This study is devoted to the measurement of colloid concentration in drops, as they occur in numerous industrial applications, such as paint sprays, ink-jet printing or the production of pharmaceutical products. A Monte Carlo ray-tracing approach has been used to study the light scattering from such particles, in which the polarization and intensity of all rays impinging onto a defined detector aperture are collected, allowing the signal generation arising from the drop passing through a plane wave or focused Gaussian beam to be simulated. When using a highly focused incident beam, the scattering corresponds to the optical arrangement of the time-shift technique, allowing the measurement of drops with suspended particles to be explored. For validation of the ray-tracing code, comparisons between the simulated time-shift signal and the measured time-shift signal were made, yielding excellent agreement. A model has been formulated to describe these signals; hence, to allow colloid concentration to be estimated directly from the received time-shift signals.