The legibility of the text on wires is an obvious necessity as an aid to identification. There is a common movie trope in which the hero must identify one of several different wires to prevent a catastrophe and is told to specifically cut the red wire. This has been so overdone that nowadays it has become a joke. This idea works well with many viewers is because it is believable that wires are color-coded to represent hazards. However, in reality many essential wires are marked with standardized writing, something that would make this entire cinematic event much simpler. SAE ARP 5607A-2005 - Legibility of Print on Aerospace Wires and Cables details guidelines needed to properly mark any wires that are used for aerospace purposes.
SAE ARP 5607A-2005 is a report intended to mark cable and wire insulations for aerospace purposes to meet legibility requirements. It focuses entirely on the legibility of human-readable characters and does not cover bar code and other machine-readable systems. Two generic types of variables affect legibility. These are stimulus variables and environmental variables. Stimulus variables are the factors that involve the wire markings themselves. This standard identifies guidelines for several key stimulus variables, which include: character size, contrast, font, character orientation, inter-character spacing, and character formation and fidelity.
SAE ARP 5607A-2005 does not focus on environmental variables. Environmental variables are completely dependent on the observer and the surrounding area around the observer. Important environmental variables are lighting observer-stimulus distance, orientation of observer to stimulus, clutter, visual acuity of observer, state of mind, and visual exposure duration. These should be controlled by the observer and not the manufacturer of the wire.
Character size is a prime factor affecting legibility. It is unfortunate that almost all wire markings are constrained, since the dimensions of the wire impose limitations on the maximum character print size. There are many different ways to measure character size, including height, width, and ratios between the two. In general, the height of the characters should not exceed 90% of the diameter of the wire. This standard reports that it is normal to print the text in point size 9 or 10, which equates to the character heights of 0.09 and 0.1 inches, respectively.
According to the standard, the three acceptable fonts for aerospace wire marking are Arial, Univers, and Tahoma. Their clarity makes them the most desirable. In terms of character orientation, it is ideal to use the traditional western form of writing, in which characters are organized horizontally so that they can be read from left to right. Additional guidelines are detailed in the standard, which all should be followed to properly mark aerospace wires for legibility.
Upon following these recommendations, the wire print is considered to meet legibility requirements if it can be correctly deciphered with the unaided eye under particular conditions. After wiping the wire with a soft, lint-free cloth, the wire must be viewed from a distance of 15 inches in an ambient light of 30 foot-candles (30 lumens per square foot). The seen text should be recorded and must match the actual text on the wire.