###### Abstract  
The evaluation of both visual and nonvisual effects from the spectral power distribution (SPD) of outdoor light is critical in lighting design. The dome-light SPD characteristically changes continuously depending on the seasonality, orientation, altitude of the sun or hour of the day. Traditional photonic parameters, such as the illuminance, luminance or correlated colour temperature (CCT), have been widely studied, but presently, there is no melanopic measurement or evaluation method. This article discusses the processes involved in establishing a simple method to determine the SPD of daylight and solar radiation over the skydome in a location to accurately account for the effects of both phototopic and circadian levels around a location. Once per month for one year, natural daylight was spectrally measured in the city of Zaragoza (Spain); radiometric and photometric characteristics were analysed by season; and circadian effects were calculated in terms of standard parameters described by the Commission International de l'Eclairage (CIE), factors recommended by normative and scientific backgrounds. Finally, we suggest that the best parameter is the melanopic versus photopic irradiance ratio, which achieves reliable results at simplifying and correlating calculations. daylight; circadian light; spectral power distribution +
Footnote †: journal: _Sciences_  
0000-0001-5996-3899-3  
The use of irradiance data makes luminance calculations more accurate, as it has been described [4; 5]. These general sky types represent a range of different sky conditions from overcast to cloudless with varying levels of direct sunlight. They are approximately illustrative of common sky settings with varying levels of cloudiness and turbidity, and they are commonly used in indoor lighting design software. Knowledge of the irradiance distribution of the sky is essential to achieve a good natural lighting design in indoor areas. It is important to know how the characteristics of the sky change in different periods of the year, depending on the climate of the specific place in which we want to conduct the measurements, the types of clouds, the light distribution, the illuminance, the geographical location, the time, etc. Daylight greatly influences the energy efficiency of a building. Despite the advances in artificial lighting, it is still difficult to reduce the energy consumption of large buildings. According to the Office Energy Saving and Efficiency Guide, 40% of the world's energy consumption is produced by buildings. A good estimate of daylight will help reduce electric light consumption. Using the universal method provided by the CIE, the luminance of the sky and thus daylight can be partially predicted. Currently, both irradiance and illuminance levels, mainly at vertical and inclined planes, from the sky are needed for proper building design [3; 5; 6; 7], but studies seeking to resolve the problem of finding artificial lighting with similar behaviour to the solar SPD have been partially developed. The correlated colour temperature (CCT) and the colour rendering index (CRI) are parameters used to visually characterize light and lighting strategies, but light reaching the eye level has two important objectives: first, this light is responsible for forming images; and second, this light is part of the exposure [8], meaning it is essential in the synchronization of the circadian system, the system in charge of setting our biological clock on time each day in accordance with the cycles of day and night (light-dark) [9]. Regulating the amount of light that reaches the corneal level and improving environmental conditions could have positive impacts on quality of life, wellbeing, and ageing-related concerns. These lighting projects, which consider the potential effects of light on people and optimize them to achieve the greatest wellbeing, are called human-centric lighting (HCL) projects.