Non-opsin-based biological vision: the physics of infrared light sensing in the pit viper

A unique set of sensory organs in certain snakes allow for sub 100 ms reaction with 5 degree accuracy in low light environments without the use of opsin based conformation-change photon detection that is nearly ubiquitous in natural systems. The capabilities of these organs center around a thin membrane sparsely populated with thermally sensitive 'pixels' capable of detecting mK temperature differences. Each membrane is situated in a shallow cavity or 'pit', the entrance of which is smaller than the area of the heat detecting membrane thus creating a rough pinhole camera. While much has been learned about the biochemistry of these sensory organs, questions of the mechanism of photon absorption, the reasons behind their geometry, how accurate imaging is obtained from a noisy environment, and how visual information is reconstructed all remain unanswered. To address these, we employ an experimental realization of a pit membrane using an InGaAs detector translated within a model pit. While exploring different optical geometries, we reconstruct thermal images using a geometrically inspired single step algorithm with noise parameters related to input and detector noise. Understanding infrared sensing in pit vipers promises new insight into the physics and biology of light sensing in natural systems, beyond the more common opsin based mechanisms.