In brass instruments the player's lips are used as a control valve that allows an oscillating flow of air into the instrument. The behaviour of this valve, the lip-reed, is determined by both the pressure difference across the lips and the open area between the lips. Computer simulations of brass instrument playing frequently represent the lips as an oscillating system with one degree of freedom, representing the height of the lip opening. This study aims to provide experimental evidence to determine values of an exponential coefficient q defining the area-height function for the lips of trombone players. A high speed digital video camera and transparent mouthpiece have been used to produce detailed images of the motion of the vibrating lips of a range of musicians and an artificial mouth over a large set of playing frequencies and sound levels. These images are analysed to investigate how the open area varies as a function of the height between the lips.