Melanopic EDI Explained: Definition, Units, and Why Lux Is the Wrong Metric

Melanopic EDI (short for melanopic equivalent daylight illuminance) is the number that actually tells you whether a light source will disrupt your body clock, and it is the metric CircadianShield uses to time its display filtering instead of relying on lux or Kelvin alone. To see why melanopic EDI exists, start with what the older unit, lux, was built to measure. Photometry, the science of measuring light, has a long history tied to human visual perception. The standard unit of illuminance, the lux, is defined as the intensity of light weighted by the photopic luminosity function: a curve that peaks around 555 nm (yellow-green) and reflects how the average human eye perceives brightness in daylight conditions.

Lux is a useful and well-validated metric for tasks like evaluating workplace lighting levels, designing reading lamps, or ensuring a space meets building code requirements. But it was designed to answer one question: how bright does this light appear to a person?

Lux cannot answer a different but equally important question: how much will this light shift your circadian clock? Melanopic EDI is the metric built to answer that second question instead.

Melanopic EDI and the Mismatch Between Vision and Circadian Response

Melanopic EDI exists because the visual system and the circadian system rely on different photoreceptors, a mismatch CircadianShield's display filtering is built around. Rods and cones (which determine lux measurements) are most sensitive to green-yellow wavelengths. The ipRGCs that drive circadian entrainment via melanopsin are most sensitive to blue wavelengths around 480 nm.

This creates a systematic mismatch. Two light sources can appear equally bright by any photopic measurement, identical lux readings, while having dramatically different effects on the circadian system. A 3000K warm white LED (weighted toward amber and red) and a 6500K cool white LED (with a strong blue component) could be tuned to produce the same lux reading. But the cool white LED would deliver two to three times the melanopic stimulation of the warm white LED.

Traditional photometry cannot capture this difference. You need a different weighting function, one tuned to the melanopsin absorption spectrum rather than the cone response.

CIE S 026:2018: The Standard That Defines Melanopic EDI

Melanopic EDI comes from CIE S 026:2018, the standard CircadianShield's filtering methodology is built on. In 2018, the International Commission on Illumination (CIE) published CIE S 026:2018, formally titled "CIE System for Metrology of Optical Radiation for ipRGC-Influenced Responses to Light." This standard formally defines five alpha-opic quantities for characterizing light in terms of its biological effects:

• Sc-EDI (S-cone-opic equivalent daylight illuminance), weighted to the short-wavelength cone response
• Sm-EDI (M-cone-opic equivalent daylight illuminance), weighted to the medium-wavelength cone response
• Sl-EDI (L-cone-opic equivalent daylight illuminance), weighted to the long-wavelength cone response
• Srho-EDI (rhodopic equivalent daylight illuminance), weighted to the rod photopigment response
• Smel-EDI (melanopic equivalent daylight illuminance), weighted to the melanopsin absorption curve of ipRGCs

Of these five, melanopic EDI (Smel-EDI) is the metric most directly relevant to circadian health, it quantifies exactly the component of a light source that will stimulate the circadian pacemaker and potentially suppress melatonin.

CIE S 026:2018 provides the first internationally standardized system for quantifying the photobiological impact of light on the human circadian system. Melanopic EDI is now the recognized metric for evaluating circadian-effective illumination in buildings, workplaces, and consumer products.

CIE S 026/E:2018, CIE System for Metrology of Optical Radiation for ipRGC-Influenced Responses to Light. Commission Internationale de l'Eclairage, 2018.

How Melanopic EDI Is Calculated and Measured

Calculating melanopic EDI, the number CircadianShield's twilight engine targets at every phase, starts with the full spectrum of the light source rather than a single brightness reading. Melanopic EDI is computed by multiplying the spectral power distribution of a light source (how much energy it emits at each wavelength, measured in watts per nanometer) by the melanopsin spectral efficiency function, then integrating across the visible spectrum. The result is then normalized to an equivalent illuminance from a D65 standard daylight illuminant.

In practical terms, this means:

• A light source that emits predominantly in the 460-490 nm range (blue) will have a high melanopic EDI relative to its photopic illuminance
• A light source that emits predominantly in the 580-700 nm range (amber, red) will have a very low melanopic EDI even at high photopic illuminance
• Two sources at the same lux level but different spectral compositions will have different melanopic EDI values, potentially by a factor of 3 or more

Getting a real melanopic EDI reading this way requires a calibrated spectroradiometer that records the full spectral power distribution, because a standard lux meter or a phone's ambient light sensor only captures the photopic (visual-brightness) response and cannot see the underlying spectrum. CircadianShield works around that by using each display's known spectral output at every color temperature setting, so it computes melanopic EDI for the viewing distance and angle you actually use without needing a physical meter in front of the screen.

