Circadian principles require a new light language (MAGAZINE) (UPDATED)

Sept. 8, 2020
The solid-state lighting industry must extend its vocabulary and grasp of the fundamentals of circadian lighting before applying so-called ‘healthy lighting’ designs, writes ALLISON THAYER.

Once upon a time, lighting was simple. There was a constant cycle of bright sunlight during the day and dim moonlight or firelight at night. This 24-hour cycle of bright and dim kept our bodies active and our minds awake during day and asleep at night. Then we put roofs over our heads to provide shelter, but in so doing we also disconnected from the natural cycle of light and dark. As buildings became bigger and deeper, we created candles to help us see even in the daytime.

With the invention of electric lighting, light got cheaper so buildings could be of any depth or height. To be cheaper than candles, electric lighting needed sophisticated engineering and metrics for design and mass production. So, we created the footcandle to measure how much light we produced and correlated color temperature (CCT) to measure how warm or cool the illumination appeared.

This simple language of footcandles and CCT worked really well for the lighting technology and science of a century ago (see sidebar “Current vocabulary”). Those days are over for two important reasons: LEDs came along, and we discovered that lighting wasn’t just for seeing anymore (Fig. 1). Instead of persisting with a lighting language based on candles and how hot they appear, we need to create a new language that represents the impact of these light sources and the new science on our health and wellbeing.

Current recommendations for the visual system

The initial function of electric lighting in the built environment can be summarized as lighting the work horizontal surface with an aesthetically-pleasing color of illumination. To accomplish that, lighting recommendations were focused on designs with evenly-spaced incandescent, fluorescent, or high-intensity discharge (HID) lamps directing their flux downward.

Recommended illuminance values were based on the amount of light on the task required for sufficient visual performance, such as reading a book or writing. Because most of the tasks were paper based, light on the work plane was the fundamental concern, making horizontal illuminance (EH) the primary metric. Based upon a number of studies conducted in the 20th century, 30 and 50 footcandles (fc) on a horizontal task surface is recommended for good visual performance, depending upon the difficulty of the task.

FIG. 2. The lighting industry has long looked to chromaticity values close to the blackbody locus to inform development of lamps with varying correlated color temperatures (CCTs) for white light.

People in different cultures and markets preferred their horizontal illuminance to come in different color tones. White light could be “cool,” “neutral,” or “warm,” and the CCT metric was developed to quantify those color tones. CCT is based upon its relationship to an ideal curve describing the temperature of a flame in kelvin. The blue part of a candle flame is hotter than the yellow part of the flame. Cool electric light sources that look blue-white have higher CCT values than warm sources that look yellow-white. As seen in Fig. 2, the so-called blackbody locus describes how the color of a flame changes with temperature from yellow-white at 3000K to blue-white at 6000K. Each line segment represents a different CCT, but it is important to know that each point, or chromaticity, on those line segments looks different even though they have the same CCT designation. Moreover, the further each chromaticity point is from the blackbody, the less likely it is to look like a candle flame.

Current vocabulary

Correlated color temperature (CCT): A specification for white light sources used to describe the dominant color tone along the dimension from warm (yellow and reds) to cool (blue) appearance. Lamps with a CCT rating below 3200K are usually considered warm sources, whereas those with a CCT above 4000K are usually considered cool in appearance. Temperatures in between are considered neutral in appearance. Technically, CCT extends the practice of using temperature, in kelvin (K), for specifying the spectrum of light sources other than blackbody radiators. Incandescent lamps and daylight closely approximate the spectra of blackbody radiators at different temperatures and can be designated by the corresponding temperature of a blackbody radiator. The spectra of fluorescent and LED sources, however, differ substantially from blackbody radiators yet they can have a color appearance similar to a blackbody radiator of a particular temperature as given by CCT.

Footcandle (fc): A measure of illuminance in lumens per square foot. One footcandle equals 10.76 lx, although for convenience 10 lx is commonly used as the equivalent.

Recommendations: To ensure adequate illumination and safety to the occupants of a space, the Illuminating Engineering Society (IES) created a standard that recommends horizontal illuminance (EH) levels for common indoor activities.

Applying research advances

During the 20th century, technology manufacturers attempted to develop lamps with chromaticities on or nearly on the blackbody, so CCT worked well as a shorthand description of white-light color variations. With the development of LEDs, however, all bets were off. For example, research with warm LEDs showed that chromaticities on the blackbody were less white than those of the same CCT below the blackbody. Moreover, that same chromaticity below the blackbody more closely resembled a cool LED light source than it did the light source on the blackbody of the same CCT. Therefore, CCT is no longer a good way to describe the color tone of white LED light sources (Fig. 3).

