Abstract: A method and apparatus are provided to continuously monitor and measure the average size of individual foam cells contained in a foam structure, such as polystyrene foam for example, as it is being fabricated. Such an arrangement permits the prompt adjustment of process conditions and/or the concentration of nucleating agents in the extrusion system in the event the foam cell size deviates from a preselected norm. The method comprises illuminating the surface of a foam sheet, receiving and collecting at least a portion of the light which is diffused and reflected from the foam sheet with a detector photocell, electrically measuring the amount of light energy received by the photocell and converting such measurements into a cell size measurement.