By means of photography with high-frequency photoflash and a computer-aided image analyzing system, liquid dispersing features, such as droplet-diameter distribution in the inner chamber of a single vertical sieve tray cap are probed in a square cap of 100 mmx100 mm section, which is set over a Phi70 mm orifice in the tray of a 500 mmx500 mm square column. Photos of the internal cap are taken, illuminated by photoflash from a slit in the upper cover of the cap. Data provided by image analyses show that the distribution of liquid droplets in the cap during operation possesses upper limit log-normal distribution. The coefficients in the upper limit log-normal distribution function are satisfactorily modeled through energy analysis and correlation. Especially, maximum diameter (upper limit of free-floating liquid droplet diameter) of stable liquid droplets, valued 6.426 mm, is derived from theoretical method.