Human Centric Lighting (HCL) and Circadian Parameters

Human Centric Lighting (HCL) and Circadian Parameters

For hundreds of thousands of years, humans grew up under natural light which varied its intensity and colour temperature based on the time of day/night (sunlight, moon light, starlight). The human’s circadian rhythm adapted to this variable light and created the feelings of alertness, sleep, relaxation and activity. Figure 1 shows the variability of sunlight in both colour temperature and intensity throughout the day.

Figure 1: Variation of sunlight throughout the day

With the advent of electric lighting by Edison in 1879 and later, the emergence of fluorescent lighting and LED lighting in the 20th and 21st century, humans are confined to live under unvarying light while indoors. In today’s technological society, in some countries, humans spend about 90% of their daylight time indoors Ref 1,2. The absence of tunability in intensity and colour temperature of the basic artificial light sources could potentially have an impact on human health. The impact could be in the form of disturbing human’s biological rhythms Ref 3 (feeling of fatigue when one needs to work and insomnia when one needs to sleep). Further studies have shown that the impact could be more serious and could lead to breast cancer and circadian rhythm sleep disorders Ref 4,5.

The circadian rhythms are set by the suprachismatic nucleus (SCN) inside the brain that acts as a clock. The intrinsically photosensitive Retinal Ganglion Cells (ipRGCs) are photoreceptors that receive light for non-visual applications and reside in the retina. ipRGCs are part of the 5 photo-receptors, the others being 3 cone photo-receptors and also the rhodopic photo-receptor. The ipRGCs contain melanopsin which absorbs light and sends signals to SCN to set the circadian clock. The melanopsin sensitivity to light is described by the action spectrum C(l) which has a peak at 490 nm and is heavily biased towards the blue slide of the spectrum. Figure 2 plots the melanopsin sensitivity C(l) along with the photopic sensitivity V(l) which has a maximum  at 555 nm. 

Figure 2:  Comparing sensitivity functions of Melanopic and Photopic action spectra  (Ref 3)

Human-Centric Lighting (HCL) requires lighting system designs with a certain specific set of biological, visual and behavioral responses in mind. HCL systems are usually tunable in colour temperature and intensity to suit a specific setting and provoke a specific response such as alertness, focus, relaxation, sleep, communication, etc.

In order to quantify the interaction between melanopic sensitivity C(l) and Spectral Power Distribution S(l), CIE has established certain new standards in their CIE S026 document (Ref 6). Although CIE has standardized the melanopic radiant flux which is defined as the radiant flux weighed by the melanopic action spectrum, other metrics are required to quantify the ratio of the melanopic radiant flux to photopic radiant flux. The Melanopic Efficacy of Luminous Radiation (MELR) is the new parameter defined by the CIE-026. It is defined by equation 1 Ref 3.

                                    Where the numerator is the melanopic radiant flux and the denominator is the product of the photopic radiant flux and the maximum Luminous Efficacy of Radiation (LER). The value of Km is 683 Lumen/watt which makes the unit of MELR as Watt/Lumen. In practice MELR is expressed in terms of mWatt/Lumen by multiplying the numerator of equation 1 by 1000. MELR quantifies the amount of Melanopic power relative to visible light and higher MELR values represents more blue in the spectrum.

                                    Another factor defined by CIE S026 is the ratio of MELR of the light source to the MELR of a standard light source which is the CIE D65 (6500 K Daylight source). This new parameter is called Melanopic Daylight Efficacy Ratio (MDER) and is represented by equation 2 below Ref 3.

                                    Since the MELR of D65 is constant and is equal to 1.326 mWatt/lumen, MDER and MLER are related by equation 3 and one is able to calculate one if the other is known Ref 3,

                                    One can not characterize the melanopic effect of light only by spectral distribution of the source as MLER and MDER parameters are derived from. In reality the melanopic effect of light not only depends on the shape of spectrum, but also on the total amount of energy falling on the outer surface of the eye Ref 3. For this reason, CIE S026 has defined a new parameter called Melanopic Equivalent Daylight Illuminance (MEDI).  If the melanopic radiant flux (Unit of Watts/m2) is divided by the MELR of D65 (Watt/Lumen), one would obtain MEDI in units of Lumen/m2 or Lux.  Equation 4 shows the relation Ref 3.

