Transient defluidization occurs when fluidizing gas is switched from a lower density gas to a higher density gas. The intensity of this phenomenon increases with decreasing particle size and with increasing difference between gas densities. In order to investigate the mechanism of this effect, images of the transient phenomenon were captured using a video camera and visualized using digital image conversion. Gray-scale pictures were obtained based on the transmittance of back light through the emulsion phase in a thin two-dimensional fluidized bed. The gray-scale pictures were transferred into pictures with a color gradation. The generation, expansion, and elimination of defluidization after switching of the fluidizing gas can clearly be seen by observing the reconstructed color movies. (C) 2013 Elsevier B.V. All rights reserved.