FACS is a research tool useful for measuring any facial expression a human being can make. It is an anatomically based system for comprehensively describing all observable facial movement.

Each observable component of facial movement is called an Action Unit or AU. All facial expressions can be broken down into their constituent AUs. Each emotion is connected to a specific combination of expressions.

I am a certified FACS decoder and able to use this method both in market research and in information. Check my works here.