In railway signaling, an aspect is the visual appearance of a signal as seen by the train crew. It is the method used by the signal system to communicate information to train operators through visual displays.
Signal aspects can be created using several visual elements, including the color of lights, the number of lights illuminated, the arrangement of those lights, whether the lights are steady or flashing, and the position of mechanical signal arms.
Common signal colors include red, yellow, green, lunar white, blue, and other colors depending on the railroad and signal system. Different combinations of these colors create different aspects.
The arrangement of lights is also important. Lights may be displayed vertically, horizontally, diagonally, or in other patterns that help communicate specific operating conditions to train crews.
Some aspects use flashing lights rather than steady lights. Flashing aspects often provide additional information such as speed restrictions or advance warning of conditions ahead.
Semaphore signals create aspects by changing the position of a movable arm. The angle of the arm determines the aspect being displayed and therefore the information being communicated.
Position light signals use patterns of lights arranged in specific shapes. Searchlight signals use a single lens that changes color to display different aspects.
Every signal system is designed to provide aspects that are clear, recognizable, and easy to distinguish under various operating conditions, including daylight, darkness, rain, fog, and other environmental factors.
The aspect itself is only the visual display. The meaning associated with that display is called the indication, which is defined in the railroad's operating rules.