Light as a biological essence or what HCL, which only changes the color tone of the illumination (CCT), cannot do

In recent years, the term “Human Centric Lighting” has come to the attention of architects, designers, luminaire manufacturers and customers in the context of lighting. Human Centric Lighting (HCL) or also “Integrative Lighting” as defined by ISO/CIE is a lighting principle that considers not only the visual but also the non-visual effects of light on people and positively influences their physiological and mental needs. It was designed to mimic natural daylight, which changes its intensity, spectral power distribution and alternate correlated color temperature (CCT) during the day and evening. Unfortunately, HCL is most often associated with LED luminaires, which only change their color (CCT) during the day, and with “Tunable White” lamps. However, in this article you will read why CCT is not the only variable we should focus on when designing interior lighting in the context of the biological effects of light.

What is the key when designing interior lighting

When designing lighting, it is important to focus on 4 basic parameters of light, following the example of nature: a) temporal pattern: timing and duration of light exposure, b) spectral power distribution (SPD) of a given light source, c) light intensity in radiometric and photometric units (illuminance, irradiance, luminous flux…) and d) spatial pattern in the room (see Fig. 1). By manipulating these parameters, not only visual effects can be influenced: visual perception, visual experience and comfort, and subjective sensation of light, but also the non-image forming (NIF) effects of vision, which, with an increasing number of scientific publications, are becoming crucial in relation to health, or more specifically to circadian rhythms, neuroendocrine responses (hormonal response regulated by the central nervous system) and human behavior (alertness, cognitive performance, vitality, mood…). Visual effects are primarily mediated by rods and cones, while NIF effects are mediated by intrinsically photosensitive retinal ganglion cells (ipRGCs). However, the 4 variables mentioned above do not contribute equally to NIF effects, and therefore some of them are not nearly as good indicators of the so-called biological potency of light.

Fig. 1: The 4 most important variables that should be considered when designing interior lighting: a) temporal pattern: timing and duration of light exposure, b) spectral power distribution (SPD) of the light source, c) light intensity, d) spatial pattern of light. The information in the figure was taken from the study (2).

Biological potency of light – NIF

The following 2 approaches are currently most used to quantify the biological potency of light. The first is the CIE S026 method based on the spectral response of photopigments in rods, cones and ipRGCs. Therefore, the α-opic Equivalent Daylight Illuminance (EDI) has been introduced. But for the biological potency, the response from the ipRGCs and thus mainly the following quantities: melanopic Equivalent Daylight Illuminance (mel-EDI) and possibly the Equivalent Melanopic Lux (EML) are crucial. These two quantities can be derived from the melanopic Daylight Efficacy Ratio (mel-DER). This approach is currently the only internationally accepted standard for characterizing the biological effects of light. While it is consistent for calculating the response of photoreceptors to a light signal, it no longer takes into account what happens to the transmitted signal at the level downstream from the retina, e.g., if mel-EDI is increased by 50%, we cannot say for sure whether melatonin expression is actually reduced by 50%, etc. In fact, the biological responses of the organism to light tend to have a sigmoidal dependence expressed in a log-linear diagram, see Fig. 2. The second approach deals with the suppression of melatonin synthesis at night using the Circadian Stimulus (CS) metric. However, melatonin suppression may not predict changes in other non-visual effects of light (increase in alertness, phase shift of circadian rhythm etc.) well enough. Hence, neither of these approaches is completely perfect and future work will need to find a quantification method that reflects all the non-visual effects of light.

Fig. 2: Sigmoidal dependence of biological response on quantity of light. The horizontal axis represents the amount of light in arbitrary units on a logarithmic scale, e.g., irradiance (W), illuminance (lux), mel-EDI (lux) or dose (J/m2). The vertical axis represents the biological response to light scaled linearly: suppression of melatonin synthesis, phase shift, subjective sleepiness etc. At very low and very high amounts of light (e.g., dose) there is only a very slight difference in biological response, the light stimulus is either below threshold or high near saturation. The diagram was taken from study (2).

People who spend most of their time indoors generally have a reduced contrast between day and night perception, due to reduced illuminance during the day and increased illuminance at night compared to our ancestors and people spending most of their time outdoors in daylight. The goal of HCL should therefore be to create an environment where the contrast between day and night is maintained. However, beyond that, the light spectrum itself (SPD) is key.

Human Centric Lighting (Tunable White) and its challenges

We are in a new era of HCL thanks to revolutionary advances in chronobiology. Until now, only changes in the correlated color temperature (CCT) and possibly the illumination during the active part of the day have occurred. CCT is readily available information that can be found in product specifications, describes how we perceive the color tone of light, and should only be applied to light sources that are formally white. We typically perceive light with a higher CCT as colder light, whereas light with a lower CCT we perceive as warmer light. However, CCT does not tell us anything about what the SPD of a given light source looks like; it is a one-dimensional reduction of SPD, which by itself cannot reliably predict the biological potency and health effects of light on humans. Many scientific studies (5-8) have compared the effects of light of different CCTs on cognitive performance, subjective satisfaction, mood and sleepiness of office workers or students in schools, even though the light sources lacked emission in the melanopic cyan wavelength region. In addition, a systematic review (9) summarized several other studies whose results are mixed and lack consistency. The problem is that luminaires with different spectral power distribution can have identical CCT based on its definition, as can be seen in the chromaticity diagram in Fig. 3. This is then the reason why the results of the studies are not consistent. Reporting SPD or CCT in combination with Duv is a solution to achieve replicability of studies and meta-analyses with carrying the conclusions into practice. Duv is the distance of the chromaticity coordinates of a given source from the nearest point on the Planckian locus, see Fig. 3.

