This guidance document constitutes part 1 of ISO 5659. The main measurement made when carrying out testing to ISO 5659-2 is the amount of light transmitted by the smoke as a fraction (or percentage) of the initial light transmitted by the optical system. The minimum percent light transmitted is then used to calculate the maximum specific optical density for each mode of specimen exposure, e.g irradiance of 25 kW/m² with pilot flame or irradiance of 50 k/Wm² without pilot flame. Part 2 describes a static (or cumulative) single-chamber test procedure.