[Human-Centered Lighting IV] What Actually Counts as Healthy Light?

For most of the day, the sun provides light at 6500 K, this blue-white light suppresses melatonin and keeps us awake. Melatonin is known as the 'sleep hormone,' sending signals to the body to relax and get ready to sleep. Without it, we stay more alert.

Both the presence and color of light affect our biological clock or circadian rhythm, an effect known as melanopic vision, which refers to the biological impact of light and how it influences melatonin production. Natural daylight is rich in blue light, which suppresses this hormone. As the day winds down, twilight takes on warmer orange and red tones, and the blue light gradually fades. In response, our bodies start producing more melatonin to help us sleep.

When our ancestors mastered fire, it brought more light in the evening and at night. However, because fire is warm-colored light, very similar to evening sunlight, it didn’t negatively affect humans' natural sleep patterns.

Photopic vision refers to our ability to see objects during the day or in well-lit spaces, basically how light affects our sight (see Figure 10). The Melanopic Ratio (MR, Melanopic/Photopic ratio, M/P ratio, or MP ratio) measures how light influences our circadian rhythm—that is, how much a light source stimulates our biological clock. Essentially, the higher the MR ratio, the stronger the circadian effect of the light, and the more it can boost alertness and wakefulness.

Figure 10 - Comparison of human photopic and melanopic response curves, with the peaks representing the eyes' peak response to light. Visually, the human eye responds most strongly to green or yellow-green light; biologically, blue light causes the strongest response, suppressing melatonin production and promoting alertness.

*Note: The Melanopic Ratio is commonly referred to in the industry as the Melanopic / Photopic ratio or “M/P” ratio, while the CIE uses the “MR” ratio. The M/P in melanopic / photopic ratio should not be confused with “mesopic luminance,” which is sometimes also called the “M/P ratio” and relates to outdoor lighting associated with human night vision. In this white paper, we use MR to represent the melanopic ratio to reduce confusion.

Candles, like those used for lighting in the past, have a CCT of around 1900K, giving off a very warm light. Recently, developments in LED technology have made it possible to fine-tune the CCT and spectrum of bulbs and fixtures (Table 1), and artificial light can come in different brightness levels (lux) and CCT. Warm light bulbs have a CCT of about 2200K and a low MR ratio, with less blue/green light, making them ideal for relaxing at night and preparing for sleep.

In contrast, light sources with a cool 6500K CCT tend to have a higher MR and more blue/green light, which can suppress melatonin production. They are suitable for daytime use in schools, factories, offices, control rooms, and other places, helping to stay alert, focused, and productive. Figure 11 compares the color configurations (SPD) of LEDs at different CCTs, and Figure 12 shows how the human eye perceives light sources with different CCTs.

Table 1 - CCT and MR of the LED demo light source

Figure 11 - Shows how LED light sources with three different CCTs customize their spectra to achieve the desired MR ratio

Figure 6 - The CIE 1931 color space maps all colors visible to the human eye. The numbers along the 'horseshoe-shaped' edge indicate the wavelengths of monochromatic light that define the boundary of the space. Inside the space, every perceivable color has coordinates called CIE x and CIE y, which also include the blackbody (Planck) locus coordinates. Tc (K) is the blackbody temperature, commonly referred to as CCT.

Figure 12 – How light of different CCTs appears to the human eye: 6500K looks like a 'cool' blue, while light in the 3000K range and below appears as 'warm' orange-yellow. Related to CIE coordinates.

As shown in Figures 12 and 6, the terms used to describe human perception of color temperature are 'warm' (low CCT) and 'cool' (high CCT), which is opposite to the actual measured color temperature of blackbody radiation sources. For blackbody sources, higher numbers mean higher temperatures (in Kelvin). In this white paper, unless stated otherwise, any reference to warm or cool light refers to perceived CCT, not the temperature of a blackbody source.

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