Natural sunlight is crucial for our health and well-being. Despite this, we spend over 90% of our time indoors, where our primary source of light is artificial. The downside is that artificial light often lacks the beneficial effects of sunlight. In an ideal world, all artificial lighting should strive to mimic the rhythm and characteristics of natural sunlight as closely as possible. With Melanopic EDI, it becomes possible to compare the effects of artificial light on humans with those of natural daylight. But how does this work in practice?
Melanopic EDI (Melanopic Equivalent Daylight Illuminance) is a method used to explain how much a light source affects the circadian rhythm of humans. It is based on the idea that light not only impacts us visually but also non-visually, influencing our biological system. This was highlighted when researchers in the early 2000s discovered a previously unknown function of the human eye: it could decode daylight in a way that helps the body understand the time of day.
Visual and non-visual perception
Our eyes possess two crucial abilities - photopic vision and melanopic vision. Photopic vision is what most people recognize as "sight." It allows us to perceive colors, shapes, and motion, enabling us to see the world around us. On the other hand, melanopic vision cannot discern shapes and colors; in fact, it doesn't "see" in the traditional sense. It is primarily designed to respond to daylight and signal to our brain when it is day or night. Our melanopic vision plays a vital role in regulating our circadian rhythm, forming the basis for discussing the melanopic strength of light.
The melanopic strength of light
Several years ago, a group of researchers published a comprehensive publication outlining recommendations for Melanopic EDI at different times of the day. These recommendations emphasized the importance of high levels of Melanopic EDI during the day and minimal to none at night. Currently, there are no requirements for Melanopic EDI, leaving it up to individual producers to decide whether to report the Melanopic effect of their light sources. Despite the lack of mandatory regulations for designers and planners to design based on specific Melanopic EDI levels, the growing understanding of its significance highlights the importance of evaluating the light we are exposed to in supporting our circadian rhythm. By measuring or calculating the Melanopic strength, we can gain insights into how light impacts our biological system, with a closer look at Melanopic DER for a more comprehensive understanding.
What is Melanopic DER?
Melanopic DER or MDER (Melanopic Daylight Efficacy Ratio) reveals the impact of artificial light on human circadian rhythms compared to natural daylight. In essence, MDER is utilized to calculate the melanopic strength of a lighting solution and quantify its effect on the circadian rhythm. This can be visualized as:
Therefore, MDER can be described as a metric that indicates how effective an electric lighting solution is in influencing the circadian rhythm compared to natural daylight. Both melanopic EDI and melanopic DER are rooted in our melanopic vision. When comparing how our melanopic (non-visual) and photopic (visual) systems respond to light, it is evident that they react quite differently. The human eye is most sensitive to green or yellow-green wavelengths in terms of photopic vision, while our melanopic vision exhibits the strongest response to blue wavelengths. In essence, it is the bluish light that exerts the most significant impact on our circadian rhythm regulation, even though the blue color may not be visibly prominent to the eye.
In general, the higher the MDER measured for a light source, the more potent its effect on our circadian rhythm, as a high MDER equates to a significant presence of blue-rich light. Therefore, when exposed to electric light with a high MDER, we are likely to feel more alert and productive, as it triggers melanopsin in the eye just like sunlight does. Since daylight serves as the cornerstone of our circadian rhythm, it is the qualities of natural light that establish the framework for MDER.
The concept of daylight and the CIE D65 standard
Natural daylight varies greatly depending on whether it's a clear sunny day or a rainy day, as well as the time of day or year. That's why there exists a specific standard for daylight, ensuring that the melanopic strength of artificial light is calculated consistently for all. For this reason, the CIE D65 standard is chosen as the general reference for daylight, which practically represents daylight with a color temperature of 6500K. As the reference, daylight also has an MDER of 1. When the MDER for a light source is less than 1, it means that the melanopic efficiency of the light is lower than that of natural light. In other words, it affects the circadian rhythm less than daylight does at the same illuminance level. Conversely, if the MDER is higher than 1, it indicates that the melanopic efficiency is higher than that of natural light.
How is the melanopic effect of light measured?
The melanopic effect of a lighting solution can be measured in various ways, depending on the equipment available for use.
- Firstly, Melanopic EDI can be directly measured using a modern spectrometer. Simply conduct a standard measurement with the spectrometer and read the measured Melanopic EDI value directly from the device. If a spectrometer with this feature is not available, alternative methods can be utilized.
- Secondly, the melanopic effect of a light source can be measured using a spectrometer (without a melanopic function) and the calculation template CIE S026 toolbox. The CIE S026 toolbox, available from the International Commission on Illumination (CIE), assists in calculations and conversions related to the non-visual properties of light. In essence, this toolbox can be utilized to measure and quantify the spectral impact of light on human physiology and behavior.
- Thirdly, MEDI can be measured using a standard lux measurement and the light source's MDER. Knowing a light source's MDER allows for easy calculation of the lighting solution's MEDI, ensuring it meets both photopic and melanopic light requirements. To calculate MEDI in this manner, simply multiply the light source's photopic lux (E) by its melanopic efficiency (MDER), resulting in the equation: MEDI = E x MDER.
When measuring MEDI, it is crucial to consider where and how the measurements are taken. A key rule of thumb is to ensure measurements are done vertically and at eye level to obtain the most accurate results. Additionally, MEDI measurements are typically conducted at a relevant height based on whether one is sitting or standing in the position being assessed. This means that MEDI should be measured at vertical sitting eye level and/or vertical standing eye level, which could range from around 1.20 to 1.70 meters in height.
Understanding the melanopic vision
When light enters our eyes, the blue wavelengths in the light activate a light-sensitive protein known as melanopsin*, which signals to our brain that it is daytime and time to be active. Evolutionarily, our eyes have evolved to use these blue wavelengths in daylight to inform our bodies about the distinction between day and night. This synchronization allows our circadian rhythm to align with the natural rhythm between night and day. As evening approaches and night falls, darkness has historically prevailed in the natural cycle of the day. During the dark period of the day, our bodies produce the hormone melatonin, also known as the "dark hormone." Melatonin is not directly linked to sleep. While many take melatonin pills to aid in sleeping, there is actually no scientific evidence to suggest that it improves sleep. However, the melatonin produced by our bodies at night contributes to many essential and rejuvenating processes that occur within our bodies while we sleep.
Exposure to light at night quickly halts the production of melatonin. This is because light activates melanopsin in the eye, which suppresses the production of melatonin. Light at night can quickly disrupt the body's rejuvenation and cleansing processes. Biologically, our bodies struggle to comprehend the presence of significant light during nighttime, causing light at night to interfere with crucial bodily processes. The intriguing aspect is that the more light we receive during the day, and the more we respect darkness at night (maintaining this rhythm day by day), the more melatonin is typically produced. This enhances our ability to recharge and strengthens our circadian rhythm.
Similar to air and water, light is a fundamental part of our biological makeup, which should be considered in our indoor lighting – and this is where melanopic EDI comes into play, allowing us to monitor and regulate it.
* The protein melanopsin has given its name to melanopic vision and the corresponding measurement units and values.