Figure 2.

Schematic of photon absorption process by a population of microvilli in a fly photoreceptor. Microvilli (blue bristles) are stacked densely, layer by layer, at the side of the cell, forming the light-guide (rhabdomere). Light input at a time bin, Δt, is the number of photons to be absorbed. These photons (green dots) enter the rhabdomere from above, propagate through its length, and are randomly distributed over 30,000 microvilli. Photons either hit a microvillus, being absorbed, or miss it, continuing to travel down to the next layers for possible absorption. (A–E) illustrates how photon absorption dynamics can differ at a particular time. (A) A microvillus that does not capture a photon; (B) A microvillus that captures one photon; (C) Two photons fall onto the same microvillus at the same time; (D) A photon can pass one microvillus layer, but (E) is captured at another layer. Light absorption statistics, which describe the probabilities of (A–E), follow Poisson distribution, if the assumptions in Section Light Absorption by Poisson Process are made.