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The Lighting Passport measurements and the WELL Building Circadian Lighting Standards

Light is one of the main drivers of the circadian system. Light enters the eyes and is transferred to the suprachiasmatic area of the brain which regulates different physiological rhythms throughout the tissues and organs in the body. It also affects hormone levels and the sleep-wake cycle (Reference 1). Although the image forming photo-receptors are rods and cones, intrinsically photosensitive Retinal Ganglion Cells (ipRGCs) are non-image forming photoreceptors that receive the light and pass the information to the brain. Humans and animals have internal clocks that synchronize physiological functions on roughly a 24hour cycle called the circadian rhythm. This cycle responds to a number of external stimuli that align the body clock to the solar day. Light is the most important of these stimuli and keeps the internal clock synchronized in a process known as the circadian photoentrainment (Reference 1). Through the action of ipRGCs, light of higher intensity and shorter wavelength promotes alertness and focus, while the absence or shortage of this light will put the body in a rest mode by producing melatonin. The biological effects of light on humans can be measured by Equivalent Melanopic Lux (EML) which is an alternate metric that is weighted to the circadian photoreceptors (ipRGCs) instead of cones and is used by the CIE (International Commission on Lighting) S026.

Figure 1 shows the location of ipRGCs in the retina.


Figure 1: ipRGCs are non-image forming photoreceptors and are located at the back of the eye. 

All forms of light and not only sunlight can contribute to circadian photoentrainment. Considering the fact that most people spend much of their working day indoors, insufficient illumination or improper lighting design can lead to a drift of circadian phase, especially if paired with inappropriate light exposure at night. Humans are very sensitive to light and under normal condition, light exposure in the late night or early morning will shift the circadian rhythms forward (Phase Advance) whereas exposure in the late afternoon or early night will shift the circadian rhythms backward (Phase Delay). To maintain optimal, properly synchronized circadian rhythms, the body requires periods of both brightness and darkness. Note that it is not only color temperature and intensity but also light spectrum that affects circadian entrainment.

The WELL Standard light features are shown in figure 2.  

Figure 2: The WELL Standard Light Features

The WELL Building Standard covers eleven features, all of which are important to consider. This article focuses on the circadian light recommendations by the WELL Building Standard.  The WELL Building Standard for light provides illumination guidelines that are intended to minimize disruption to the body’s circadian system and enhance productivity. It also supports good sleep quality and provides good visual acuity where needed.

The WELL Building Standard defines EML values for different parts of the day that are suitable for circadian photoentrainment. These EML values are listed below: 

Daytime EML >=250 lx
Evening time EML <= 10 lx
Night time EML <=1 lx

The Lighting Passport spectrometer from Allied Scientific Pro follows the international CIE S026 standards and provides measurements of Melanopic Equivalent Daylight Illuminance (MEDI) and Melanopic Daylight Efficacy Ratio (MDER). These parameters are not the same as EML and MP ratio as required by the WELL Standard. An article in the Illumination Engineering Society (IES) blogs discusses the differences between the MP ratio and MDER (Reference 2). The article explains that WELL standard uses a reference light source which is equal energy at all wavelength (CIE illuminant E) (Reference 3) as a reference whereas CIE S026 method uses the daylight CIE D65 light source at 6500K color temperature. The article calls the MP ratio used by WELL Standard as MP(3) and the MDER used by CIE S026 as MP(4) and provides a conversion factor between the two (Reference 2). 

MP(3)=1.1 MP(4) Eq (1)

In a previous blog (Reference 4) and an article by Cerpentier (Reference 5), expressions for Melanopic Efficacy for Luminous Radiation (MELR), Melanopic Daylight Efficacy Ratio (MDER) and Melanopic Equivalent Daylight Illuminance (MEDI) were defined. Using those equations and dividing MEDI by MDER, one gets 


Replacing the expression for MELR (source) from the previous blog (Ref) , one gets

 Since EML/MP(3) is also equal to Illuminance, it implies that if a linear relation is found between MDER and MP(3), the same linear relationship is also true between EML and MEDI. Therefore, 

EML=1.1 MEDI Eq (4)

Therefore, for inspectors who are measuring the WELL Standard for circadian light in buildings, measurement MEDI using the Lighting Passport and simply multiplying the MEDI reading by 1.1 should do the job. These WELL Building light factors are not currently in the Light Passport apps but will be soon integrated. Figure 3 show the circadian parameter output of the lighting passport as measured by SGM+ or SGE+ apps.

Figure 3: Melanopic EDI as provided by the Lighting Passport needs to be multiplied by 1.1 to get the EML value required by the WELL standard

It is to be noted that the circadian features have an extra cost that can be paid through the app.

For more information on the lighting passport models, refer to the following link:


1- The WELL building standard v1 with 2016 addenda.
5- Fundamental Spectral Boundaries of Circadian Tunability, J. Cerpentier, IEEE Photonics journal, Vol. 13, No 4. August 2021.

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