A chopper controls the radiation's path, alternating it between the sample, the blank, and a shutter. If it hits the transparent section, it will go straight through the chopper and then pass through the cell containing the sample. It is then bounced by a mirror onto a second rotating disk. This disk is rotating such that when the light arrives from the first disk, it meets the mirrored section of. The simplest instrument for molecular UV/Vis absorption is a filter photometer (Figure 13 4 1), which uses an absorption or interference filter to isolate a band of radiation. A chopper is a circular-shaped device split into quarters (refer to the figure below).