• Reactions to light increase the permeability of the stromal membrane to cofactors such as Mg++ that are necessary for the Calvin cycle. An important aspect of the issue lies in the hypothetical "radiation temperature" assigned to the light beam. This concept arises from Planck's vision of assigning an entropy, and therefore a temperature, to radiation. However, Planck was very clear that there is only one thermodynamic radiation temperature: that of the black body at equilibrium (Planck 1912). In fact, he states that since the rays of radiation, used to define a temperature, which pass through a point can be arbitrary, there are an infinite number of such "temperatures". Almost all previous discussions have used these arbitrary “temperatures” in thermodynamic equations requiring equilibrium