Characterising the melanopic/spectral microclimate of indoor spaces

Today, people spend 90\% or more of their lives indoors. It is therefore imperative that their health and well-being is considered a priority in architectural design. Daylight, its availability and spectral characteristics have recently taken a more prominent role in design. Today, daylight design is trans-disciplinary and must not only consider energy performance, visual task and comfort but also matters of health and well-being. The discovery of the intrinsically photosensitive Retina Ganglion Cell (ipRGC) and subsequent findings suggest a much wider role for daylight in delivering healthy buildings. The human circadian system is known to be responsible for the temporal regulation, over approximately 24 hours, of a range of physiological processes and drives the rhythms of body temperature, hormone secretion and metabolism. Published research on circadian physiology has demonstrated that the impact of light on the human circadian system depends on its intensity, spectrum, spatial distribution, timing, duration, pattern and photic history. The primary environmental stimulus to this system is light exposure to the eye. The internal human body clock requires environmental cues to synchronise to the day/night solar cycle on a regular basis so as to avoid imbalances in the circadian system that have been shown to have negative health and well-being consequences.A considerable body of research is now providing insights to the many potential implications of good and bad lighting design decisions, but much of this has focused on visual physiology and non-image forming (NIF) issues. What is less often considered is the nature of the spectral microclimate, i.e. that environment whose specific characteristics resolve to directly influence the spectral properties of light which, in turn, affect the occupants' circadian system and the entrainment potential of a typical microclimate.Research has begun to identify the most influential design parameters on the circadian entrainment potential of a microclimate. Most findings are based on computer simulations and those that have studied the effects under real daylight conditions are very few and limited. As a result, very little is known about the specific spectral nature of daylight in real settings.This research undertook to study the effect of a specific range of design variables on circadian entrainment under real daylight conditions. The research developed and employed a novel monitoring system consisting of five spectrometers (four internal views and one external) to measure the spectral environment of a scale model using four internal wall colour finishes from four view perspectives. A dataset of 2,689 reading sets was generated to represent the modelled scenario under 11 sky conditions for each hour of the day, between 08:00 and 17:00, in four view directions (1,584 individual configurations) and under three possible sun conditions. The analysis of the dataset involved both the results of monitoring and an assessment of the metrics available. In addition to existing and widely used metrics, a number of additional relationships were proposed to support both analysis and interpretation. It was found that the use of more than one metric was typically needed to make more revealing interpretations of the data. It was also found that even with a relatively modest number of variables, the spectral interaction between daylight and the modelled space was complex and, at times, challenging to represent.Under daylight conditions, variations in sky type resulted in only modest changes to melanopic and photopic spectral irradiance. It was shown that, in the absence of the direct solar component, there were only modest changes to the melanopic content of daylight, decreasing as cloud cover increased. The direct solar component produced a very different result, dominating in both intensity and spectral character but also relatively static in its spectral properties. These characteristics translated directly into the internal microclimate where other variables played a more significant role in modifying the spectral properties. Of these, the chosen wall colours were the most impactful, exemplified by their respective spectral reflectance profiles. These, together with the total area of wall available to reflect irradiance and the extent to which the window was visible from each respective sensor, had a defining influence on the measured spectral profiles.