What is the Melanopic Ratio?

Photopic vision refers to what we see during the day and in a well-lit space, the visual impact of light. Rods and cones in the human eye receive light and send signals to the brain. Learn more about how the human eye sees light and color in this article. 

Melanopic Vision

In the past 20 years, researchers have discovered other photosensitive cells in the eye—cells not involved in visual processing. In the retina, scattered above the color- and shadow-sensitive rods and cones are the retinal ganglion cells (RCG), also called photosensitive retinal ganglion cells (ipRGCs). The ipRGCs form the main neural pathway to the brain’s hypothalamus region, the location of our circadian clock, the mechanism that aligns our internal body cycles—our circadian rhythms.

To align those rhythms, as light enters the eyes it fires a photosensitive protein called melanopsin in each ipRGC. In a multistep process (shown in the image below), melanopsin suppresses the sleep-inducing hormone melatonin during the day and gradually ramps up melatonin production as light fades into night. Why? Because melanopsin responds most to the primary colors of daylight—blue and cyan. Thus, the human eye responds to light as a visual perception stimulus, but also triggers our bodies to respond hormonally by releasing melatonin. What’s more, the RGCs control circadian effects even in the visually impaired.

The Melanopic Ratio^* (MR)—which is also called the Melanopic/Photopic ratio, M/P ratio, or MP ratio—quantifies the effect of light on circadian rhythm: how much a light source stimulates the human circadian system. Basically, the higher the MR ratio of a light source, the stronger the circadian effect it has, and therefore, the more strongly it promotes alertness and wakefulness.

The image below compares human photopic (visual) and melanopic (circadian) response curves, where the peak of the curve represents the eye’s peak response to light. Visually, the human eye responds most strongly to green or greenish-yellow light. Biologically, blue light elicits the most powerful response, suppressing melatonin production and promoting wakefulness.

Melanopic Ratio of Standard LED Light Sources

Candles, as we used for lighting in the past, have a color coordinated temperature (CCT) of around 1900 K, which is quite a warm light. More recently, LEDs have made it possible to fine-tune the CCT and color spectrum of light bulbs and luminaires, as shown in the table below.  

Artificial light sources can be designed at many different brightness levels (lux) and with different CCTs. Warmer light bulbs have a CCT around 2200 K and a low MR. They have fewer blue/cyan wavelengths and are best suited for nighttime relaxation and preparation for sleep.

By contrast, a light source with a cool CCT around 6500 K would tend to have a high MR with more blue/cyan, suppressing melatonin production. It would be ideal for daytime use and supporting alertness and productivity in schools, factories, offices, control rooms, etc. The graphs below compare the color profile (SPD) of different light sources.

The following illustration demonstrates how the light sources with different CCTs might look to a typical human viewer. “Cool” blue at 6500 K, and “warm” yellow-orange as light moves into the 3000 K range and below. The illustration includes correlated CIE coordinates. Note that the terminology for human perception of color temperature as “warm” (low CCT) and “cool” (high CCT) light is the opposite of actual measurements of black-body color temperature. For black-body radiators, higher numbers mean higher temperatures (degrees Kelvin).

For the purposes of this discussion, any mention of warm and cool light refers to the perceptual CCT, not black-body temperature. Learn more about color coordinated temperature (CCT) and black body radiators.

* NOTE: The Melanopic Ratio is also commonly referred to as the melanopic/Photopic ratio or “M/P” ratio in the industry. CIE uses the term “MR” ratio. The melanopic/photopic ratio M/P should not be confused with mesopic luminance, also sometimes referred to as the “M/P ratio,” which describes outdoor lighting in relation to human night vision. In these articles, we primarily use MR for the melanopic ratio to minimize confusion.

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