During the day, the signal lights need to be brighter than they are at night so that drivers can see them clearly from a certain distance. At night, such a brightness would be too bright for drivers (like meeting a driver on a road who has his high-beams on). As such, the brightness of the outside signals must be controlled using the Day/Night buttons.
During the day, the Day (Tag) light should be lit. At night, the Night (Nacht) light should be lit. Toggling between day and night is done by pressing the correct button (Tag or Nacht) together with the Station Button.
If there is a discrepency between what the outside photosensor is detecting and the brightness selected on the signalling panel, the "correct" light will blink, accompanied by a ringing bell. In cases where the photosensor is tricked by weather conditions (fog, dust, eclipse, etc) the middle Reset (WUT) button can be pressed to suppress the ringing. The light will continue to flash until there is no longer a discrepancy.
These buttons affect the brightness of the real signals in the yard; they do not affect the brightness of signals as shown to the dispatcher. For that, use the Panel Brightness buttons.