Artificial skylights in building design

Artificial skylights are electric lighting systems designed to reproduce the appearance and, in many cases, the biological effects of natural daylight in interior spaces that have limited or no access to real windows or roof glazing. Unlike conventional luminaires, they aim to recreate the visual character of the sky and sun, and increasingly to support the occupant’s circadian rhythm through the spectrum and intensity of the light they emit.

They are used where genuine daylight is difficult or impossible to introduce, such as basements, deep-plan floors, healthcare facilities, transport interchanges and retail interiors, and are one response to the growing interest in daylighting, biophilic design and human-centric lighting.

[edit] Background

Access to daylight has long been recognised as important to the quality of the built environment, and is addressed in areas such as daylighting, the WELL Building Standard and biophilic design. Daylight supports visual comfort, influences mood and helps regulate the body’s internal clock. However, many buildings contain spaces that cannot be adequately daylit because of their depth, orientation, use or location below ground.

Artificial skylights emerged as a way to provide some of the perceived benefits of daylight in these situations. Early decorative approaches used backlit images of the sky. More recent systems attempt to reproduce daylight more convincingly, both in appearance and in spectral quality.

[edit] How artificial skylights work

Contemporary artificial skylights generally fall into two broad categories.

• Optical scattering systems recreate the blue of the sky by scattering light in a way that mimics Rayleigh scattering in the atmosphere, often using a nanostructured diffusing layer to produce a realistic sky colour and an apparent sun. This approach was popularised by manufacturers such as CoeLux.
• LED spectral systems use arrays of tunable multi-channel LEDs, sometimes combined with optical layers that create an illusion of depth, so that the panel appears to open onto sky at infinity rather than being a flat surface on the ceiling. Manufacturers in this category include Innerscene and others.

Common technical features across systems include:

• Spectral tuning to approximate the colour and correlated colour temperature (CCT) of daylight, shifting from cooler, blue-rich light during the day to warmer tones in the evening.
• Depth or infinity effects, achieved optically so that the apparent light source seems far beyond the plane of the ceiling.
• UV-free output, since the biological benefits being targeted do not require ultraviolet radiation.
• Programmable control, allowing the light to follow a daily cycle or be adjusted to a schedule.

[edit] Circadian lighting and human health

A significant driver behind artificial skylights is the science of circadian lighting. In addition to the rods and cones used for vision, the human retina contains intrinsically photosensitive retinal ganglion cells (ipRGCs) containing the photopigment melanopsin, which are especially sensitive to blue-enriched light. These cells influence the timing of the circadian rhythm, alertness and the release of hormones such as melatonin.

Bright, blue-enriched light during the day and dim, warm light in the evening are associated with better-regulated sleep-wake cycles. To quantify this, the lighting industry uses metrics such as melanopic equivalent daylight illuminance (melanopic EDI), standardised by the International Commission on Illumination (CIE) in CIE S 026. Artificial skylights that vary their spectrum and intensity across the day are intended to deliver appropriate melanopic stimulus at the right times.

Research has linked daylight exposure in buildings to improvements in sleep quality and wellbeing among office workers, and daylight and views are frequently cited as desirable in healthcare and workplace environments. It should be noted that the long-term health outcomes of artificial daylight systems specifically are still an area of ongoing research, and manufacturers’ claims should be assessed against independent evidence.

[edit] Applications

Artificial skylights are specified in a range of settings, including:

• Healthcare — patient rooms, treatment areas, imaging suites and other windowless clinical spaces, where the appearance of daylight may support patient experience and staff wellbeing.
• Workplaces — deep-plan offices and basements without access to perimeter glazing.
• Retail and hospitality — creating a sense of openness in interior spaces.
• Transport and public buildings — concourses and waiting areas below grade.
• Residential — basements, bathrooms and internal rooms.

[edit] Specification considerations

When specifying artificial skylights, relevant factors include:

1. Visual realism — how convincingly the system reproduces the appearance of sky and sun, including apparent depth and viewing angle.
2. Spectral and circadian performance — CCT range, colour rendering (for example CRI or the IES TM-30 metrics), and melanopic EDI delivered at the occupant’s eye.
3. Controls and daily cycle — whether the system can automatically follow a daylight-like schedule and integrate with building controls.
4. Physical integration — panel depth, weight, mounting method and compatibility with ceiling systems, which affect installation.
5. Certification and compliance — electrical safety and regional certifications such as UL listing and CE marking.
6. Standards alignment — the contribution the system can make towards credits in schemes such as the WELL Building Standard, including feature L03 Circadian Lighting Design.

[edit] Standards and guidance

Artificial skylights intersect with several standards and guidance documents, including the WELL Building Standard v2 (particularly its circadian lighting features), CIE S 026 on the measurement of light for its effects on the circadian system, and guidance from the Illuminating Engineering Society (IES) on colour quality and light for human health. They also relate to broader daylighting standards such as EN 17037 Daylight in buildings, which sets out recommendations for daylight provision, view and sunlight exposure.

[edit] Limitations and considerations

Artificial skylights do not provide the full range of benefits of real daylight, such as genuine views out, solar heat gain where desired, or the free provision of light during daylight hours. They add electrical load and capital cost, and the strength of the wellbeing claims made for different products varies. As with any human-centric lighting intervention, specifiers are advised to evaluate independent performance data and to consider artificial skylights as one part of a wider lighting and daylighting strategy rather than a complete substitute for access to the outdoors.

[edit] Related articles on Designing Buildings

• Daylighting
• Biophilic design
• Circadian rhythm
• WELL Building Standard
• Human-centric lighting
• Lighting of buildings
• Colour rendering index
• EN 17037 Daylight in buildings

[edit] References

1. International WELL Building Institute — WELL Building Standard v2, feature L03 Circadian Lighting Design. https://www.wellcertified.com
2. CIE S 026/E:2018 — System for Metrology of Optical Radiation for ipRGC-Influenced Responses to Light, International Commission on Illumination. https://cie.co.at
3. Lucas, R.J. et al. (2014) — Measuring and using light in the melanopsin age, Trends in Neurosciences, 37(1), pp. 1–9. https://doi.org/10.1016/j.tins.2013.10.004
4. Boubekri, M. et al. (2014) — Impact of windows and daylight exposure on overall health and sleep quality of office workers: a case-control pilot study, Journal of Clinical Sleep Medicine, 10(6), pp. 603–611. https://doi.org/10.5664/jcsm.3780
5. EN 17037:2018 — Daylight in buildings, European Committee for Standardization.

Innerscene SPD Analyzer — free browser-based tool for calculating CCT, CRI and melanopic EDI (per CIE S 026) from spectral power distribution files. https://innerscene.com/tools/spd-analyzer