Do high CRI LED lamps have a lower M/P ratio?

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Do high CRI LED lamps have a lower M/P ratio?

With the proliferation of LED lamps, there has been quite a bit of concern over the potential negative effects of the blue light wavelengths produced by LEDs. One of the most tangible and commonly discussed aspects of blue light is its potential to impact the circadian rhythm and affect one's quality of sleep.

Our 95 CRI, flicker-free light bulbs have been very popular among our customers and a commonly asked question is whether or not they contain blue light, and if the high CRI rating is beneficial for limiting blue light exposure.

We ran some calculations on our products' light spectrum and compared their M/P ratio values to other competing products with lower color rendering to see how they stack up.

What is the M/P ratio?

A recently developed metric called the M/P ratio is gaining momentum as an effective way to measure the amount of blue light energy a light source contains. M/P is short for melanopic lux / photopic lux, and it attempts to characterize a light source's potential to affect circadian rhythms. In essence, the M/P ratio describes the ratio between the amount of light energy that promotes alertness (melanopic curve) versus the amount of light energy that produces the perception of brightness (photopic curve).

The photopic curve may sound technical, but it is actually the same curve used in calculating the luminous output values (lumens) that characterize light bulb brightness. Generally, brightness is "good" - it helps us see things visually and is the reason we use light bulbs in the first place.

On the other hand, the melanopic curve is less about the overall brightness of a light source, and more about the relative amounts of blue light energy in the light source. As you would expect, this curve is centered closer to blue light energy wavelengths (see chart below).

It may help to think of the M/P ratio as a way to describe the number of "brightness lumens" vs "alertness lumens." For example, an M/P ratio of 0.50 suggests that for every 100 brightness lumens emitted by the bulb, 50 alertness lumens are emitted. If you're looking to limit the amount of circadian impact, a lower "alertness lumens" value would be preferred, so it would follow that a lower M/P ratio would also be desired.

Calculation Results

We tested and extracted the spectral power distribution (SPD) data from three LED lamps with CRI values of 82, 91 and 97, as well as a halogen lamp. All lamps had a correlated color temperature value of approximately 3000K.

Lamp 1 - Brand X (82 CRI)

CCT: 2976K
CRI / R9: 82 / 13
M/P ratio: 0.513

Lamp 2 - Brand Y (91 CRI)

CCT: 2947K
CRI / R9: 91 / 56
M/P ratio: 0.546

Lamp 3 - Waveform Lighting CENTRIC HOME A19 (97 CRI)

CCT: 2987K
CRI / R9: 97 / 86
M/P ratio: 0.548

Lamp 4 - Halogen Lamp (100 CRI)

CCT: 3000K
CRI / R9: 100 / 100
M/P ratio: 0.581

Our results show that as CRI increases, the M/P ratio increases as well, suggesting that high CRI LED lamps may actually have a stronger circadian impact than their lower CRI counterparts.

Why do high CRI lamps have a stronger circadian impact?

Are you surprised by these results? We were, too! After all, we think of high CRI as almost always a good thing from a spectral perspective, so we similarly would expect high CRI light sources to also be good for limiting circadian impact.

As it turns out, however, the M/P ratio does not take any color rendering aspects of a light source into account. It is, ultimately a mathematical equation that simply compares the amount of brightness-producing light energy to alertness-producing light energy. If a light source is really effective at producing light energy that falls under the photopic curve, but has a relatively low energy output under the melanopic curve, that alone will give it a low M/P ratio.

Seen from this angle, our results start to make more sense. As we've explored before, a high CRI LED is actually not as efficient at producing perceived brightness, with one of the primary reasons being that high CRI LED sources emit less yellow light and produce more blue, cyan, red and deep-red wavelengths.

Below is an extreme but illustrative example of how a very low CRI light source can result in a very low M/P ratio. In the chart below, we see a 590 nm amber LED source, compared against the melanopic curve (blue) and photopic curve (green). Most of the amber LED's light energy falls under the photopic curve (green) and very little falls under the melanopic curve (blue), resulting in a very low M/P ratio of 0.117.

It should go without saying that an amber LED has an awful CRI and would not be suitable for general indoor lighting. Strictly speaking, however, it may be an effective way to limit circadian cycle impacts owing to its very low M/P ratio. Intuitively, we can also confirm this by verifying that very little light energy is produced by the amber LED in the region under the melanopic (blue) curve.

The next graph shows a halogen lamp's spectrum charted against the same melanopic and photopic curves. Unlike the amber LED, the spectrum is very smooth and gradual. The amount of energy increases as wavelength increases from the violets, blues, greens, yellows and reds. Overall, the distribution is dominated by orange and red wavelengths (which is consistent with what we visually observe in a halogen lamp's warm and yellowish hue).

What you will also notice, however, is that there is an abundance of light energy under the melanopic (blue) curve. We also see, compared to the amber LED, the amount of yellow light energy is a bit lower, but the amount of deep orange and red wavelength energy is very high. The result is that there is a higher amount of melanopic light energy, and a lower amount of photopic light energy, resulting in a much higher M/P ratio of 0.581.

Another way to think about this is from the perspective of narrow vs full spectrum light. As we saw above, a narrow spectrum amber LED's color rendering could be improved by increasing the width of the spectrum. By increasing the width, we would need to add more red wavelength energy, as well as green, cyan and blue energy. On a relative basis, this would be reducing the photopic wavelength energy, and in absolute terms, increasing the amount of melanopic wavelength energy. As you can see, the increase in M/P ratio values is almost inevitable as a spectrum becomes more full or complete.

Final Thoughts

High color rendering and full spectrum light sources are often associated with positive health benefits, but our testing results show that they may be more impactful on circadian cycles than lower CRI lamps.

Keep in mind that M/P ratio values are very helpful, but other factors such as time of day, proximity to, and duration of exposure to the light source arguably has a much larger impact on circadian cycles.

One analogy we frequently use is caffeine content in coffee. The M/P ratio can be thought of as being equivalent to the caffeine content in milligrams per cup of coffee. If you are concerned about caffeine's impact on sleep quality, it can be helpful to drink coffee with lower caffeine content, but you'll also know that when and how many cups you drink the coffee matters as well.


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