The frequently asked questions shown below cover basics and terminology related to street lighting and blue light.

What is “blue light”?

“Blue light” is a term often used as shorthand to describe a variety of ranges of wavelengths that play key roles in the health issues raised by the AMA. But the term can lead to confusion, because there’s no consensus definition of blue light; light colors vary along a continuum, and there’s no single, discrete definition of blue or any other color. For the sake of accuracy, it’s important that any time the term “blue” is used, it’s clearly defined, since different ranges of the spectrum apply to different concerns.  

Figure 1 provides a list of definitions of blue and other spectral colors from four different reference documents.  Together, these documents categorize blue light as falling somewhere in the range from 424 nm to 500 nm, but the specific ranges reported differ significantly. [1]

Figure 1. Wavelength ranges for monochromatic light as reported in four source documents.  Source:
Figure 1. Wavelength ranges for monochromatic light as reported in four source documents. Source:

“Blue light” is also a term sometimes used by astronomers to describe lighting wavelengths that are scattered in the atmosphere at night and result in skyglow that interferes with the observation and appreciation of night skies. Along these lines, the Cégep de Sherbrooke, a Canadian university with a focus on astronomy and atmospheric science, broadly defines the blue range as 405-530 nm and recommends a metric called “% Blue,” which sums the radiant power in that range, dividing by the total power emitted between 380 and 780 nm. [2]

None of the cited ranges for blue are more definitive than others, which means that generic terms such as “blue light,” “blue-rich LEDs,” and “blue content” are not very specific and in fact can be misleading, given that the term “blue” itself is not a defined quantity in terms of spectrum, visual outcome, or nonvisual outcome. 

Moreover, the associated health and other impacts under discussion are caused by particular wavelengths of light, not by colors. The relevant wavelengths for any given effect don’t necessarily coincide with what the human eye perceives as a particular color. The effects linked to “blue light” in the AMA release, for example, in actuality extend into violet, indigo, cyan, and green. Referring only to blue incorrectly discounts the effects of these other wavelength regions.

These FAQs thereby avoid use of the term “blue light,” except where necessary to address specific use of the term in public media.

  1. Source:
  2. http://galileo.graphycs.cegepsherbrooke.qc.CA/app/en/home

What is a spectral power distribution (SPD)?

The spectral power distribution (SPD) of a light source is the amount of radiant power it emits at different wavelengths across the visible spectrum.  An SPD can be represented as a table of radiant-power values, or as a graph similar to those in Figure 2. In addition to determining the apparent color of the light, the SPD determines how the source affects the appearance of objects it illuminates, as well as its potential scattering characteristics within the atmosphere [1] and the potential for associated health effects (such as those brought up in the AMA guidelines). The SPD is the fundamental light-source information used in color science.

SPDs can vary widely, even within a given light-source technology. Two metal halide lamps that have correlated color temperatures (CCTs) of 4000 K can have very different SPDs, for example, as shown in Figure 2. This is also true of LED products, so it’s important to keep in mind that no single SPD is entirely representative of a given light-source technology.

Figure 2  Two 4000 K metal halide SPDs showing variability in wavelength content, with one high-pressure sodium SPD for comparison

Figure 2.  Two 4000 K metal halide SPDs showing variability in wavelength content, with one high-pressure sodium SPD for comparison.

  1. This FAQ focuses primarily on potential health consequences of light at night. Discussion of the relationship between light at night and sky glow is addressed separately by DOE.

What is correlated color temperature (CCT)?

Most white-light sources emit a range of wavelengths, which, when combined, produce the color of light perceived by the human eye. Correlated color temperature (CCT) is a shorthand way to describe the light’s color, in terms of its apparent “warmth” or “coolness.” CCT is expressed in kelvin (K), and the value corresponds to the color of light emitted by a heated mass (a theoretical blackbody radiator) at that physical temperature (although the light source is not actually operating at that physical temperature). So an LED with a CCT of 3000 K will appear to give off a color of light close to that of a tungsten filament operated at a physical temperature of 3000 K (tungsten being very close to an ideal blackbody radiator). The challenge is that light sources with very different SPDs can have the same CCT, as illustrated by the two 4000 K MH products in Figure 2 (see previous question).  Similarly, sources with the same CCT can look different. For this reason, CCT is only a rough gauge of the actual spectral content of a light source.

What are the five types of photoreceptors in the human eye, and what is an action spectrum?

