METRICS
Daylight is a complex phenomenon that can be evaluated in different ways, so there are a variety of metrics for assessing whether good daylighting is being achieved or not. Measuring daylighting can directly show whether visual needs will be met and further inform whether non-visual needs are likely to be supported. As with any rating scheme or legislation, the daylighting standard EN 17037 also requires the use of certain metrics to show that its performance targets are met. So, what are the options for measuring daylight?
KEY LEARNINGS
- Explain why the ‘glass to floor area ratio’ is not a suitable daylighting measure.
- Understand a range of simple daylighting metrics.
- Gain an appreciation of more advanced daylighting metrics.
Glass to floor area ratio, is this a metric?
Glass to floor area ratio is arguably the most commonly known ‘metric’ for daylight provision. Historically, it has been used in building regulations/codes. But is it really a metric?
Quick and simple calculation methods are useful tools – if they guide you to appropriate design decisions. If they don’t guide you to a reliable solution, why use them?
Glass to floor area ratio is less of a metric and more like a rule of thumb. It can guide you at a surface level but should not be used to justify whether a space has sufficient daylight.
Some countries have legislation that tries to ‘correct’ the percentage by taking into consideration shading on the window, or other variables. Despite this, the ratio is not proven to accurately represent daylight levels. A simple ratio does not account for variables such as window placement, orientation, external obstruction, glass specification, climate and more.
In the below scenario three rooms with the same glass to floor area ratio are shown side by side with their corresponding daylight factors. It is easy to see just how large the difference in light quantity can be between spaces all having the same glass to floor area ratios.
1 face window: Median DF 0.70%
2 facade windows: Median DF 1.38%
1 roof window, 1 facade window: Median DF 2.68%
1 face window: Median DF 0.70%
2 facade windows: Median DF 1.38%
1 roof window, 1 facade window: Median DF 2.68%
ILLUMINANCE-BASED METRICS
Illuminance Illuminance is the measure currently used by most performance indicators to determine daylight availability in the interior. Expressed in lux (lumens* per square metre, or lm/m2), it measures the amount of light received on a surface.
For the purposes of evaluating a design proposal, illuminance can be predicted by simulation tools. When assessing actual illuminance, it can be measured using a luxmeter.
When using a luxmeter to measure actual illuminance, it is important to keep in mind that the reading relates to a single moment in time. By contrast, when using metrics such as daylight autonomy (see below), illuminance levels need to be recorded regularly over a year to understand how they change over days and seasons.
This highlights the importance of simulation software in providing means to predict illuminance levels over time and further inform design choices.
Daylight factor
As a daylight availability metric, the daylight factor (sometimes referred to simply as DF) expresses the daylight available at a point on a work plane inside a room, as a percentage of the daylight available under unobstructed overcast sky conditions1.
The higher the daylight factor, the more daylight is available in the room.
The magnitude and distribution of the daylight factor within a space is affected by elements of the building and its design, including the following2:
- The size, distribution, location and transmission properties of the facade and roof windows.
- The size and configuration of the space.
- The reflective properties of the internal and external (surrounding) surfaces.
- The degree to which external structures obscure the view of the sky.
Daylight factor has some limitations, predominantly that its calculation is based solely on overcast sky conditions so does not consider the effect of direct sunlight and subsequently orientation of a building. In addition is does not typically take into account a building’s location or variations in sky luminance. However, it is possible to link daylight factor levels to occurrence of illuminance levels using data from weather files (more specifically the median external diffuse illuminance), as proposed in EN 17037.
Daylight factor = (Ei / Eo) x 100%
Daylight factors can be calculated for:
- A specific point/s within a room Some regulation uses daylight factor and specifies that the point measured should be a particular distance from glazing/depth of room (Sweden, Czech)
- A median for the room as a whole this largely replaced ‘average’ DF as the preferred DF metric with the introduction of EN 17037. As it ensures 50% of a room meets the target DF, it follows a closer logic to the more advanced compliance metric of spatial daylight autonomy.
- An average for the room as a whole When using average DF it is important to interpret the result to ensure good daylight provision throughout a room – a room with very high DF in a small portion of the room can bring up the average and become misleading to the quantity of light in a large percentage of the room
Median Daylight Factor
The median daylight factor is good indicator because it also informs on the spatial distribution of daylight, and thus allows you to evaluate that the required daylight levels are available in 50% of the room.
In practice, this means a particularly deep room with floor to ceiling windows on only one façade, may have met an average DF target but may not be possible to reach the median DF target without either increasing the floor to ceiling height (and subsequently the window height), or introducing windows from a second façade or roof.
The Daylight Visualizer report functionalities support the evaluation of EN 17037 requirements with daylight factor simulations, making it easy to determine what levels of illuminance (for 50% of daylight hours, 50% of the work plane) the median DF level correspond to.
Average Daylight Factor
Typically, rooms with an average daylight factor of 2% or more can be considered daylit, but electric lighting may still be needed to perform visual tasks. A room can be considered ‘strongly daylit’ when the average daylight factor is 5% or more, as electric lighting will most likely not be used during daytime3. Keep in mind daylight factor is the percentage of available daylight – so an average of just 2% of the available daylight externally needs to fall onto the work plane of a room for it to typically be considered ‘daylit’.
The diagram below shows the daylight factor being higher close to a window, while towards the back of the room electric lighting is required to perform tasks. This shows the need to consider daylight uniformity alongside daylight factor if you have particular activities occurring throughout the room (e.g. in a classroom).
