Simulation of phototactic swimming using a rotating filter system. (a) Illustration of the counterclockwise rotation as viewed from behind the spheroid of a swimming spheroid. The black arrows indicate the swimming direction. The fluid streamlines were analyzed from regions 1-3. (b) Experimental setup of the simulation. Filter 1 (F1): 74% transmissivity (T), 2 × 14° = 28° sector cutout; Filter 2 (F2): 74% (T), 2 × 25° = 50° sector cutout; Filter 3 (F3): 41% T, 2 × 55° = 110° sector cutout. The sandwich construction results in T1 = 100% (gap, no filter), with T2 = 74% (F1), T3 = 55% (F1 + F2), and T4 = 23% (F1 + F2 + F3). (c) Schematic representation of the changing light conditions in the eye during rotation. (d) Directivity of the eye viewed from the flagellar end of the cell. The blue line is a polar plot of tangential electric energy density at the plasma membrane overlaying the eyespot . α is the half-beam width, which is the angle between the half-maximum directivities of the eye. The red line reflects the result from (e). (e) The blue line represents the linear plot of tangential electric energy density. Owing to the reflection symmetry, only 0° to 180° is shown. Filter combinations and the cutting of filters were calculated to match the blue line, with the most appropriate combination shown as a red line. (f)-(h) Relative length of the tracks of polystyrene beads analyzed in regions 1 to 3 using the setup shown in (b). Vertical, dotted gray lines indicate the approximate minimal length of the tracks ~300 ms after the maximum light intensity. (i) The light intensity was recorded by an oscilloscope. The red vertical lines in (f)-(i) indicate the maximum light intensity.