The notion of a "dark illuminant" is not so strange when one understands that every point on the surface of the sphere is a potential illuminant, the direction of illumination being a function of the position on the spherical surface, and the magnitude of the illumination being proportional to the analog value of an illumination variable at that point. A "dark illuminant" therefore simply means a potential illuminant that is currently "off", i.e. representing low if any illumination from that direction. Consider the representation in the retina, where there exist "off-cells" that represent a non-existent entity, the absence of a signal that was formerly present. A real neural signal represents a non-existent entity. In the same way, a "dark illuminant" is a signal that represents a low level of illumination. Therefore a percept of "being lit by a dark illuminant" is equivalent to a percept of weaker ambient illumination. It is described as a "dark illuminant" because a dark surface suggests a low level of illumination from the normal direction, and therefore unlike in physics, where darkness is only the absence of light, in this system a dark surface propagates a signal representing darkness along the surface normal which then alters the value of the illuminant node.