Fig. 3: Cutout from the CIE 1976 (u’,v’) chromaticity diagram. The black lines represent constant CCT values. The Planckian locus (the red curve) represents the chromaticity coordinates of the blackbody radiation at different temperatures. Duv is the distance of the chromaticity coordinates of a given light source from the nearest point on the Planckian locus. The figure was taken from study (4).

The conclusion is that the correlated color temperature (CCT) is a measure of the visual perception of light colour and should immediately raise doubts whether it can be related to non-visual effects.  Conversely, a luminaire with an optimally designed SPD combined with the appropriate illuminance value at the right time is crucial in the evaluation of biological efficacy. During the day, the organism (except for nocturnal creatures) needs bright white light with a balanced spectrum, including short cyan wavelengths (ideally those corresponding to the absorption ≈ action spectrum of melanopsin, with a maximum around 480 nm). Such light provides the necessary biological signal for the organisms compared to dim light or light with a dip in the melanopic region used at the same time. Sunlight and LED light sources may have the same CCT value but completely different SPD (see Fig. 4), and thus completely different effects on the organism. The differences are particularly noticeable in the cyan and red regions of the spectrum. Lighting that is intended to support non-visual effects of vision in places where people are expected to be active, whether they will be performing cognitive or physical tasks (i.e., where they are not relaxing or sleeping), can be static with no change in SPD and intensity over the course of the day if the parameters are properly set to the daily optimum of natural sunlight (illuminance, CCT, and especially SPD).

Fig. 4: HCL based on Tunable White light sources and comparison of spectral power distribution of sun and LED light sources with CCT: a) 2700 K (left), b) 6500 K (right).

Full-spectrum light sources replicate natural daylight and are the ideal solution for indoor lighting during daytime activities to preserve human health and support the working vitality and mood of users. This is mainly because all effects of light on living organisms have not yet been investigated and any variations of artificial lighting compared to natural light indoors are not desirable for the long-term stay of users to avoid any negative impacts not yet recognized or investigated in detail. Among other things, this also offers significant potential for improving the activities and resulting outcomes of organizations.

Mgr. Tereza Ulrichová, Spectrasol

The studies on which the article is based:

  1. K. Houser and T. Esposito, ‘Human-Centric Lighting: Foundational Considerations and a Five-Step Design Process’, Frontiers in Neurology, vol. 12, p. 630553, Jan. 2021, doi: 10.3389/fneur.2021.630553.
  2. K. Houser, P. Boyce, J. Zeitzer, and M. Herf, ‘Human-centric lighting: Myth, magic or metaphor?’, Lighting Research & Technology, vol. 53, no. 2, pp. 97–118, Apr. 2021, doi: 10.1177/1477153520958448.
  3. T. Esposito and K. Houser, ‘Correlated color temperature is not a suitable proxy for the biological potency of light’, Sci Rep, vol. 12, no. 1, p. 20223, Nov. 2022, doi: 10.1038/s41598-022-21755-7.
  4. D. Durmus, ‘Correlated color temperature: Use and limitations’, Lighting Research & Technology, vol. 54, no. 4, pp. 363–375, Jun. 2022, doi: 10.1177/14771535211034330.
  5. R. Lasauskaite and C. Cajochen, ‘Influence of lighting color temperature on effort-related cardiac response’, Biological Psychology, vol. 132, pp. 64–70, Feb. 2018, doi: 10.1016/j.biopsycho.2017.11.005.
  6. Y. Yuan, G. Li, H. Ren, and W. Chen, ‘Effect of Light on Cognitive Function During a Stroop Task Using Functional Near-Infrared Spectroscopy’, Phenomics, vol. 1, no. 2, pp. 54–61, Apr. 2021, doi: 10.1007/s43657-021-00010-5.
  7. W. Luo, R. Kramer, M. Kompier, K. Smolders, Y. de Kort, and W. van Marken Lichtenbelt, ‘Effects of correlated color temperature of light on thermal comfort, thermophysiology and cognitive performance’, Building and Environment, vol. 231, p. 109944, Mar. 2023, doi: 10.1016/j.buildenv.2022.109944.
  8. R. Lasauskaite, M. Richter, and C. Cajochen, ‘Lighting color temperature impacts effort-related cardiovascular response to an auditory short-term memory task’, Journal of Environmental Psychology, vol. 87, p. 101976, May 2023, doi: 10.1016/j.jenvp.2023.101976.
  9. C. Wang et al., ‘How indoor environmental quality affects occupants’ cognitive functions: A systematic review’, Building and Environment, vol. 193, p. 107647, Apr. 2021, doi: 10.1016/j.buildenv.2021.107647.