Abstract: A smoke detector tester is disclosed herein having a hand held aerosol container for holding under pressure a quantity of spray emitted from the container by a finger valve in the form of an aerosol cloud of known particle distribution within the sensing area of a smoke detector undergoing test. The particle size distribution within the aerosol cloud simulates the aerosol particle sizes that are emitted during the early stages of the process of combustion. The tester thus provides a true functional test of the smoke detector's sensing ability.