Circadian Lighting - Melanopic Flux, M/P Ratio, and Melanopic Daylight Efficacy Ratio (Mel, M/P, MDER)

The subfield of circadian lighting in human-centric lighting is gradually being understood and mastered, and lighting test reports display the circadian lighting parameters of concern: Melanopic Flux, M/P Ratio, Melanopic Daylight Efficacy Ratio (MDER), and spectral energy distribution at 1nm intervals in the 380-780nm range.

The three parameters of circadian lighting can be calculated from the spectral energy distribution Φ_e(λ) at 1nm intervals. By inputting the spectral data into the CIE S 026 alpha-opic Toolbox calculation tool, the corresponding data can be obtained.

Melanopic Flux

Definition:

Represents the luminous flux of light emitted by a light source that stimulates the melanopic receptors in the human body, reflecting the potential ability of light to regulate the human biological clock.

Calculation Method:

Melanopic Flux (abbreviated as: Mel). It is obtained by integrating the spectral power distribution weighted by the melanopic sensitivity curve S_mel(λ) (the ipRGC response curve is the melanopic sensitivity curve). Generally speaking, the higher the melanopic flux, the higher the melanopic illuminance that the luminaire can achieve, directly acting on ipRGC cells, thereby affecting melatonin synthesis.

M/P Ratio

Definition:

This ratio characterizes how much melanopic-sensitive component is contained in the spectrum. It is the ratio of melanopic flux to luminous flux, used to evaluate the comprehensive impact of light on human physiological rhythms and visual functions.

Calculation Method:

(Reference: Appendix L1, Page 79 of Well V2 version of the American Building Standard)

Where:

• Melanopic function prefactor: 832lm/W
• Maximum efficacy coefficient of photopic spectral V(λ): 683lm/W
• Φ_e(λ): Spectral curve of the light source under test
• V(λ): Photopic spectral curve
• S_mel(λ): Melanopic sensitivity curve (ipRGC response curve)

In the S026 standard, a similar characterization quantity is defined, called α-opic luminous radiation efficiency (ELR). For the melanopic sensitivity curve, it is denoted as K_mel,v, and its calculation formula is:

Its unit is mW/lm. The relationship between K_mel,v and M/P Ratio is:

The higher the M/P Ratio, the more effectively the same luminous flux can suppress melatonin synthesis. Generally, situations requiring alertness often require luminaires with higher M/P Ratio, such as: learning and working environments.

Melanopic Daylight Efficacy Ratio (MDER)

Definition:

Measures the ability of lighting to simulate the effect of natural daylight on human circadian rhythm regulation, reflecting the ratio of the melanopic flux of a light source to the melanopic flux of standard daylight with the same illuminance.

CIE selects standard daylight D65 as the reference spectrum. According to the formula

Calculate the melanopic luminous radiation efficiency of D65:

Table A.1 in the S026 standard has its calculated value, which is 1.326mW/lm. The full English name of MDER is Melanopic Daylight(D65) Efficacy Ratio, and its calculation formula is:

Where: the numerator represents the ability of the light source under test to suppress melatonin, and the denominator represents the ability of standard illuminant D65 to suppress melatonin.

The relationship between MDER and M/P is linear, as shown in the following calculation formula:

If the MDER ratio is greater than 1, it means that compared to standard illuminant D65, the light source under test has a stronger ability to suppress melatonin, and vice versa.

MDER can be directly converted with building lighting illuminance. At this time, the commonly used parameter for circadian lighting is Melanopic Equivalent Daylight(D65) Illuminance (abbreviated as: MEDI, denoted as E_mel,D65). In building lighting with specific circadian requirements, whether to suppress or not suppress melatonin, the required illuminance under standard illuminant D65 can be specified. Lighting designers can directly calculate how much illuminance luminaires need to be selected to meet lighting standard requirements using MDER, based on comparing the melatonin suppression ability of the light source with that of D65. The calculation formula is:

At this point, the practical significance of MDER becomes very clear. Generally, buildings need to be compared with natural daylight. If the same alerting effect is to be achieved, for products with MDER greater than 1, lower illuminance can be used, and vice versa, higher illuminance is required.

References:

• [1] Kang Yuzhu. Concepts and Calculations of Circadian Lighting Characterization Parameters[J]. China Lighting Appliances, 2021, 1(1)
• [2] CIE S 026/E:2018 Standard "System for Metrology of Optical Radiation for ipRGC-Influenced Responses to Light"