Charged colloidal particles are important for practical applications in diverse fields. Electrophoretic ink, a promising technology to implement flexible reflecting displays (‘electronic paper’), is based on the movement of charged colloidal pigment particles in an electric field [Comiskey et al, Nature 394 (1998) 253]. For electrophoretic deposition, a method to apply uniform coatings on metal surfaces, the charge of small particles in a liquid is used to manipulate them [Besra et al, Progress in Materials Science 52 (2007) 1]. In biotechnology, enzymatic reactions are investigated in real time by using charged colloidal silica beads [Galdener et al, Biophysical Journal 80 (2001) 2298], and charged particles in colloidal crystals are used as a model system for atomic systems [Leunissen et al, Nature 437 (2005) 8]. These are only a few examples in which it is important to be able to measure accurately different properties of charged colloidal particles, such as their charge and size. A number of methods exist to measure properties of colloidal solutions. These methods usually yield average values for a large number of particles. For many applications however, it is essential to measure, preferably in real time and dynamically, the properties of individual colloidal particles. This is useful primarily to investigate the physical mechanisms resulting in a charge on the particles. It also allows using individual colloidal particles as a probe to measure (time and place dependent) electric fields and concentrations of various molecules in a liquid.