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Current and future challenges for evaluating and measuring daylight in Sweden

Updated: Jan 23, 2023


This synopsis is a summary of some observations and conclusions regarding the adequancy of building performance metrics for evaluating daylight, with a specific focus on Swedish conditions. The synopsis presents both current and future challenges of simulating daylight to fulfill the building regulations and green building certifications, and for providing adequate daylight and electric light levels in Sweden. It discusses the relation between simulated values and actual daylight levels in buildings, the non-visual effects of daylight and electrical lighting during winter in Sweden, as well as the need for including future weather conditions in current simulations. In addition, it discusses potential recommendations to mitigate these challenges, taking into consideration the current climate policy in Sweden, which aims to have zero net emissions of greenhouse gases by 2045 [1].


The motivation for writing this synopsis was triggered by three main questions that the authors' of this study often discuss with different stakeholders along the design process including architects, property owners, and specialists. These questions are:

- What are the light levels needed from both daylight and electric light in indoor spaces to fulfill our biological (non-visual) needs for light throughout the different periods in Sweden.

- Is there a correlation between simulated and onsite measured values in Sweden considering the constant fluctuati ons in weather conditions and extremly dark winters?

- Is it possible to design electric lighting and daylight harvesting systems using current wheather files and simulation methods in Sweden?


To investigate these questions, a 3 bedroom (4 rooms according to the Swedish classification system) apartment in Stockholm was evaluated through both computer simulations and onsite measurements. The apartment, home to one of the authors, offered valuable insights on the perceived daylight conditions during different times of the year, as well as the possibility for onsite measurements. The apartment is on the ground floor of a multi-storey residential building with a 60m distance to the surrounding context and approximately 11-25% window-to-wall ratio (WWR) in different rooms and 11-19% window to floor ratio (WFR). As a reference, the WFR values meet the minimum WFR (10%) present in Danish standards [11]. With this into account, the apartment can be considered well-daylit during most of the time of the year. This was validated by computer simulations as the apartment met all the requirements for the different certification systems, such as Miljöbyggnad 3.1 gold (Median Daylight Factor (DF) ≥ 1,3 %), BREEAM SE (Average DF 2.1% in Kitchens, and 1.6% in regularly occupied spaces). In addition to that, the apartment also met LEED 4.1 requirements and the pre-conditions in the WELL certification system (check appendix A for all requirements and results).

Onsite measurements during the winter months of November, December and January were carried out on-site under different sky conditions. In december, under an overcast sky at noon, the measured illuminance levels at the desk by the window (highlighted in yellow in the floor plan) was as low as 76-116 lux horizontally, and 76-86 lux vertically at eye level by the desk. Similar values were also observed during 23rd February at 09:00 am under an overcast sky, where illuminance levels reached 176 lux horizontally at the desk plane, and 162 lux at vertical eye level. However, much higher illuminance values were recorded on 18th February at 11:00 am under an overcast sky, as the illuminance levels reached 2560 Lux horizontally and 1900 lux vertically.

Even though that the frequency of these occurances were not recorded, as measurements were done when the observer perceived low light levels, the measured data showed that illuminance levels can vary greatly with different overcast skies, reaching indoor levels below 100 lux, despite sitting very close to an unobstructed window, facing south. This huge variation that the different overcast skies have is not at all considered now when simulating the Daylight Factor (DF) requirements to evaluate compliance with Swedish regulations, on the contrary, the simulations seem to be overestimating the daylight results in comparison to reality for both Swedish and international regulations.

In addition to this, in an attempt to understand a bit more about the varying conditions we have in Stockholm (latitude of 59 °N), a wearable light sensor (Lys) [19] was worn over the period of a year, as an empirical test to get more data by the author who occupied the apartment. The used sensor measured light levels from both daylight and electric light, analyzed their spectral composition, and then compared it with the light intake needed

to meet our circadian stimulus, based on an algorithm. The sensor then visualizes the results in a mobile application, indicating if you have or have not met your light intake, during the day, as well as showing if you have been exposed to low levels of light before going to sleep.

The results showed the challenges we experience at this latitude, as in December, regardless of the sky conditions, the "wake up" goal was never achieved, as light levels outdoor are not high enough. It was also observed that the office lighting did not compensate for the low daylight levels, even if the user sat by the window throughout the whole day. On the other hand, during summer it was hard to reach the sleeping goal, as daylight levels are still too high late in the evening. It is therefore important, especially during winter time, to actively seek to compensate the low light levels that we are exposed to during the mornings, for example by going for a longer walk on the way to the office, or taking a lunch break outdoors. This also shows the need to carefully design electric lighting, specially at these latitudes, to complement the lack of daylight, but even more important, for optimizing daylight in spaces where people sit for long periods of time while having a view out and more access to natural light.

Furthermore, simulations were done to calculate the equivalent melanopic lux (EML) from daylight in the investigated apartment using a software called ALFA. The results showed some discrepancies when repeating the same simulation with the same settings, which decreased the reliability to derive concrete conclusions, similar to the findings presented in a similar study [2]. Therefore the results from the EML simulations done in ALFA will not be discussed in this synopsis, but are presented in Appendix A.


