The velocities of individual actively swimming cells of the green alga Chlamydomonas nivalis can be measured with the use of spatial displacements of their successive images (by a 'tracking' method). This paper proposes a new tracking method based on the use of laser light; it is common for the tracking of passive particles, but the case of actively swimming cells presents specific problems. The first problem is the potential influence of the laser light on the behaviour of the cells. A key result of the study is that for carefully chosen parameters the effect of the laser light on the algal swimming is negligible (below the measurement error). Hence, the self-swimming velocities can be measured; sample results are given. The proposed method permits a large measurement volume, and thus allows the velocities of several hundreds of cells to be measured simultaneously. From these data an approximation for the probability distribution function of the velocities of the cells can be derived; this function is crucial for the construction of mathematical models of algal motility. The laser-based tracking method is applicable and useful for measurements of algal motion in still fluid. The method thus presents new opportunities in this area of research.
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