## 1 Introduction  
To tailor the properties of nanoparticles and composite materials comprising such nanoparticles, control of the particle size is of vital importance. Nanoparticles are used in products for a wide variety of applications ranging from performance polymers to pharmaceutics, food ingredients, cosmetics and coatings and paints [1; 2; 3; 4; 5; 6]. Very often, such well-defined nanoparticles are produced in bottom-up liquid phase syntheses using e.g., redox or sol-gel type reactions. A detailed study of the nucleation and growth of such particles would provide valuable information on their mechanism of formation, and could furthermore be used for process optimization and scale up. For such a study, real-time measurements of the particle size are required. For production scale synthesis, such real-time measurements could also provide data that enable rational intervention in a production process, hence minimizing the number of off-spec batches and waste. Dynamic Light Scattering (DLS) is the standard technique for determining the size of nanoparticles in the range of a few nanometers to a few microns dispersed in liquids [7; 8]. It makes use of a laser to study the time dependent scattering of small particles in liquid dispersions. Since such particles undergo Brownian motion, the distance between the scattering particles in the liquid constantly changes, which results in a fluctuating scattering intensity [9]. Analysis of these fluctuations yields information about the average size and size distribution of the nanoparticles. Since in conventional DLS experiments no external flow is imposed on the dispersion, the fluctuating scattering intensity solely correlates to the diffusion of particles, and does not comprise translational information. It can thus be used to determine the diffusion coefficients of the particles, and from that the particle size distribution through use of the Stokes-Einstein equation. Consequently, such conventional DLS techniques do not allow for real-time measurements on flowing or moving dispersions. DLS measurements rely on single scattering, which means that each photon should only be scattered once in the dispersion sample before it reaches the detector. Since this requires low particle concentrations, dispersion samples typically require dilution with additional solvent which is inconvenient and may even influence the state of the dispersed colloids. To overcome this, a significant amount of research was performed in the past decade focusing on suppressing multiple scattering. It showed that multiple scattering suppression in DLS measurements can e.g., be accomplished by using two lasers operating at different wavelengths, and two detectors having distinct bandpass filters to capture scattering information from each single laser. This technique is generally referred to as two-color DLS [10]. Another more simple and robust arrangement of the beam-detector pairs is possible by displacing them symmetrically in a third dimension above and below the scattering plane, which is known as the three-dimensional (3D) cross-correlation geometry [11]. Using this geometry, two scattering experiments are performed at the same time and on the same scattering volume using two beams and two detectors. The signals seen by the detectors are cross-correlated, and only photons that are scattered once will produce correlated intensity fluctuations on both detectors. In that way, the information on single scattered photons can be extracted, and used to determine that particle size distribution in turbid dispersions. However, an important drawback of this 3D technique is that each photon detector not only receives the desired information from its correspondingly positioned laser, but also undesired information from the second laser. Thus, cross-talk occurs between the two light scattering experiments. This drawback is overcome through temporal isolation of the two beam-detector pairs in modulated 3D cross-correlation DLS.