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  • Writer's pictureRick Utting

Blue Light Content: Understanding the Rabbit Hole

Updated: May 1

Known for natural beauty and world-class stargazing, Hawaii has had a long relationship with warmer color light.

I recently noticed a variety of definitions for the terms Blue Light Content and Percent Blue.  I must admit, I was a little surprised to discover the inconsistencies because blue light content is not a new term; it’s readily found in research, standards, ordinances, and even LED manufacturer cut sheets.  So, what is blue light content and how did we get here?

Blessed with a regionally dark ocean and high altitude, Hawaii County (aka the Big Island) is internationally known for its Mauna Kea observatories and world-class astronomical viewing.  To protect this rare treasure against detrimental light pollution, Hawaii has had a long relationship with restricting the spectral content of street and area lights. In 2022, Maui County followed suit and enacted ordinance #5434 calling for outdoor lighting to contain less than “2% Blue Light Content.” This really caught my attention. I thought the requirement sounded concise, pithy, understandable, and, if correct, an elegant consumer-like solution. It made me feel like I’ve heard this before. Perhaps that’s because mainstream consumer products have asserted information to the general public about healthy ways to avoid blue light by using TVs, cell phones, and sunglasses using blue-light filter technology.  At the time, I didn’t know why Maui was enacting its blue content requirement, or how it was measured, but it sure sounded like a good idea. This is where the rabbit hole began.

I don’t think anyone will disagree that electrified lighting has revolutionized human capability over the past 120 years. Population growth, urbanism, transportation, and modern conveniences have all driven its demand. In short, electric lamps, the most common form of artificial light at night (ALAN), have transformed the way we live, work, and play. This includes outdoor activities at night.

The purpose of the Maui ordinance cites the preservation of dark skies (important for cultural, astronomical, biodiversity, and tourism value) and the protection of wildlife (seabirds, sea turtles, and other reptiles, amphibians, mammals, or invertebrates). In fairness, the ordinance also mandates shielding requirements for most lighting types, so the burden of success is not completely on the less than 2% blue-light content requirement.

When I spoke with an astronomer, an ecological scientist, and a roadway engineer about Maui’s blue light content requirement, I received three valuable, but different, perspectives. Each profession has a different purpose for caring, or not caring, about blue (i.e., shorter wavelength) emissions.  At a minimum, the three perspectives certainly have different definitions for which wavelengths are of interest to their work.


The astronomical perspective stated that for outdoor lighting, where the moon and starlight are nature’s original lighting solution, naturally occurring sky glow is relatively quiet below 556 nm (emission line from oxygen). According to Richard Wainscoat, astronomer and educator at the University of Hawaii, this is an ideal range for ground-based astronomy to peer through Earth’s atmosphere, unless it’s polluted with artificial light and sky glow. This may be one reason so much attention is given to the 400-490 nm peak within many InGaN LED light sources, and why astronomers have an interest in reducing artificial 400-550nm blue and green emissions that are more prone to atmospheric scattering. Therefore, should we define blue light content as 400-550 nm?  Will we include green in our definition for  Blue Light Content? Would 400-520nm better align with and encompass blue light content?  And when exactly does visible light transition from violet to ultra-violet? Is it 380 nm or 400 nm? Can we see light below 400 nm?  If we can’t see it, why do we spend energy to produce it? Maui chose to define blue light content as 400-500 nm. Is that right? Of note, Richard will stress spectral wavelength restrictions should not stand alone, they must work together with reduced light quantities and shielding to satisfy observatory needs.

In the ecological view, ecological scientist Dr. Travis Longcore, an adjunct professor at UCLA, explained that ALAN does have an impact on terrestrial wildlife and nocturnal ecosystems. Wavelength sensitivities between taxonomic groups do span into UV (i.e., below 400 nm) emissions and vary between reptiles, amphibians, birds, arthropods (including spiders and insects), and non-human mammals. Thus far, predicting ecological responses with one broad blue-light-content brush has not statistically been a great indicator of disturbance to all creatures. Does the Maui definition of 2% blue light content between 400-500 nm work for their seabird, sea turtle, and other ecological needs?

 The roadway engineer asks, if the peak of photopic sensitivity is 555 nm (yellowish-green), and the peak of scotopic sensitivity is 505 nm (turquoise), can we actually improve nighttime performance by leveraging blue light content instead of avoiding it?  Are there additional benefits of improved efficacy and energy savings? Can blue content improve contrast enough for humans under low-light conditions that we can lower light levels?  If safety is not impacted, are human uses fair tradeoffs versus astronomy, birds, sea turtles, and other wildlife?

ANSI/IES TM-37-21 Description, Measurement, and Estimation of Sky Glow cites a Pacific Northwest National Laboratory (PNNL) dataset which defines “% Blue” using 380-500 nm. Additional versions of the PNNL dataset used by the DOE define blue content as 405-530nm.  The difference in percent blue between these two definitions is 8 to 9 percent for the same 2700K source (e.g., 6-7% versus 15%). So, which is correct?  If attempting to achieve the Hawaii requirement, you can start to see the importance of having a standard definition for not only the numerator range but also the range of the denominator. For most LED sources, a tighter numerator range and an exorbitant denominator range, beyond the visual spectrum, will artificially lower “Blue Light Content.”   

Figure 1 – Percent Blue versus CCT (as published in ANSI/IES TM37-21 table 3-3 and defined as 380-500 nm).
Figure 1 – Percent Blue versus CCT (as published in ANSI/IES TM37-21 table 3-3 and defined as 380-500 nm).

Two ramifications from the current confusion are 1) it may be difficult, if not impossible, to satisfy multiple benefits with one definition for blue light content; and 2) without a standard definition, how does one specify and purchase products across a level playing field?  The range of emissions in the numerator must have a very specific goal and serve a particular purpose.  The function of the denominator range is only to determine a mathematical percentage worthy of comparing two alternatives.

I don’t expect the terms blue light content or percent blue it to go away anytime soon. By not defining them, I am concerned we are fostering a range of uses that may lead to the accidental belief that less blue light content will solve your problem. When our needs and reasons for measuring blue light content differ from one another, how will we settle on just one definition? The astronomer, the ecologist and the roadway engineer all agree blue light content is not a proxy for lowering light levels that are targeted and shielded. Instead, it is a component intended to work as part of an overall system within the Five Principles of Responsible Outdoor Lighting.

To be clear, I am not being critical of the Maui ordinance. Even if the impact of the blue light content requirement misses one of its intended purposes, I applaud the effort. It has raised awareness and moved outdoor lighting closer to the mainstream. Before additional definitions for blue light content emerge, I’d like to see our industry answer if this is a rabbit hole or a term we can use, what will it be used for, and what will the definition be.

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