                                    There have been some recommendations by scientists working in the field regarding the adequate MEDI required at different times of the day (Ref 7). It is imperative to note that the measurement of MEDI should be done at the plane of the eye while in a sitting position. The height of the eye from the ground should be at approximately 1.2 m. Figure 3 shows the scenario

                                    Figure 3: The arrow shows the direction of Melanopic Lux measurement

                                    Illuminance measurements are done perpendicular to the plane that is being illuminated. In the case of the eye, we are generally standing up or sitting down, and the surface of the eye is therefore considered a vertical plane and the sensor needs to be held horizontally in the position of the eye measuring vertical illuminance. Recommendations by the scientists from the University of Manchester are the following Ref 7:

                                    •       Daytime MEDI ≥ 250 Lux

                                    •        Evening MEDI  ≤ 10 Lux

                                    •        Sleep time as low as possible,  ≤ 1 Lux

                                    Another standard that quantifies the circadian light has been designed by Lighting Research Center (LRC) formerly of the Rensselaer Institute and now part of Mount Sinai school of medicine. It is called the Circadian Stimulus (CS) (Ref 8). In this scale a parameter called Circadian Light is defined which is normalized and quantifies the amount of circadian light (CLA 2.0) received by the eye. CLA 2.0 is a 2021 modification of CLA from a 2005 model. CLA had a fixed value of 1000 for CIE illuminant A (Incandescent lamp at 2864 K) in 2005 model but for the 2021 model, CLA 2.0's value has changed to 813. The model has two states and compares the amount of yellow and blue light intensity. Depending on which one is bigger, the model uses different expressions to calculate the CLA 2.0. The expression involves the use of the melanopic action spectrum corrected for crystalline lens transmittance, photopic action spectrum, s-cone-opic action spectra, rhodopic action spectrum and macular pigment transmittance, etc. (Ref 8). Once CLA 2.0 has been calculated, one can easily calculate the CS using the following formula,

                                    Where t is time duration in hours and f is the spatial distribution factor which could have a value of either 2, 1 or 0.5. Nominally, CS is measured with t=1 and f=1. CS is measured on a scale of 0.1 to 0.7 where for example 0.5 signifies a melatonin suppression of 50% in one hour. Figure 4 demonstrates the variation of percent melatonin suppression with CLA 2.0

                                       Figure 4: Variation of percent melatonin suppression with CLA 2.0

                                    Allied Scientific Pro offers different models of the Lighting Passport spectrometer (Standard Pro, Essence pro) which can measure all these new circadian parameters as devised by CIE’s S026 standard and the LRC’s 2021 model which contains CLA 2.0. For more information about these spectrometers, please refer to 


                                    By monitoring the circadian parameters, the Lighting Passport spectrometer can help provide healthy, beneficial and circadian friendly light to every environment such as schools, hospitals, residentials, offices, etc.


                                    1- Human-Centric Lighting: Foundational Considerations and a Five-Step Design Process, K.Houser and T. Esposito, Frontiers in Neurology, January 2021.

                                    2- Human-Centric Lighting, Stan Walerczyk online presentation,

                                    3- Fundamental Spectral Boundaries of Circadian Tunability, J. Cerpentier, IEEE Photonics journal, Vol. 13, No 4. August 2021.

                                    4- Effects of artificial light at night on human health: A literature review of observational and experimental studies applied to exposure assessment, Y. Cho, Chronobiol. Int. vol. 32, no. 9, 2015

                                    5- Early life exposure to artificial light at night affects the physiological condition: An experimental study on the ecophysiology of free-living nestling songbirds, “Environ. Pollut., vol, 218, 2016.

                                    6- CIE System for Metrology of Optical Radiation for ipRGC-influenced Responses to Light. CIE S 026/E:2018

                                    7- Recommendations for daytime, evening, and nighttime indoor light exposure to best support physiology, sleep, and wakefulness in healthy adults, T Brown, PLOS Biology, march 17, 2022

                                    8- Modelling Circadian Phototransduction: Quantitative Predictions of Psychophysical Data, M.Rea, Frontiers in Neuroscience, Volume 15, February 2021.

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