The IR sensitive membrane of the Crotaline pit organ was modeled numerically to help interpret electrophysiologic measurements of the pit organ response to a calibrated infrared source simulating a biological target. The model results are compared to electrophysiologic measurements for an on-axis exposure (target normal to the pit organ axis, oriented for maximum response).
Additional model studies were conducted to: 1) estimate the field of view of the pit organ and 2) estimate the expected temperature rise in the membrane from the target at varying distances. The pit organ model was based on detailed measurements of its geometry. The membrane illumination irradiance difference from background thermal radiation (in W/mm2) was calculated from a quasi-analytical solution for the radiation coupling factor, Fjj. The illumination function was used to estimate temperature rise neglecting infrared heat transfer between the membrane and surrounding pit organ tissues. That is, the membrane was assumed in thermal steady state with the snake body and the environment outside of the target. The mammalian target is thus assumed to represent a small perturbation to the thermal steady state condition. This matches the electrophysiologic data, and is reasonable since the snake is cold blooded and snake body temperature is very close to its surroundings. The membrane includes blood flow effects, but it turns out that the membrane blood flow is strictly capillary in nature and changes the effective lateral thermal conductivity rather than providing significant heat transfer. The membrane is “optically thin”, being only about 5 wavelengths in thickness, and the specific optical properties of the interior layers were estimated from relative water content.