Healthy Lighting: Equivalent Melanopic Lux (EML) and Melanopic Ratio - WELL Building Standard V2 Requirements

Healthy lighting is an important development direction in the lighting industry. People no longer simply consider the lighting function of light, but also pay attention to the comfort and health of light. The International WELL Building Institute (IWBI) has developed lighting guidelines as part of its WELL Building Standard, aiming to quantify and standardize certain aspects of worker health in the workplace. The WELL Building Standard ensures that lighting design regulates circadian rhythms, thereby regulating sleep, to improve the health of building occupants. This article provides a detailed introduction to two important key indicators of healthy lighting: Equivalent Melanopic Lux (EML) and Melanopic Ratio (M/P Ratio), as well as WELL Building Standard V2 requirements and calculation methods for workplace lighting, compliant with CIE S 026:2018 standard requirements. Equivalent Melanopic Lux (EML) and Melanopic Ratio (M/P Ratio) are core parameters for evaluating healthy lighting performance in the WELL Building Standard.

Part 1: What is Equivalent Melanopic Lux (EML)? - WELL Building Standard Healthy Lighting Evaluation Indicator

Equivalent Melanopic Lux (EML) is a photometric parameter used to quantify the degree of stimulation of light sources on melanopic light response (definition cited from WELL Building Standard V1 Appendix Units and Measurements), compliant with CIE S 026:2018 standard requirements. The EML value is weighted according to the response of intrinsically photosensitive Retinal Ganglion Cells (ipRGCs) to light, converting the spectral stimulation of light sources to quantitatively describe the biological effects of light on humans, providing support for circadian rhythm health. Equivalent Melanopic Lux (EML) is one of the core indicators for evaluating healthy lighting performance in the WELL Building Standard, compliant with CIE S 026:2018 standard requirements.

Light with higher EML increases alertness and promotes wakefulness, suitable for daytime work environments; light with lower EML promotes melatonin secretion and reduces alertness, suitable for nighttime rest environments. Equivalent Melanopic Lux (EML) and melanopic illuminance are important evaluation indicators for healthy lighting and circadian lighting design, compliant with WELL Building Standard V2 requirements, helping designers choose appropriate lighting solutions based on different usage scenarios and times. By measuring Equivalent Melanopic Lux (EML), the impact of light sources on melatonin secretion can be evaluated, thereby optimizing lighting design to support human circadian rhythm health, compliant with CIE S 026:2018 standard requirements.

Part 2: WELL Building Standard V2 Requirements for Workplace Lighting

WELL Building Standard V2 has clear requirements for workplace lighting to ensure that lighting design in workplaces can support employee health and productivity, compliant with CIE S 026:2018 standard requirements. According to WELL Building Standard V2 requirements:

• Vertical illuminance at all workstations (including daylight): Should reach 180 EML or above daily from 09:00 to 13:00, ensuring sufficient melanopic illuminance in daytime work environments to increase alertness and work efficiency, compliant with WELL Building Standard V2 requirements.
• Artificial lighting vertical illuminance at all workstations: Should reach 240 EML or above, ensuring that artificial lighting can also provide sufficient melanopic illuminance in the absence of natural light, supporting employee circadian rhythm health, compliant with WELL Building Standard V2 requirements.

These requirements reflect the WELL Building Standard's emphasis on healthy lighting, quantifying Equivalent Melanopic Lux (EML) to ensure that lighting design can effectively regulate circadian rhythms and improve the health and work efficiency of building occupants, compliant with CIE S 026:2018 standard requirements.

Part 3: How to Calculate Melanopic Ratio? - WELL Building Standard Calculation Method

Regarding the calculation of melanopic illuminance, the concept of Melanopic Ratio (M/P Ratio) is introduced here, based on the conversion of ordinary visual lighting parameters (lx, lux) to Equivalent Melanopic Lux (EML), compliant with CIE S 026:2018 standard requirements. Melanopic Ratio (M/P Ratio) is an important parameter in healthy lighting design, used to convert traditional visual illuminance to melanopic illuminance, helping designers evaluate the impact of light sources on circadian rhythms and melatonin secretion. In the WELL Building Standard, Melanopic Ratio (M/P Ratio) is a key indicator for evaluating healthy lighting performance of light sources, compliant with CIE S 026:2018 standard requirements.

