La Nature se dévoilant devant la Science

Consider the space S of incoherent beams. They may be
scaled (e.g, via neutral density filters) and added (e.g., via
superposition of irradiance on a white, Lambertian screen).
There is a null element, the “empty beam”, which is
simply no radiation at all.
S would be a linear space were it not for the fact that
the radiance has to be non–negative.
Thus real (actual) beams fill the “positive part” S + of
the linear (infinitely dimensional) “space of beams” S.
“Colors” (to be explained!) are elements of a 3 D lin-
ear space C. (The “trichromacy” was first suggested by
Thomas Young.) 7