Melanopic EDI vs Melanopic Lux: Same Measurement, Two Names

Melanopic EDI and melanopic lux describe the same number, a point of confusion CircadianShield's own users run into when reading spec sheets or research papers. CIE S 026:2018 defines melanopic EDI (abbreviated Smel-EDI, or informally M-EDI) as an illuminance quantity, so it is reported in lux, exactly like ordinary photopic illuminance. Writing "a melanopic EDI of 250 lux" and "250 melanopic lux" both point at the identical measurement: the illuminance a D65 daylight source would need to produce the same melanopsin stimulation as the light source being measured.

A related term, melanopic DER (daylight efficacy ratio), is not the same thing, and it is worth knowing before comparing two products. DER is melanopic EDI divided by photopic illuminance, a ratio that describes how melanopically efficient a light source's spectrum is independent of how bright it is. A warm 1800K screen and a cool 6500K screen can be set to the same photopic lux, but the 6500K screen will have a far higher melanopic DER, and therefore a far higher melanopic EDI at that same brightness. CircadianShield's twilight-phase engine tracks DER internally so it can hold melanopic EDI to a target instead of just a target color temperature.

Why Color Temperature Is Not Enough to Control Melanopic EDI

Melanopic EDI is not the same thing as color temperature, even though the two are often confused. Color temperature, measured in Kelvin and familiar from display settings and smart bulb apps, is a useful shorthand but an imprecise proxy for melanopic stimulation. It describes the color appearance of a light source by comparing it to the emission of a theoretical black body at a given temperature, but it does not specify the actual spectral power distribution.

Two light sources can have the same correlated color temperature (CCT) while having very different spectral shapes, and therefore different melanopic EDI values. This happens because CCT is calculated from a tristimulus match to the human visual system, not from the actual underlying spectrum. Light sources with very different spectral distributions can appear identically "warm" or "cool" while differing substantially in their blue content.

This matters practically because display filter software that only shifts color temperature is not necessarily minimizing melanopic output. A display set to 2700K using a simple matrix transform still delivers more melanopic stimulation than a display actually modeled on the spectral characteristics of a 2700K light source, because the underlying LED blue spike is attenuated but not eliminated. Apple's Night Shift is the clearest example: it shifts color temperature on a clock schedule but was never built to model or report a melanopic EDI value, which is the gap tools that actually target the metric are built to close.

What the Research Recommends for Melanopic EDI

Melanopic EDI targets by time of day come from a 2022 paper by Brown et al. in PLOS Biology, which CircadianShield's twilight-phase schedule follows. The paper synthesized evidence from leading circadian research groups and published specific melanopic EDI recommendations by time of day:

• Daytime (indoor): At least 250 melanopic lux to maintain alertness and support circadian entrainment
• Evening (1-2 hours before bed): Below 10 melanopic lux to avoid melatonin suppression
• Night (sleeping hours): Below 1 melanopic lux

We recommend daytime indoor melanopic EDI of at least 250 lux, transitioning to less than 10 lux in the evening and less than 1 lux at night. These targets are achievable with current technology and represent meaningful improvements over typical indoor lighting practice.

Brown TM et al. Recommendations for daytime, evening, and nighttime indoor light exposure to best support physiology, sleep, and wakefulness in healthy adults. PLOS Biology. 2022;20(3):e3001571.

Applying Melanopic EDI to Display Filtering

Melanopic EDI is the number CircadianShield targets on both Mac and Windows, and the implications for display filtering generally are clear from the same logic. Effective software should not merely shift color temperature, it should model the display's actual spectral output at each color temperature setting and compute the resulting melanopic EDI delivered at a typical viewing distance and angle. It should then target specific melanopic EDI values aligned with the time of day and the recommendations above.

During morning hours, when melanopic stimulation supports wakefulness and SCN entrainment, the display should maintain a high melanopic output. During late evening, melanopic EDI should be reduced below 10 lux equivalent. In the middle of the night (for those who use screens during sleep disruptions), below 1 melanopic lux is the appropriate target.

This requires knowing the spectral characteristics of the display being filtered, which varies between manufacturers, display technologies (OLED vs IPS vs VA panels), and even between individual display units. A single generic color temperature curve cannot be equally correct for all displays.

The Bigger Picture for Melanopic EDI

Melanopic EDI, not lux, is where circadian light evaluation is heading, and the shift represents a maturation of the field CircadianShield was built around. We now have the tools to measure light exposure precisely enough to make meaningful recommendations for display design, architectural lighting, shift-work scheduling, and personal health devices. This same photoreceptor mismatch is why color temperature and melanopic EDI diverge across a full day, a topic covered in more depth on CircadianShield's circadian rhythm science guide and science page.

For anyone evaluating products that claim to protect sleep or support circadian health, the right question to ask is not "does this reduce blue light?" but rather "what melanopic EDI does this deliver, at what time of day, and how does that compare to the CIE S 026 recommendations?" That is the standard the science has set, and it is the one worth holding products to.

Melanopic EDI: Common Questions