The diagram below shows the daylight factor being higher close to a window, while towards the back of the room electric lighting is required to perform tasks. This shows the need to consider daylight uniformity alongside daylight factor if you have particular activities occurring throughout the room (e.g. in a classroom).
Daylight uniformity
Daylight uniformity refers to how evenly daylight is distributed within a room or space. An average daylight factor could indicate good levels of daylight availability, but if that average was achieved because one part of the room was receiving a lot more daylight than others and skewing the result, then the result could be a misleading indicator of ‘good daylighting’.
Daylight uniformity can be expressed through two different methods as a ratio between the:
- lowest level of light and the mean (or average) level of light within a room
or
- lowest level of light and the max level of light within a room
The higher the difference in illuminance values the less uniform the light is in the evaluated space. If the values are significantly different this can also act as a red flag to check the glare probability index within the same space.
It is important to consider the use of a space before setting a target uniformity ratio – a sports hall or a classroom furnished with individual desks throughout will likely have a higher need for uniform lighting (typically 0.6), compared to a living room for example.
ADVANCED METRICS
Daylight autonomy Daylight autonomy is another daylight availability metric. As it is a climate-based metric, it is a great choice for analysing daylight levels based on real weather conditions and the effect of the sun (and subsequently building orientation).
It is expressed as the percentage of occupied time (or daylight hours4) that a target illuminance is met by daylight at a point in a space. For example, a target illuminance of 300 lux and a threshold daylight autonomy of 50% would mean that daylight levels were above the target illuminance for 50% of the occupied time (or daylight hours).
By specifying that the ‘occupied time’ should correspond to daylight hours, EN 17037 ensures that Daylight autonomy remains a suitable metric for projects at any latitude.
Spatial daylight autonomy Spatial daylight autonomy calculates the percentage of space that meets the target illuminance (50% of floor area EN17037 specifies).
Useful daylight illuminance Useful daylight illuminance is also a daylight availability metric and is simulated using weather data and occupancy hours. Useful daylight illuminance is very closely linked to daylight autonomy, however instead of taking a singular target illuminance it considers a range of daylight levels that provide useful illumination at the work plane.
It is expressed as the percentage of the occupied time when a target range of illuminances are met by daylight, at a particular point in a space.
Occupancy hours for UDI are assessed based on inputs about when the space will be occupied (for example, it is typically as described in LEED between 9 a.m. and 3 p.m. simulated with a clear sky on two different days during a year5).
Daylight illuminances in the range of 100 to 300 lux are considered “effective” either as the sole source of illumination or in conjunction with artificial lighting. Daylight illuminances in the range 300 to 3,000 lux are often perceived as “desirable”6.
Spatial useful daylight illuminance Spatial useful daylight illuminance is expressed as a percentage of space (eg. 75%) that meets the target illumination range (eg. 300 – 3000 lux) over the specified time period (eg. 9am - 5pm).
LUMINANCE-BASED METRICS
Luminance Where illuminance measures the light received at a surface, luminance measures the amount of light reflected or emitted from a surface. Luminance allows us to evaluate visual comfort and glare at different positions in a room, and it is expressed in candelas per square metre (cd/m2).
The candela relates to human perception; it is a measure of the visible light received by the eye, which is different from the lumen measuring the total light shining from an object7.
Daylight glare probability
Daylight glare probability is defined in an empirical formula connecting measurable physical quantities (luminance of glare sources, illuminance at eye level, solid angle of the glare source) with the glare experienced by subjects (considering field of view and line of sight).
It is expressed as a value where the higher the DGP, the more likely it is that glare will be experienced.
Table E.1 – DGP values can be categorised in following ranges
DGP is typically evaluated over an annual period to account for the temporal behaviour of the occurrence of glare. An annual evaluation can then reveal the amount of time (when the space will be occupied) throughout the year where a target DGP value is exceeded, flagging if further glare protection is required.
As DGP measures illuminance at eye level on a plane perpendicular to the line of sight, and a person’s line of sight is of course a highly dynamic reference point in a room, the DGP should investigate the expected worst-case position(s) within a space.
It is important to keep in mind that DGP is only a relevant metric to apply to rooms that are mostly side-lit and where activities are comparable to reading, writing, or using display devices. Furthermore, it is not a suitable metric for very deep or dark rooms with small daylight openings and cannot be calculated from positions far away from daylight openings8.
Putting the fundamentals of daylighting into practice
Daylighting is a powerful tool that can transform architecture, and the experience that occupants have within buildings. Understanding the fundamentals of daylight means that you can begin to implement them within your building designs. The next section of this guide looks at daylighting in project planning, which is the best time to put the fundamentals into practice so that daylight can be a guiding design principle.
1 Hopkins, R. G. (1963) Architectural Physics: Lighting, London: Her Majesty’s Stationery Office. · 2 Mardaljevic, J., Andersen, M., Roy, N., Christoffersen, J. (2012) Daylighting, Artificial Lighting and Non-Visual Effects Study for a Residential Building · 3 CIBSE (2002) Code for Lighting, Oxford: Chartered Institution of Building Services Engineers. · 4 En 17037 specifies that occupied time should correspond to daylight hours (based on local climate data) · 5 LEED V4 for Building design and construction, 2019 · 6 Mardaljevic, J., Andersen, M., Roy, N., Christoffersen, J. (2012) Daylighting, Artificial Lighting and Non-Visual Effects Study for a Residential Building.· 7 https://www.nist.gov/si-redefinition/candela · 8 CEN European Daylight Standard (EN 17037:2018), Annex E.