be visually perceived. In other words, it determines the magnitude and direction of the luminous flux. The absolute value of the light vector is the ratio of the luminous energy transferred across an area ΔS in a unit of time to ΔS under the condition that the transfer (the direction of the light vector) is perpendicular to ΔS. The concept of the light vector is used mainly in theoretical photometry for the quantitative description of light fields and is the photometric analogue of the Poynting vector. The divergence of the light vector determines the volume density of absorption or emission of light at a given point of a light field.
Sometimes, especially in older scientific literature, the electric field strength vector E of an electromagnetic wave is called the light vector.
L. N. KAPORSKII