In the early 1970s, David Lee(1974) provided evidence that time to contact is specified by an optical variable, which he termed tau (τ). He also showed that tau can be quantified mathematically by relating object size, the distance of the object from the person, and the angle subtended at the person by the object size and distance. In mathematical terms, tau is the inverse of the relative rate of change of the visual angle subtended by the moving object, given the speed of approach of the object is constant.