What is m-DER and how does it work?

m-DER is the abbreviation of Melanopic Daylight Efficacy Ratio, which is defined by the CIE and introduced in theCIE S 026/E:2018document. Also called the Melanopic DER, this metric is invented for evaluating the circadian effect compared to natural light.

The m-DER is a mathematical calculation and the formula of is complicated, however, we can simplify it for easier understanding.

Generally, the m-DER can be considered as the circadian effect comparison between your artificial light and the natural light, the higher the m-DER result is, the more circadian effect your light is.

What is the circadian effect?

After research, it is known that the work efficiency is related to the sunlight at different times of a day. People are less productive in the early morning and dusk, but are more productive in midday, this phenomenon is called the circadian effect. There is a metric called M/P ratio for evaluating the circadian effect, the higher the M/P ratio is, the more stimulatory effect will be, and the more excitement or focus you will gain.

What is “natural light”?

Natural light is generally recognized as the sunlight, but to evaluate the circadian effect quantificationally, we should choose a standard and consistent reference, hence, the CIE D65 (Daylight 6500K) is selected to be this reference.

Why do we still need m-DER beside the MP/ratio?

During the use of the M/P ratio, itis found that it lacks an objective comparison. The M/P ratio cannot intuitively show the actual impact of light on the biological clock. Even if a person is skilled in this art, like a lighting designer, cannot use the M/P ratio to quickly design indoor lighting that matches the biological clock. The m-DER is to solve this by applying the ratio of circadian effect which helps with designing the lighting simply and intuitively.

When m-DER = 1

This means the circadian effect of your lights is equivalent to the real sunlight. It is an ideal situation, but difficult to achieve by the common CCTs for general lighting.

When m-DER < 1

This means you need more lights to achieve an equivalent circadian effect to the sunlight, but there might be challenges:

1.      More lights mean more costs.

2.      More lights mean more lux, and more lux means the difficulty to balance the demanded and actual light quantity and distribution.

When m-DER > 1

Generally speaking, the various CCTs of common LED will not be greater than 1 unless significantly increasing the color temperature, but a high CCT will always cause uncomfortableness and increase the risk of high blue light hazards.After calculation, for the standard LED technology, the CCT should be higher than 7500K to achieve the m-DER > 1, but 7500K is much higher than 4000K which is widely accepted and used in the workplace.

High m-DER benefits for the lighting design

The m-DER is only for evaluating the work or study environment considering the concentrating effect, therefore it is not related to the design for sleep or relaxed lighting, after understanding this, there are the following steps to utilize the m-DER metric during the lighting design:

Step 1. Confirm the “visual lux” you need.

During the lighting design for a specific space, the designers always need to calculate and confirm the lux demand. For example, the illuminance in the office should be >500 lux according to the CIE 008/E-2001, here we take this 500 lux as the calculation factor for the next steps.

Step 2. Calculate the “circadian effect lux” you need.

Assuming the m-DER value of the light is 0.8, the calculated lux is >500, then the circadian effect lux you need is 500/0.8 = 625 lux, therefore, the actual visual lux you need is at least 625 lux than 500 lux.

Step3. Calculate how many lights you need.

With the calculated 625 lux and the necessary photometric data (IES/LDT file), designers can figure out how many lights should be used in the space. And the people who live in this space will benefit from the specific circadian effect design to help them concentrate to be more productive.

High m-DER is a key factor.

A high m-DER light can simplify the lighting design and keep a balance between the circadian effect and lux. We can simulate a comparison below when the CCT and circadian effect lux are the same, different m-DER values mean different lights.