Drastic changes in weather conditions during both winter and summer are already noticeable, which puts in question the current methods used to size HVAC systems. This was recorded by the European Environment Agency which shows the observed trend in annual temperature increase from 1960 to 2021, in addition to the predicted changes in annual temperature for two future scenarios.

These temperature variations were observed during the summer in 2022, as Europe experienced one of the hottest heat waves. In Sweden for example, Mariestad in Västergötland had the highest temperature in entire Sweden on June 26th. The figures in page 7 show the monthly average temperature during 2022, as well as the average temperature deviation from normal (1991-2020) during 2022 as recorded by the SMHI (Swedish Meteorological and Hydrological Institute) [18].


The illuminance measurements onsite carried out in the apartment resulted in surprisingly low illuminance values, despite that the apartment had considerably large glazed areas, and no close obstructions, and that the obeserver considers the apartment as a place with very generous daylight. This motivated the authors to compare digital daylight simulations with the onsite measurement, which showed that they were both very different. Along the study we identified that this can be due to several factors, including the weather files used, the fact that simulations can be overestimating the results in cases with high daylight levels, and that simulations in general have shown to be overestimating the iluminance values in comparison to reality. More indepth studies from other authors have also shown that we might be underestimating overheating potential, however all of these studies were not carried out within a Nordic context, and it is therefore a recommendation to conduct these studies as an outcome of this synopsis. In addition to that, post occupancy studies and onsite measurements are needed, as current simulations methods do not strongly reflect daylight behaviour in reality.

More research specifically focusing on the current and future weather data is needed to be able to have more comparable results between simulations and onsite measurements. This needs to be also reflected in the local standards in Sweden to push the industry to design better buildings, that can help the building industry and Sweden in meetings their climate goals for 2045. Climate based simulations with reference to daylight hours instead of occupancy hours are also recommended, similar to the requirements in the current Danish regulations.


We would like to thank Zahabia Gandhi for her contribution to this synopsis for carrying out all the simulations for different standards and green certification compliance, as well as of other investigated parameters. We would also like to thank Solemma for facilitating an educational license for Climate Studio and ALFA which were used to carry out all the simulations in this study.

Read the full report "Current and future challenges for evaluating and measuring daylight in Sweden" inside the final report, starting on page 54:


[1] Ministry of the Environment. (11 3 2021). Government Offices of Sweden. Retrieved from Sweden's climate policy framework:

[2] Pierson, C., Aarts, M., & Andersen, M. (2021). Validation of Spectral Simulation Tools for the Prediction of Indoor Daylight Exposure. CIE 2021 Midterm Meeting & Conference Light for Life-Living with Light.

[3] Levin, T. (2017). Daylighting in Environmentally certified buildings, subjective and objective assessment of MKB Greenhouse, Malmö, Sweden. Sweden: Lund University.

[4] Reinhart, C. (2017). Session-7: Daylight Performance Predictions. IBPSA Education

Webinar. Retrieved from

[5] Brembilla, E., Drosou, N., & Mardaljevic, J. (2022, May). Assessing daylight performance in use: A comparison between long term daylight measurements and simulations. Elsevier, Energy Buildings, 262. doi:

[6] Danny , H. (2007). Daylight and energy implications for CIE standard skies. Elsevier, Energy Conversion and Management, 48, 745-755. doi:

[7] Mardaljevic, J., & Christoffersen, J. (2016). ‘Climate connectivity’ in the daylight factor basis of building standards. Building and Environment. doi:10.1016/j.buildenv.2016.08.009

[8] SMHI. (2021, May 6). SMHI. Retrieved from New climate data files for calculating the energy performance of buildings:

[9] EN 17037

[10] LEED. (2022). USGBC. Retrieved from Daylight for Nordic Projects:

[11] Mathiasen, N., Frandsen, A. K., & Grønlund, L. (2022). Daylight conditions in housing–Its role and priority in Danish building regulations. Architecture, Structures and Construction, 23-37. doi:

[12] Bygningsreglementet. (2018). Bygningsreglementet. Retrieved from Light and visibility (§ 377 - § 384):

[13] C. Cuttle. A Guiding Light: The Intuitive Argument. The CIBSE Journal - Lighting

Supplement, pages 14{15, December 2012.


[15] European Environment Agency. (2022, June 15). European Environment Agency. Retrieved from Observed annual mean temperature trend from 1960 to 2021 (left panel) and projected 21st century temperature change under different SSP scenarios (right panels) in Europe:

[16] CIBSE. (2002). CIBSE. Retrieved from Weather data:

[17] Reinhart, C. (2018). Daylighting Handbook II. USA: Reinhart, Christoph.

[18] SMHI. (2022, July 30). SMHI. Retrieved from Juni 2022 - Högsommarvärmen kom till midsommar:


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