Research overview: photon recycling and sensitivity gains from the tapetum

The functional payoff of the tapetum lucidum is that it gives photons a second pass through the photoreceptor layer, raising the probability of absorption and effectively boosting sensitivity. This overview surveys estimates of how much extra light capture the tapetum provides and how that contributes to the cat's ability to see in dim conditions. The mechanism is straightforward: light not absorbed on its first pass is reflected forward, so any photon caught on the rebound is a detection that would otherwise have been lost. The survey discusses the resulting sensitivity gain alongside the cost of slight image blur from imperfect, scattered reflection. Quantifying photon recycling helps explain the cat's measured low-light advantage over humans, who lack a tapetum. It frames the reflective layer as a sensitivity amplifier whose benefit is large precisely when photons are scarce.