The lung surfactant is a film coating the alveolus of the lung. We want to characterize the structure of this film at the air-water interface at a molecular level. This will give us crucial information on the structural importance of the lung surfactant in a healthy lung. To do this characterisation we need to use neutron reflectometry for an unambiguous determination of the thickness of the film and how this changes with cholesterol and Ca2+ ions. We want to mimic the conditions in the lung by having a controlled humidity close to 100% at 37 °C. We want to follow the film formation over time and investigate the structural changes with repeating compression ¿ expansion cycles imitating the work of breathing. We want to do this study on both a clinical extract used to treat infants with respiratory distress syndrome and an extract of the endogenous lung surfactant of the new-born baby.