For illumination of the samples, an argon ion laser, operated at 488 nm (Innova 90, Coherent, Palo Alto, CA), is used together with a single mode fiber-optic system (kineFlex-, Point Source, Southampton, UK). For measurements of acridine orange, this laser is replaced by a 470 nm laser diode directly coupled to the microscope (LDH 470 with driver PDL 800-B, PicoQuant, Berlin, Germany). The collimated laser beam, at the exit of this fiber, is expanded by a Galilei telescope (with lenses of and ), and a resulting beam of 8 mm diameter is focused by a cylindrical lens (focal length: ) and deflected onto the spheroid by a 90 deg mirror, as illustrated in Fig. 1. With a numerical aperture, , the waist of the illumination beam can be limited to about 10 μm along the spheroid, as demonstrated in Fig. 2 for a test solution (2.5 μM rhodamine 6G). By an adjustable screw, the setup for beam deflection and focusing, deflection mirror and cylindrical lens, is coupled to the objective turret of the microscope, whereas, all other optical and mechanical components are fixed on the base plate of the microscope stage with a customized sample holder. Therefore, the light sheet and the objective lens can be moved, simultaneously, into vertical direction and all planes of the spheroid are imaged without re-adjustment of the microscope. A laser power of 0.042 mW, focused to a line sheet of 10 μm waist and 8 mm width, results in an irradiance of for both lasers, which corresponds to about half of the solar constant. An exposure time between 100 ms and 1 s is used for all images.