There are five currently recognized photoreceptors in the human eye.  These include three different types of cone photoreceptors, which are responsible for color and detail vision under well-lighted (or photopic) conditions. The different types of cone receptors are distinguished by their comparative sensitivities to short-, medium-, and long-wavelength light.  In addition to the cones, there are rods, which provide monochromatic vision under low lighting (or scotopic) conditions. Rods are responsible for vision at low light levels but don’t detect color or detail. The eye’s aggregate sensitivity shifts toward shorter wavelengths under scotopic conditions than under photopic conditions (Figure 3). There are also the recently discovered intrinsically photosensitive retinal ganglion cells (ipRGCs), which are crucial for relaying light information to parts of the brain controlling the biological clock. [1] Relevant primarily to circadian physiology, pupil dilation, and other nonvisual effects, the ipRGCs contain melanopsin, a photopigment that has a peak photosensitivity at 480 nm. For a variety of reasons, the peak response of the ipRGCs in vivo appears to be at about 490 nm, [2] though this value is still being refined. 

Figure 3 shows the spectral sensitivity (or action spectra) for each of the five known types of photoreceptors, which combine in various ways to allow for visual and nonvisual processes. The mechanisms underlying some of these processes are well-known, such as how the three cone photoreceptors provide for the perception of color; whereas other mechanisms, such as those involving the body’s circadian systems, are only beginning to be understood. Importantly, the responses of the photoreceptors are not static, but change based on the amount and duration of light present. [3] The role of ipRGCs at various light levels is still being investigated. The response of the ipRGCs is usually referred to as the melanopic response.

Figure 3. Spectral sensitivity (action spectra) of the five known human photoreceptors, along with Vλ, the photopic response curve used to define and quantify lumen output from a light source. Data from CIE TN 003:2015.
Figure 3. Spectral sensitivity (action spectra) of the five known human photoreceptors, along with Vλ, the photopic response curve used to define and quantify lumen output from a light source. Data from CIE TN 003:2015.

As it pertains to the issues related to “blue light,” the corresponding melanopic action spectrum encompasses a wide range of wavelengths that extends well beyond the nominal definitions of “blue.” In other words, while some portion of the melanopic response occurs with short wavelengths that are typically recognized as blue, it’s also influenced by colors outside of those wavelengths. Furthermore, it should be noted that ipRGCs don’t act in isolation when it comes to influencing the biological clock; that is, the rods and cones also play a role, although the full extent of their contributions is not fully understood at this point. [4]

  1. Lucas RJ, Peirson SN, Berson DM, Brown TM, Cooper HM, Czeisler CA, Figueiro MG, Gamlin PD, Lockley SW, O'Hagan JB, Price LA, Provencio I, Skene DJ, Brainard GC. 2014. Measuring and using light in the melanopsin age. Trends in Neurosciences, 37(1), 1-9.
  2. CIE. 2015. TN 003:2015: Report on the First International Workshop on Circadian and Neurophysiological Photometry, 2013.
  3. Joshua J. Gooley, Kyle Chamberlain, Kurt A. Smith, Sat Bir S. Khalsa, Shantha M. W. Rajaratnam, Eliza Van Reen, Jamie M. Zeitzer, Charles A. Czeisler, and Steven W. Lockley. Exposure to Room Light before Bedtime Suppresses Melatonin Onset and Shortens Melatonin Duration in Humans.  J Clin Endocrinol Metab. 2011 Mar; 96(3): E463–E472.
  4. Lucas RJ, et al, op cit.

How do LEDs create white light?

The most common white LEDs today employ blue-pump LED chips that, at the chip level, produce an SPD peak centered somewhere in the range of about 445 to 465 nm. Light from the LED chip passes through a phosphor layer that converts most of the chip’s output into longer wavelengths, typically in the green, yellow, orange, and red parts of the spectrum. The mix of these colors produces white light.

However, there are other methods of producing white light, which are less common but offer greater flexibility for adjusting the SPD. One such method is to combine LEDs of different colors — such as phosphor-converted LEDs (PC-LEDs) in combination with other LEDs that emit specific colors (see Figure 4) — or to combine multiple LEDs of various colors, which can be varied in relative output to attain any apparent color of light desired. Systems offering dynamic adjustability tend to be more expensive and thus have been employed less frequently in street lighting and other outdoor applications to date. 

Figure 4. PC amber-cyan-violet flat lens chip array (
Figure 4. PC amber-cyan-violet flat lens chip array (