Melanopic sensitivity curve and visual sensitivity curve are different spectral response curves. The melanopic sensitivity curve reflects the response of ipRGCs to light of different wavelengths, with a peak around 480nm (blue light region); the visual sensitivity curve (V(λ) curve) reflects the visual response of the human eye to light of different wavelengths, with a peak around 555nm (green light region). The difference between these two curves leads to different Melanopic Ratios (M/P Ratios) for different light sources, compliant with CIE S 026:2018 standard requirements. By measuring Melanopic Ratio (M/P Ratio), the impact of light sources on Equivalent Melanopic Lux (EML) and melanopic illuminance can be accurately evaluated, compliant with WELL Building Standard V2 requirements.

Method 1: Rough Conversion Method (Based on Light Source Type) - WELL Building Standard V2 Appendix L1

WELL Building Standard V2 provides a table of approximate conversion ratios for different light sources (formula calculation shown in the figure below, cited from WELL Building Standard V2 Appendix L1), compliant with CIE S 026:2018 standard requirements. This table divides light sources into several major categories and provides rough Melanopic Ratio (M/P Ratio) conversion values, suitable for preliminary evaluation and quick calculation of Equivalent Melanopic Lux (EML). WELL Building Standard V2 Appendix L1 specifies reference values of Melanopic Ratio (M/P Ratio) for different light source types in detail, compliant with CIE S 026:2018 standard requirements.

For example, if an incandescent lamp has an illuminance of 200 lx in a certain space, according to the Melanopic Ratio (M/P Ratio) (0.54) provided in WELL Building Standard V2 Appendix L1, the melanopic illuminance at this time is 200×0.54=108 EML. This conversion method is simple and easy to use, but has limited accuracy, because even for the same type of light source with similar color temperature, if their spectral distributions are different, the Equivalent Melanopic Lux (EML) and melanopic illuminance values should also be different, compliant with CIE S 026:2018 standard requirements. To obtain more accurate Equivalent Melanopic Lux (EML) and melanopic illuminance calculation results, the precise spectral conversion method should be used to calculate Melanopic Ratio (M/P Ratio), compliant with WELL Building Standard V2 requirements.

Method 2: Precise Spectral Conversion Method (M/P Ratio) - WELL Building Standard Recommended Method

When more precise Equivalent Melanopic Lux (EML) and melanopic illuminance calculation is needed, the precise spectral conversion method must be used, compliant with CIE S 026:2018 standard requirements. This method requires measuring the relative intensity of each wavelength of the sample and using specified formulas to calculate the accurate Melanopic Ratio (M/P Ratio, Melanopic/Photopic Ratio) through weighted calculation. Melanopic Ratio (M/P Ratio) is the ratio of Equivalent Melanopic Lux (EML) to visual illuminance, which can accurately reflect the impact of light source spectral distribution on Equivalent Melanopic Lux (EML) and melanopic illuminance. The WELL Building Standard recommends using the precise spectral conversion method to calculate Melanopic Ratio (M/P Ratio) to obtain more accurate Equivalent Melanopic Lux (EML) evaluation results, compliant with CIE S 026:2018 standard requirements.

The precise spectral conversion method considers the complete spectral distribution of the light source, so it can provide more accurate Equivalent Melanopic Lux (EML) and melanopic illuminance calculation results, compliant with CIE S 026:2018 standard requirements. This method is particularly suitable for scenarios that require precise evaluation of the impact of light sources on circadian rhythms and melatonin secretion, such as medical lighting, office lighting and other healthy lighting applications. By precisely measuring Melanopic Ratio (M/P Ratio), Equivalent Melanopic Lux (EML) can be accurately calculated, thereby evaluating whether light sources meet WELL Building Standard V2 requirements.

