In Rushton's paper on the Principle of Univariance, he states:
Thus, though the rod input has two variables, wave-length and energy, the output differs only in one respect, namely 'brightness'.
However, as far as I understand, a photon can only vary according to one dimension, namely wavelength (vibrational energy), because the wavelength and energy of a photon are interdependent.
This suggests then that the input to a cone cell, for brightness to be perceived (i.e. the intensity of light), must be a beam of photons, so that the input can vary in wavelength and amplitude (does this equate to the number of quanta being absorbed by the photopigment per unit of time?).
My question, then, is how can a discrete event (the changing of shape of a photopigment) encode a continuous value (i.e. intensity)?