Fig. 6. Absorbed number of photons per second by a photoreceptor, with pure rhodopsin (R) and with one (R+S) or two (R+2S) sensitizing pigment molecules per rhodopsin, from a sky with radiance given by Fig. 1, for a fully dark-adapted photoreceptor (h=; see Fig. 4A, inset), as a function of distal rhabdomere diameter. Photon absorption increases with rhabdomere diameter. One sensitizing pigment molecule per rhodopsin increases the absorption by 14-18% with respect to pure rhodopsin, and two sensitizing pigment molecules per rhodopsin increase the absorption by 20-27%.