We use light scattering experiments to measure, in detail, the population dynamics of Dictyostelium cells at low cell densities < 10^3 cells/ml. The experimental setup consists of a narrow laser beam that hits the rotating vial with our cell sample, the speckles from the scattering events at the angle of about 30 degrees from the transmitted direction are focused onto the photodiode using a convex lens. As the cell passes through the laser beam, due to its different index of refraction, the laser light is refracted and appears as a brief flash of light. The duration of this pulse tells us how long the cell spent in the laser beam and the peak intensity of this pulse reveals the cell size or the size of the part of the cell that grazed the laser beam. The signal is passed through a high-pass filter (hence the signal pulses as shown on the animation are derivates of bell-shaped input pulses) and sent to the microphone port on the PC sound card. Using Pulse-Recorder-Analyzer (courtesy of Dr. Marek Dolleiser, University of Sydney, AU) we collect the width, height and the initial timing of each pulse.
Spectrum of collected pulses (including noise). The pulses representing cells have height of about 10 au and width of about 2 ms. The animation shows the time evolution of this distribution during 45 hours.