The lighting industry now recognizes the non-visual effects of light on health and circadian rhythms, driving the development of standardized metrics for optimal melanopic lighting across various settings. The two most widely used are M-EDI and EML, both of which measure light’s biological impact—especially around the blue-light spectrum (~480 nm).
What is M-EDI (Melanopic Equivalent Daylight Illuminance)?
The International Commission on Illumination (CIE) recommended a circadian metric called M-EDI. It quantifies the amount of light exposure from a source that stimulates melanopsin-based photoreception in the retina, and it shares the same fundamental methodology as EML. This metric determines how much daylight would be required to produce the same biological impact as a given lighting condition. The key difference is that the magnitude of M-EDI is expressed as the illuminance of the CIE standard illuminant D65 that delivers the same stimulus to ipRGCs. Thus, the M-EDI value can be converted from the EML value by multiplying it by a coefficient of 0.9063.
Recommended levels: Daytime: >250 M-EDI; Nighttime: <50 M-EDI
What is Equivalent Melanopic Lux (EML)?
Another key metric is EML, introduced and popularized through the WELL Building Standard. Like M-EDI, EML measures the circadian effectiveness of light but is expressed in lux, modified to account for the sensitivity of melanopsin-containing ipRGCs, rather than the traditional photopic response of cones.
While traditional lux measures brightness as perceived by the human eye, EML adjusts the measurement to reflect the biological (non-visual) impact of light on the circadian system. It allows lighting professionals to assess how effectively a light source stimulates melanopsin receptors, which are essential for regulating the biological clock.
The WELL v2 Standard recommends at least four hours of EML lighting in the morning to effectively entrain the circadian rhythm. This exposure helps support healthy sleep-wake cycles, improve cognitive function, and enhance mood regulation.
Suggested ranges: Daytime: >150 EML; Nighttime: <50 EML
The choice between M-EDI and EML often depends on the context. M-EDI is rooted in scientific standardization and widely used in research and technical standards, while EML is application-focused and directly linked to practical health outcomes and WELL Building certification.
What is Circadian Action Factor (CAF)?
The human eye contains various types of optic nerve cells, one of which does not process visual information but instead responds to light stimuli. When light enters the eye, this type of optic nerve cell—known as the intrinsically photosensitive retinal ganglion cell (ipRGC)—triggers non-visual biological effects. These effects significantly influence the release of hormones such as melatonin and cortisol.
When ipRGCs are exposed to high-frequency, high-energy light, they inhibit the body’s production of melatonin, making people feel alert and energized. Conversely, when exposed to low-energy light, melatonin levels gradually increase, making people feel relaxed and sleepy. Therefore, light serves as a crucial external factor affecting the human circadian rhythm.
Gall and Bieske developed the Circadian Action Factor (CAF) as an indicator of the non-visual effects that light has on human health and well-being. It is a metric that describes how a light source influences the circadian system, which governs essential processes such as sleep, alertness, and cognitive performance. This metric helps lighting designers understand how specific light exposures impact the circadian system and contribute to circadian rhythm regulation.
CAF is defined as the ratio of the amount of light affecting the circadian system to the amount of light useful for vision. It is calculated based on the light spectrum emitted by a device. Recommended CAF values vary depending on the time of day: a high CAF is typically preferred during the day to enhance alertness and cognitive function, while a lower CAF is more suitable in the evening to support relaxation and preparation for sleep. Higher CAF values correspond to greater melatonin suppression.
Compared to color temperature, CAF is a more accurate indicator for predicting the melatonin-suppressing effects of light. Preliminary studies suggest that CAF may be a more reliable index for studying the physiological effects of light on the human body.
Gall proposes ideal combinations of individual activities, the Circadian Action Factor (CAF), and the color temperature of light sources, as shown in the image below.
What is Circadian Stimulus (CS)?
Developed by researchers at the Lighting Research Center (LRC), the Circadian Stimulus (CS) metric is a method for quantifying light’s impact on acute melatonin suppression, a marker of circadian system activation. It measures the effect of light—based on spectrum, intensity, and duration—on the human circadian system.
In contrast to the metrics proposed by the CIE (M-EDI) and IWBI (EML), the CS metric considers not only the role of ipRGCs, but also the contributions of rods and cones in non-visual physiological responses to light. Notably, the CS metric addresses the fundamental problem of subadditivity in the human circadian system. It indicates how effectively a one-hour exposure to a specific light level and wavelength stimulates the circadian system, based on its ability to suppress the hormone melatonin.
The CS value ranges from 0 (no biological effect) to 1 (maximum activation) and is calculated based on the response characteristics of melanopsin-containing photoreceptor cells in the retina. These cells regulate melatonin secretion and influence physiological functions such as the sleep-wake cycle and alertness.
By measuring CS values in real time, users can accurately assess whether their light environment aligns with the body’s natural circadian needs. For example, a high CS value during the day (recommended: 0.3–0.7) helps enhance alertness and cognitive performance. At night, the CS value should be kept below 0.1 to avoid melatonin suppression and sleep disturbances. This makes the CS metric particularly valuable for optimizing dynamic lighting systems, such as circadian-adaptive lighting solutions for workplaces, healthcare facilities, and residential environments.
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