The invention provides a method of testing an inhaler based on performing an optical analysis of a dry powder medicament plume discharged from the inhaler upon actuation. More particularly, embodiments of the invention comprise illuminating the dry powder plume with a source of electromagnetic radiation and capturing one or more images of a pattern of radiation reflected or diffracted by the illuminated plume. The images are subsequently processed to determine and/or analyse one or more geometric and/or dynamic characteristics of the plume.