The increasing application of computational modeling for two-phase flow analysis increases the demand for more accurate measurement techniques. An accurate in-line monitoring system is developed for estimating the size of rising bubbles in stagnant water. Telecentric Direct Image Method (TDIM) is applied as an inexpensive and easier implementing alternative to the complex and costly Interferometric Particle Imaging (IPI). A system from Dantec Dynamics is utilised for IPI measurements. It shows an unexpected bimodal bubble distribution for two bubble sizes with mean diameters of 189.31 um and 669.34 um, and a total mean diameter of 323.59 um with a standard deviation of 230.5 um. TDIM utilises the principle of shadow imaging, the advantage of telecentric optics and digital image processing. The mean diameter becomes 436.13 um with a standard deviation of 78.36 um. In this connection TDIM turns out more reliable than IPI, for which further measurements are necessary due to incorrect settings in the optical setup.