OSRAM’s app note on their force based sensing by optical reflection. Link here (PDF)
An optical force sensing system uses a light emitter and a light sensor that are located side-by-side below a flexible (and reflective) surface. The more forceful a press on that surface, the greater its deflection, the higher the value reported by the sensor. In other words, optical force sensors implement a (force aware) button. This application note reviews the design aspects of such a system, covering optical, electrical, mechanical and even driver and application software. It discusses various component selections from discrete solutions to fully integrated and suggest applications like keypads or even sliders.