What is Melanopic EDI (m-EDI) and how does it work?

Melanopic EDI is the “lux” perceived by ipRGCs in the retina

Melanopic EDI, or m-EDI is the abbreviation of Melanopic Equivalent Daylight Illuminance, which is introduced in the CIE S 026/E:2018 document. m-EDI is a “lux” metric but unlike the general theory of visual lux V(λ), m-EDI measures the light to our non-visual receptor of ipRGCs (Intrinsically Photosensitive Retinal Ganglion Cells) which also exist in our retina, and ipRGCs affect the melatonin secret   ion/restraint very much, thus, m-EDI is an important “lux” metric for evaluating the circadian effect to human.

m-EDI measures the light at the eyes                                               

Generally when we measure lux, we will measure it in the horizontal direction – this is to evaluate the lux to the surface. But m-EDI, as the typical human-centric lighting metric, measures the vertical direction to the eyes, and the measurements should be centric in human to follow the eye positions always.

How is m-EDI measured and calculated?

If you have a spectrometer that supports HCL (like UPRtek MK350S, Everfine 300BW-H…), then you can measure the light directly at the eyes. If you don’t have such a device, then you can just measure the standard lux – but do remember to measure at the eye level as vertical lux, then check with the datasheet or ask your supplier for the m-DER value of the luminaire, and the m-EDI is the product of vertical lux and m-DER. The fact that should be recognized is that m-EDI is an illuminance metric, but it describes the non-visual effect based on the melanopic response and features.

m-EDI = Vertical lux * m-DER

What level of m-EDI should be obtained or limited?

The m-EDI is a key metric to define if your lighting environment gains a strong, enough or weak circadian stimulus, then to obtain the wellbeing lighting target. CIE (Technica Note 015:2023), WELL Building (v2) and DIN (67600) define the recommended m-EDI levels for different scenes, and the latest white paper of “Lighting Design for Health, Wellbeing and Quality of Light, A Holistic Approach to Integrative Lighting” from IALD also summarizes the recommended this human centric lighting metric, as listed below.

m-EDI versus EML

Both m-EDI and EML measure the non-visual lux to the eyes and they are telling about the same story, the difference is that the calculations are based on different references. m-EDI is related to m-DER, which is the comparison between the measured light and the natural light (D65 spectrum); and EML is related to M/P, which is the comparison between the measured light and illuminant E (equivalent spectrum).

What should I choose for different m-EDI designs?

The key to choosing the lighting products for different m-EDI designs is highly related to choosing the correct m-DER products. At the same level of lux (vertical), a high m-DER luminaire will get high m-EDI for alertness, productivity and wakefulness, and a low m-DER luminaire will get low m-EDI for relaxation, cozy and sleepy. Following the listed m-EDI 250lx, 10lx and 1lx above, here are the calculations and references.

m-EDI > 250 lx

With the screen and closing to the window, m-EDI is easy to achieve 250 lx since the screen is positioned close to the eyes, and the natural light brings very strong illuminance even when facing away from the window. Thus, the artificial lights do not need to contribute too much extra m-EDI.

But when there is only artificial light, m-EDI 250 lx is somehow challenging since the vertical illuminance (E_v) is generally 50% - 70% horizontal illuminance (E_h), based on the typical measurement in a standard office with panel lights, and a standard 4000K LED is typical 0.6 m-DER, assuming the environment follows EN 12464-1 of E_h 500 lx, then the final m-EDI is only 150 lx, at most 210 lx.

To improve the m-EDI, we need to achieve either higher lux or higher m-DER. While, increasing lux is about whether to increase the lights/power, which increases power consumption, or increase the efficacy of the light, which means more lumen output, then more glare will be generated with meaningless extra lumen. Thus, increasing the m-DER is an ideal solution to maintain the same power and output to gain better m-EDI.

Taking Yuji Lighting Well24 Day series as an example, with its specific spectrum design, the m-DER is 38% higher than a standard LED with the same color temperature of 4000K, thus, when achieving the same 500 horizontal lux level, Yuji Lighting Well24 Day series can give 208 – 291 lux m-EDI with different lighting designs, this is meaningful when applying the certifications like WELL Building – with proper lighting design, Well24 Day series 4000K light can get at least 1 point and is feasible to get full 3 points; but the standard 4000K challenges the lighting design – it is only possible to get 1 point at the most, and is risky to get nothing if the lighting design is not suitable.

m-EDI < 10 lx

The m-EDI should be limited in the evening (like starting from 7:00 PM) or in a relaxed environment. A warm color temperature should be considered for this, and 2700K is usually chosen for such environments. The m-DER of a 2700K LED (80 CRI) light is typical at 0.38, which means the illuminance to the eyes should be limited below 10/0.38 = 26 lx. But in actual situations, the light is ideally limited to an even lower level because there can be other light sources like the TV, fireplace,…increase m-EDI.

Therefore, we need to choose the light with as low m-DER as possible. Furthermore,  low m-DER light is also meaningful to maintain sufficient visual brightness lux since people are not asleep in this scene but would read, talk, eat...Taking Yuji Lighting Well24 Nite series 2200K as an example, whose m-DER is 0.27 and is 29% lower than a standard 2700K LED light. We can understand it in this way as an example: When achieving 30 vertical lux to the eyes, Yuji Lighting Well24 2200K has 8.1 m-EDI “non-visual lux” which is effective in circadian stimulus therein this 30 “visual lux”, while the standard 2700K LED will have 11.4 m-EDI “non-visual lux” then exceed the target of < 10 lx.

m-EDI < 1 lx

At night time, especially during sleep, the ideal m-EDI is zero, while the proposed < 1 lx means when light is necessary, like getting up at midnight, the m-EDI should be controlled very carefully to not disturb the ongoing drowsiness but the light should still be bright enough to see clearly to aoivd the risk of being tripped or too dark to keep the body balance at night, especially for the elderly.

Currently, 2700K LED light is still widely used as a night light, but when the 2700K light is calculated to achieve 1 lx m-EDI, the vertical lux is only 2.6 lx – too weak to see things clearly, especially such lights are always installed under bed or in the corner, then the light is even weaker to get into the eyes.

Therefore, the ideal m-DER should be much lower than the m-DER of the standard 2700K LED of 0.38. Taking Yuji Lighting FlameWarm light as an example, with the super low m-DER of 0.04, the FlameWarm light can be 9.5x more powerful and brighter to illuminate 9.5x larger area for sufficient brightness without disturbing the drowsiness.

With such low m-DER, there can be more options for lighting design than under-bed lighting. Even for a desk lamp or floor lamp at the bedside, with a distance of 50cm and 20 vertical lux, the FlameWarm light is only 0.8 lx m-EDI, whereas the standard 2700K LED lighting will be 7.6 lx m-EDI. Apparently the low m-DER FlameWarm light is more flexible to use as wider illumination for generating enough brightness and keeping the circadian safety simultaneously.

Make good use of the metric of m-EDI

We always talk about human centric lighting but few companies give the quantitative instructions of “how to judge whether the lighting is human centric or not?” even if the relevant standard and organizations already indicate the guidance. The definition of m-EDI is useful metric and Yuji Lighting is in depth involved in human centric lighting products, this article aims to help lighting designers identify and design the correct lighting for human wellbeing, while, m-EDI is not the only "signal" to judge, we will share more in the future newsletters, welcome to subscribe and discuss with us.