Abstract: An opto-mechanical system performing fast and accurate non-contact foot measurements is proposed. By processing the measured data it is possible to get three-dimensional foot models and to design custom-made shoes. This work is subdivided into two parts: in Part I, after introducing the system operating method and describing the vision technology employed, a mathematical model of the system is developed accounting for the main internal parameters of the camera. In order to reduce scanning time and the time necessary to create the 3D foot model, the measured data have been arranged in a particular structure and a suitable acquisition procedure has been developed. A laboratory prototype has been built to test the performances of the system: a detailed analysis of its geometry and of its components is provided as well as a concise description of the simple user interface developed to run and control the system.