Lighting industry has started using some metrics to evaluate the human centric lighting especially in the circadian lighting application. We are going to look at two metrics today: one is Equivalent Melanopic Lux (EML) , the other is Circadian Stimulus(CS). The EML model was proposed by Lukas et al. and the CS model was developed by the Lighting Research Center of RPI.
Human eyes can response to light in both vision and beyond vision biology. With light we can images and information, shape and color ,feel the brightness, contrast and perception of space. Light is also biology with the light we exposure impact our attention, inner body clock, alertness, hormones, fatigue and Circadian Rhythm. Human eyes can respond to both visual an non visual stimulus at the same time. Light is the main stimulus that helps the circadian clock, and thus circadian rhythms, keep a synchronized rhythm with the 24-hour day. This impacts the functioning of the circadian system and quality of sleep. Disruption of circadian rhythm has been linked with obesity, diabetes, depression and metabolic disorders.
Our eyes have five type of receptors( Rods, S-Cones,M-Cones,L-Cones, and melanopsin-containing intrinsically photosensitive Retinal Ganglion Cells- ipRGCs – the non-image- forming photoreceptors of the eyes.) that will be response to retinal irradiance.
Each retina photoreceptor has its own unique responsive curve with different spectrum sensitivity along the light spectrum. The frequency at which the melanopic response is most efficient is 480nm (or 490nm according to some others).
The EML is a measurement of light’s effects on the circadian cycle , and it is measured 1.2 meter above the floor (assumed eyelevel for a desk-based worker), and its measured vertically. This is a departure from lighting for visual needs were the surface where a task is conducted or the target to be seen is considered. The amount of light on the vertical plane will depend entirely on the way that light is delivered into the space, as a combination of direct light from a luminaire, reflected light from walls and ceilings – and perhaps a contribution from windows.
This unit EML was proposed by Lucas and others (Lucas et al., “Measuring and using light in the melanopsin age.” Trends in Neuroscience, Jan 2014). The authors provided a toolbox which for a desired spectrum derives equivalent “α-opic” lux for each of the five photoreceptors in the eye. The authors selected scaling constants such that each of the values would be identical to each other and the standard definition of lux for a light spectrum of perfectly uniform energy (CIE Standard Illuminant E). Given a spectrum of light, each equivalent α-opic lux is related to each other by a constant-Melanopic Ratio. the The International WELL Building Institute (IWBI) have spreadsheets to aid in this calculation. With knowing specific light’s spectrum power distribution, you can calculate the constant-Melanopic Ratio, then the EML by multiply the ratio with measured or modeled lux.
The CS model is also using spectral power distribution rather than simply the visual intensity and also consider the light entering the eye at the vertical plane. The circadian stimulus model has two separate weighting functions, depending on the characteristics of the light. Warm light (<3500K) would use the “warm” weighting function and cool light (>3500K) would use the “cool” weighting model. The Circadian Stimulus (CS) model’s “cool” weighting function (blue) has a peak sensitivity at 465nm, while its “warm” weighting function (yellow) has a peak sensitivity of 485 nm. The Melanopic lux weighting function (black) used by WELL has a peak sensitivity of 490 nm.
The LRC also published a web version CS calculator with more robust and flexible functionality to help lighting professionals select light sources and light levels that will increase the potential for circadian-effective light exposure in architectural spaces. You can also download the offline Excel version calculator.
So far in WELL V2 pilot 2021, IWBI accepts both metrics for Circadian Lighting Design. From light design perspective, since both models measurements are taking on the 1.2m vertical plane, Designer can consider whether desktop luminaire, overhead lighting can be added to improve the CS or EML score. Glare, brightness perception and psychological effects of light should also be carefully addressed, and when considered in combination with visual and non-visual effects, lighting design can become more “holistic” .