Summary

Healthy lighting is an important development direction in the lighting industry, and the WELL Building Standard provides scientific evaluation indicators and normative requirements for healthy lighting design, compliant with CIE S 026:2018 standard requirements. Equivalent Melanopic Lux (EML) and Melanopic Ratio (M/P Ratio) are two important key indicators of healthy lighting, used to quantify the impact of light sources on circadian rhythms and melatonin secretion. WELL Building Standard V2 has clear requirements for workplace lighting: vertical illuminance at all workstations (including daylight) should reach 180 EML or above daily from 09:00 to 13:00, and artificial lighting vertical illuminance at all workstations should reach 240 EML or above. These requirements reflect the WELL Building Standard's emphasis on Equivalent Melanopic Lux (EML) and melanopic illuminance, compliant with CIE S 026:2018 standard requirements.

Equivalent Melanopic Lux (EML) and melanopic illuminance calculation can adopt two methods: the rough conversion method provides approximate Melanopic Ratio (M/P Ratio) based on light source types, suitable for preliminary evaluation; the precise spectral conversion method (Melanopic Ratio M/P Ratio) calculates accurate Equivalent Melanopic Lux (EML) ratios by measuring complete spectral distribution, suitable for scenarios requiring precise evaluation, compliant with CIE S 026:2018 standard requirements. Both methods have their advantages, and designers can choose the appropriate calculation method according to actual needs. The WELL Building Standard recommends using the precise spectral conversion method to calculate Melanopic Ratio (M/P Ratio) to obtain more accurate Equivalent Melanopic Lux (EML) evaluation results, compliant with CIE S 026:2018 standard requirements.

As a professional LED lighting testing equipment manufacturer, Yiming Technology deeply understands the importance of healthy lighting and the WELL Building Standard, and has developed the SPM5000 series integrating sphere spectrometer testing system, which can accurately measure all parameters required for healthy lighting, including Equivalent Melanopic Lux (EML), Melanopic Ratio (M/P Ratio), spectral power distribution, etc., and generate test reports that comply with WELL Building Standard V2 requirements, compliant with CIE S 026:2018 standard requirements. Yiming Technology's SPM5000 series testing system fully complies with WELL Building Standard V2 requirements, helping enterprises accurately evaluate the Equivalent Melanopic Lux (EML) and melanopic illuminance performance of light sources, providing reliable basis for healthy lighting design. Yiming Technology is committed to providing professional healthy lighting testing services and WELL Building Standard testing solutions for customers, jointly promoting the development of the healthy lighting industry.

IV. Frequently Asked Questions (FAQ)

1. What is the difference between EML and M/P Ratio?

Answer: EML and M/P Ratio are two different concepts in healthy lighting:

• EML (Equivalent Melanopic Lux): An absolute value representing the degree of stimulation of light sources on melanopic light response, unit: lux, used to evaluate the impact of light sources on circadian rhythms, compliant with CIE S 026:2018 standard requirements
• M/P Ratio (Melanopic Ratio): A ratio representing the ratio of melanopic efficacy to photopic efficacy, used to convert visual illuminance to melanopic illuminance, compliant with WELL Building Standard V2 requirements

Relationship: EML = Visual Illuminance × M/P Ratio. M/P Ratio is a key parameter for calculating EML.

2. Why does WELL Building Standard require 180 EML and 240 EML?

Answer: WELL Building Standard V2 requirements for workplace lighting are based on scientific research:

• 180 EML (including daylight): Ensures sufficient melanopic illuminance in daytime work environments to increase alertness and work efficiency, supporting human circadian rhythm health
• 240 EML (artificial lighting): Ensures that artificial lighting can also provide sufficient melanopic illuminance in the absence of natural light, supporting employee circadian rhythm health

These requirements are compliant with CIE S 026:2018 standard requirements, aiming to ensure that lighting design can effectively regulate circadian rhythms by quantifying Equivalent Melanopic Lux (EML).

