Optical Measurement of Contact Forces Using Frustrated Total Internal Reflection

Frustrated total internal reflection (FTIR) imaging is an interesting, important optical tool that can be used to measure the pressure exerted beneath soft contacting objects, such as biological materials, polymers, human or animal feet, and the outsoles of sneakers. Wider application of the technique has been prevented by a lack of understanding of the combined optical and mechanical processes that occur when an object contacts a waveguide. To fill this gap, the authors observe light scattering when an elastomer touches a planar waveguide, and also derive a combined theory of light scattering and contact mechanics that agrees quantitatively with the data.