Assessing melanopic equivalent daylight illuminance in office spaces using a simplified approach for predominantly cloudy climates

Daylight is essential for circadian entrainment, yet current daylighting metrics and guidance do not address this aspect. Latest recommendations for light exposure suggest a daytime melanopic equivalent daylight illuminance (EDI) of 250 lx at the eye. This paper investigates the feasibility of these recommendations using daylight data from Watford, England and computer modelling in example offices achieving the minimum and high levels of recommendation in EN17037. In predominantly cloudy climates, it is impractical to require a specific melanopic EDI every day of the year. Instead, it is more feasible to aim for a certain number of daily hours on a given proportion of annual days. This paper evaluates a metric of achieving four hours with 250 lx melanopic EDI on 90% of annual days. Daylight may be sufficient for circadian entrainment in spaces that meet the high level of recommendation in EN17037 and if facing unobstructed windows. However, facing large windows can cause glare. It is easier to achieve 250 lx melanopic EDI over any four hours throughout the day than just in the morning, although this may not guarantee adequate circadian entrainment. Balancing daylight provision targets and daylight levels for circadian stimulation while limiting glare from daylight is challenging.