3. What is the difference between rough conversion and precise spectral conversion methods?

Answer: Main differences between the two methods:

• Rough conversion method: Provides approximate M/P Ratio values based on light source types (such as incandescent lamps, LEDs, etc.), simple and easy to use but with limited accuracy, suitable for preliminary evaluation, compliant with WELL Building Standard V2 Appendix L1 requirements
• Precise spectral conversion method: Calculates accurate M/P Ratio values by measuring complete spectral distribution, high accuracy but requires professional equipment, suitable for scenarios requiring precise evaluation, compliant with CIE S 026:2018 standard requirements

The WELL Building Standard recommends using the precise spectral conversion method to calculate M/P Ratio to obtain more accurate Equivalent Melanopic Lux (EML) evaluation results.

4. How to choose healthy lighting testing equipment?

Answer: When choosing healthy lighting testing equipment, consider:

• Support for CIE S 026:2018 standard (EML and M/P Ratio calculation)
• Spectral measurement range 380-780nm (covering visible light range)
• Spectral resolution ≤5nm (ensuring measurement accuracy)
• Ability to measure spectral power distribution (SPD)
• Automatic calculation of EML and M/P Ratio (compliant with WELL Building Standard V2 requirements)
• Generate test reports compliant with WELL Building Standard V2 requirements

Yiming Technology SPM5000 series integrating sphere spectrometer testing system fully meets the above requirements.

5. What are the testing conditions for healthy lighting?

Answer: Healthy lighting testing should be conducted under the following conditions:

• Within ±5% of rated voltage
• Stable operating state (30 minutes warm-up)
• Ambient temperature 25℃±5℃
• No external light interference
• Test distance compliant with standard requirements
• Measurement per CIE S 026:2018 standard

These conditions ensure the accuracy and repeatability of test results, compliant with WELL Building Standard V2 requirements.

6. What happens if products do not meet WELL Building Standard requirements?

Answer: If products do not meet WELL Building Standard V2 requirements (EML below 180 or 240), they will not be able to obtain WELL Building Standard certification, affecting the WELL certification score of projects. Enterprises need to improve product design, increase Equivalent Melanopic Lux (EML) levels, and retest for verification. By using testing equipment compliant with CIE S 026:2018 standard, the healthy lighting performance of products can be accurately evaluated to ensure compliance with WELL Building Standard V2 requirements.

V. Glossary

Term	English	Definition
EML	Equivalent Melanopic Lux	Equivalent melanopic illuminance used to quantify the degree of stimulation of light sources on melanopic light response, unit: lux, compliant with CIE S 026:2018 standard requirements
M/P Ratio	Melanopic Ratio	Ratio of melanopic efficacy to photopic efficacy, used to convert visual illuminance to melanopic illuminance, compliant with WELL Building Standard V2 requirements
ipRGCs	Intrinsically Photosensitive Retinal Ganglion Cells	Light-sensitive special retinal cells that participate in circadian rhythm regulation
Melanopic Sensitivity Curve	Melanopic Sensitivity Curve	Response curve of ipRGCs to light of different wavelengths, peak around 480nm (blue light region), compliant with CIE S 026:2018 standard requirements
Visual Sensitivity Curve	Visual Sensitivity Curve (V(λ))	Response curve of human eye to light of different wavelengths, peak around 555nm (green light region)
Circadian Rhythm	Circadian Rhythm	Human 24-hour biological clock regulating sleep-wake cycles, affected by light, healthy lighting aims to support circadian rhythm health
Melatonin	Melatonin	Hormone secreted by the pineal gland regulating sleep, low EML light promotes melatonin secretion, high EML light inhibits melatonin secretion
WELL Building Standard	WELL Building Standard	Building standard developed by International WELL Building Institute (IWBI), focusing on the impact of building environment on human health, V2 version requires 180-240 EML for workplace lighting
CIE S 026:2018	CIE S 026:2018	Healthy lighting standard published by International Commission on Illumination (CIE), specifying calculation methods for Equivalent Melanopic Lux (EML) and M/P Ratio
Healthy Lighting	Healthy Lighting	Lighting design that improves occupant health and well-being by regulating circadian rhythms, also known as circadian lighting or human-centric lighting