A combine harvester includes a grain pan that is arranged to catch a crop stream. The grain pan is driven in a fore and aft oscillating manner to convey the crop stream rearwardly across a conveyance surface to a rear edge. The grain pan is provided with an upright panel extending in a fore and aft direction on the conveyance surface. A grain cleaning system is arranged to receive the crop stream from the grain pan. A crop stream analysis system is provided for analysing a vertical section of a crop material layer disposed on the grain pan. The analysis system includes a vertical array of photoelectric sensing devices mounted to the panel. Each photoelectric sensing device is configured to sense a reflectance of crop material disposed against the panel. A processor is configured to receive reflectance signals from the photoelectric sensing devices and determine a material stratification status from the reflectance signals.