Melanopic EDI Explained: Why Lux Is the Wrong Metric

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?

It cannot answer a different but equally important question: how much will this light shift your circadian clock?

The Mismatch Between Vision and Circadian Response

The fundamental problem is that the visual system and the circadian system use different photoreceptors. 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 Changed the Field

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

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

Why Color Temperature Is Not Enough

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.

What the Research Recommends

A 2022 paper by Brown et al. in PLOS Biology - which synthesized evidence from leading circadian research groups - 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 This to Display Filtering

The implications for display filtering are clear. 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

The shift from lux to melanopic EDI as the primary metric for circadian light evaluation represents a maturation of the field. 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.

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.