wheel 308 the direction and amplitude of the pre loading force may be adjusted by correspondingly designing the properties of the pre loading spring 310 which may be a helical spring for example the aperture wheel 308 is provided with the aperture holes 304 which are formed as through holes in the aperture wheel 308 furthermore a part of a circumferential edge of the toothed aperture wheel 308 has a number of engagement sections 314 e g formed as indentations between adjacent teeth the engagement sections 314 and the position adjustment sphere 302 are matched so that the position adjustment sphere 302 can be engaged in a mechanically stable way in each of the engagement sections 314 fig7 shows a scenario in which the position adjustment sphere 302 is engaged in one of the engagement sections 314 it will be appreciated by a skilled person that the selection of a specific engagement section 314 in which the position adjustment sphere 302 is presently engaged corresponds unambiguously with one of the aperture holes 304 being selected or actuated a drive unit such as stepper motor 312 is provided for driving the aperture wheel 308 via a gear mechanism which will be described below in more detail in other words the stepper motor 312 provides the mechanical energy by which the aperture wheel 308 is rotated thus the rotation energy is provided by the stepper motor 312 the above mentioned gear mechanism comprises as can be best seen in fig3 and fig4 a toothed first gear wheel.