Dependence of the photophoretic velocity for different illumination intensities. a) Intensity dependence of the median of the photophoretic velocity distribution for the indicated size range for black-coloured silica particles (all sizes in μm). The solid black line indicates the slope as expected by theory (arbitrary offset). b) Intensity dependence of the width of the photophoretic velocity distribution for the indicated size range for black-coloured silica particles (all sizes in μm). The solid black line indicates the slope as expected by theory (arbitrary offset). c) Relation between the median velocity and the width of the velocity distribution for the black-colored silica particles. The typical error for the single data points is indicated in figure a).