With car lights, function came first. Then came aesthetics. Over the years, this superficial interest grew into a major factor driving lighting design. In 2012, the NY Times noted that lights on new vehicles appeared as “liquescent strands of rubies and diamonds.” However, to promote driver awareness and visibility, automobile illumination needs to follow a set of consistent rules to assure the safety to those operating, traveling in, or in close proximity to automotive vehicles.
A surface vehicle standard, SAE J 578-2020: Chromaticity Requirements For Ground Vehicle Lamps And Lighting Equipment, defines and offers a means for the control of colors used in the external lighting equipment of motor vehicles. This includes lamps and reflex reflectors.
How We See Colors
Defining colors might seem strange. Red is red, and blue is blue, right? However, colors are not as absolute as people may believe. In fact, color is in the eyes of the beholder. Quite literally—once the light reflected off an object enters through the cornea, bends through the pupil, hits the lens, and focuses on the retina, rod and cone cells detect and respond to send signals to the brain. After the nerve impulses are processed, you see a color.
Humans have three types of photo pigments—red, green, and blue. Monkey, ape, and hominin ancestors became trichromatic epochs ago to be able to discern ripening red fruits among diverse foliage in forest canopies. Today, human trichromatism allows us to perceive colors that reflect off almost any object we can see, including car lights. While there are some abnormal outliers—people who are colorblind lack or have malfunctioning cones, and up to 12 percent of females have four cone types in their retinas—most people are similar in their inherent capabilities to process colors.
It is fair to assume that the light traveling into people’s eyes as they stare at the same object is identical, regardless of the person. However, since we can be wired differently in a physiological sense, and certainly in terms of the contents of our brains, things can start to differentiate from there. For instance, shades and blends of colors are subject to varied perception. Just look at the now-infamous “the dress” Internet phenomenon from 2015. Furthermore, many relativist social scientists insist that color perception varies among cultures.
Color Specifications in SAE J 578-2020
With all this in mind, standardizing color makes sense. To help control the colors employed in the external lighting equipment of motor vehicles, SAE J 578-2020 covers the boundary equations and boundary vertex points for light emitted from a device in the following colors: red, yellow (amber), selective yellow, white (achromatic), green, blue, and signal blue.
In addition, SAE J 578-2020 addresses two methods for checking the color of light from the device for compliance with color specifications: the tristimulus method and the spectrophotometric method.
Users who want to make use of the spectrophotometric method, which can be used as a referee approach when the commonly used methods produce questionable results, should refer to ASTM E308-22: Standard Practice For Computing The Colors Of Objects By Using The CIE System for more details.
Please note that, while SAE J 578-2020 applies to the overall effective color of light emitted by the device in any given direction, it is not meant to be used for the color of the light from a small area of the lens. Similarly, it does not apply to pilot, indicator, or tell-tale lights.
Changes to SAE J 578-2020
SAE J 578-2020 revises the previous edition of the same standard, SAE J 578-2019: Color Specification. As you can easily note from the title, one of the most apparent changes made to this standard is the change to its name, which occurred to better reflect the document’s content.
Additionally, the standard went through the following changes:
- New section 3.1.6, “Blue-Green,” to define a “blue-green” color space for use by automated driving system (ADS) lamps (as specified in SAE J3134).
- To minimize the likelihood of blue-green sources being mistaken for signal blue sources, the signal blue color space boundary toward green was changed from y = 0.320 to y = 0.280.
- References to warning device reflex reflectors were removed.
- The sentence concerning operating temperature was deleted from 4.1, “Method of color measurement.”
- The sentence referring to a referee approach was deleted from 4.1.2, “Spectrophotometric method.”
- New section A.4, “Correlation Between CIE 1976 and CIE 1931,” in Appendix A.
- A.3, “Orange Fluorescent Information Guideline,” from SAE J 578-2019 was removed, since the color fluorescent orange is outside the scope of this document.
Furthermore, Figure 1, “CIE 1931 chromatacity diagram,” was also updated to reflect several of the above changes.
SAE J 578-2020: Chromaticity Requirements For Ground Vehicle Lamps And Lighting Equipment is available on the ANSI Webstore.