FIG. 3. The vocabulary of footcandles and CCT is outdated and incomplete with the advance of LED sources into the lighting market, and adequate only to describe the visual system’s response to light.

Research on circadian rhythms has demonstrated that exposure to the natural 24-hour sunrise and sunset is ideal for health and wellbeing. Indoor lighting during the day is not nearly as bright as it is outdoors, and with self-luminous displays (e.g., tablets), the night is no longer as dim as moonlight or firelight. What may be bright enough for the visual system may not provide enough stimulus to activate the circadian system. In fact, the circadian clock is in a totally separate part of the brain than the visual cortex, but we have no standard language to describe bright or dim with regard to time of day. Like we did in the 20th century for vision, we now need to engineer electric lighting for supporting circadian rhythms with metrics that define bright days and dim nights.

To address this shortfall, the Lighting Research Center (LRC) developed a way to engineer lighting in terms of its ability to affect circadian rhythms. Circadian light (CLA) is a bit like CCT because it describes the color of the light source for the circadian system, and circadian stimulus (CS) is like footcandles because it describes how much circadian light is being delivered.

FIG. 4. The lighting community must embrace new vocabulary including circadian light (CLA) and circadian stimulus (CS) in order to adequately describe and quantify non-visual effects of light on the circadian system.

To quantify metrics such as CCT and footcandles, the spectral power distribution (SPD) of the light source must be known. SPD defines the amount of energy produced by the light source at every wavelength. Similarly, to engineer circadian lighting one needs to start with the SPD of the light source to determine CLA and CS (see sidebar “Additional vocabulary”). Although CCT and footcandles, and CLA and CS start with SPD, they cannot be substituted one for the other since they were developed for entirely different purposes (Fig. 4). It would be like measuring miles per gallon used by a car to measure the speed of the car. Both are related to how well the car performs, but the two metrics provide very different types of information about the car’s performance. The SPD of the light source must be known to calculate circadian light — that is, CLA. From this spectrally-weighted optical radiation, the impact of that light on the circadian system — that is, CS — is determined.

Additional vocabulary

Circadian light (CLA): Irradiance weighted by the spectral sensitivity of retinal phototransduction mechanisms stimulating a response from the biological clock.

Circadian stimulus (CS): The response of the circadian system to different amounts of CLA from threshold, CS=0.1, to saturation, CS=0.7.

Recommendations: UL 24480 recommends a CS ≥0.3 for at least two hours in the morning or early afternoon and CS <0.1 at night. Both ambient lighting and self-luminous displays need to be considered.

Recommended metrics for the circadian system

Similar to those 20th century studies used to develop recommended footcandles for visual tasks, laboratory and field studies aimed at developing recommended CS levels for supporting circadian rhythms were conducted. From those studies and a public consensus process, UL 24480 recommends a CS ≥0.3 during the day and CS <0.1 at night. The LRC also provides a web-based calculator to ensure that the light source SPD (spectrum and amount) will provide the recommended levels of CS in UL 24480. The calculator can be found on the LRC’s Light and Health web page at

FIG. 5. Key takeaways compare the current terminology used to discuss lighting metrics for general illumination, versus recommended additional vocabulary that will refine understanding of circadian lighting metrics.

Moving forward with circadian lighting

In the time before we had evidence of the impact of light on human circadian rhythms, footcandles and CCT were good enough for lighting — one term referred to how much light we had to see, and the other referred to the color of the light we see. With the advent of LEDs and research into the non-visual effects of light entering the eye, we need to extend our lighting vocabulary to include circadian-effective light (CLA) and circadian stimulus (CS) — one term refers to how sensitive the circadian system is to different light sources, and the other refers to how effective the light is for stimulating the human circadian system. The important point is that we cannot use visual metrics to represent non-visual responses, so we need new metrics to engineer and apply lighting for the circadian system. In subsequent articles, we will go into more detail.

Editor’s note: In Part 2 of this circadian lighting series, Allison Thayer will analyze commercially-available LED SPDs, exploring how target circadian stimulus values might be determined and reached. This will effectively explain the impact of spectra on circadian stimulus.

Get to know our expert

ALLISON THAYER, MS, BA, is a research specialist at the Lighting Research Center (LRC) at Rensselaer Polytechnic Institute. She holds a bachelor’s in architecture, and a master’s of science with a concentration in lighting from Rensselaer. Thayer has played a main role in the development of the structure and content of the Healthy Living website.

*Updated Oct. 13, 2020 4:35 PM for figure caption formatting.

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