The Design Day and Annual Metric table reports averages, maximums, minimums and a variety of annual metrics for each luminaire zone. For each of the representative clear or cloudy sky design day times, and an annual average and maximum, the table reports the workplane illuminance. For the various annual metrics the table reports that given metric’s average, maximum and minimum values. The radio buttons to the left of the table can be used to select a condition or annual metric. The selected metric value for each point on the workplane grid will then be displayed in the plot and the isometric chart and summarized in the spatial binning table at the top of the page. The Interactive View button can be used to generate a rendering of any design day selected (note that it cannot render the annual average, annual maximum or annual metric selections).
The bottom rows of the daylight results table display a number of annual daylighting performance metrics. The metrics displayed and their definitions are as follows:
Annual Average – When selected, the grid and plot will display average illuminance for each point throughout the year. The annual illuminance is not just an average of the “Design Day” conditions that have been simulated and shown in the table but derived from an annual climate based simulation (using the selected weather file), hence giving a more accurate annual picture.
Annual Maximum – When selected, the grid and plot will display maximum illuminance recieved at each point throughout the year. The maximum illuminance is not just taken from the “Design Day” conditions that have been simulated and shown in the table but derived from an annual climate based simulation (using the selected weather file), hence giving a more accurate annual picture.
Daylight Saturation (DS) – is the fraction of a given location’s ‘annual daytime lighting load’ that is met with daylight. It defines the maximum lighting savings that could be obtained while ensuring the target illuminance is always met at a given point. The ‘annual lighting load’ for a space or point is considered to be how much light is required to maintain a target illuminance annually during a set period: all daytime hours or occupied daytime hours. It is similar to DA, but gives credit to hours where there is partial daylight saturation meeting a portion of the target illuminance (E) in addition to the hours where the target illuminance has been exceeded (100% saturated). This is an annual daylight quantity metric useful for determining the daylight “potential” of a space, especially for spaces with continuous dimming daylight responsive electric lighting control. The target illuminance can vary and is a function of the space and design goals (in SPOT it is set to be the design illuminance) and is written as a subscript when reporting DS (ie. DS_30fc_). DS is used in the SPOT performance reports as well as the Collaborative for High Performance School (CHPS) reports.
Daylight Excess (DE) – a measure of excessive daylight in a space and reports the fraction of annual daytime or occupied daytime hours that exceed a certain threshold. The maximum acceptable illuminance threshold for a space is considered to be some multiplier of the target illuminance, typically 10x. For example, if a space has a target illuminance of 430 lux (40fc), then DE430×10 would be a measure of how many hours an analysis point exceeds 430×10 lux = 4300 lux (400 fc) throughout the year. While potential daylight glare in a space is a function of view angle and the viewed luminance, excessive illuminance can be a good indicator of daylight glare potential, excessive solar heat gains and just excessive daylight in general. Illuminance calculations can be a good proxy for luminance based glare potential for a range of viewing angles. The high illuminance threshold can vary and is a function of the space and design goals (in SPOT it is set to be the design illuminance x10) and is written as a subscript when reporting DE (ie. DE_300fc_ or DE_30×10fc_). DE is used in the SPOT performance reports as well as the Collaborative for High Performance School (CHPS) reports.
Daylight Autonomy (DA) – a measure of the fraction of annual hours that a given target illuminance (E), often 215-430 lux (20–40 fc), is exceeded by daylight. This is a common metric used for evaluating the quantity of daylight available for a given space particularly when evaluating on/off daylight responsive electric lighting control systems. The target illuminance can vary and is a function of the space and design goals (in SPOT it is set to be the design illuminance) and is written as a subscript when reporting DA (ie. DA_30fc_).
Useful Daylight Illuminance Combined (UDIa+s or UDIc) – a measure of the amount of useful daylight illuminance throughout the year. UDI combined is the summation of UDI autonomous (UDIa), defined as the fraction of the year where the daylight illuminance is between 300 and 3000 lux (~30-300 fc), and UDI supplementary (UDIs) defined as the fraction of the year where the daylight illuminance is between 100 and 300 lux (~10-30fc). The UDIs calculation is binary, reporting the number of hours the daylight falls into the 100-300lux bin rather than giving “partial credit” like the Daylight Saturation metric. The resulting UDIc value is a good measure of the amount of useful daylight throughout the year for a space targeting around a 300lux (30fc) ambient lighting level and is used as the primary metric in the UK’s Priority Schools Building Programme (PSBP).
Useful Daylight Illuminance Exceeded (UDIe) – a measure of excessive daylight illuminance in a space throughout the year. UDIe is defined as the fraction of the year where the daylight illuminance is above 3000 lux (~300fc). This is a measure of excessive daylight that has a high glare potential in the space, similar to the DE metric.
Daylight Factor (DF) – a historic metric used to estimate the quantity of daylight in a space. It is simply the ratio of interior illuminance at a point to the exterior global illuminance at the same time under a standard CIE overcast sky. It is a static metric in that rather than looking at multiple climatic conditions throughout the year it is based on the one cloudy sky distribution. However, given this rather large simplification, it can still be useful metric in comparing the relative daylight quantity between designs and for climates where cloudy conditions truly dominate.
Spatial Daylight Autonomy (sDA) – a new approach for reporting Daylight Autonomy for a given area or space as a single number representing the area that exceeds a certain DA threshold, such as 50%. This idea of a single ‘spatial’ number can be used to report all the daylighting metrics and various spatial bins are reported for each metric in the spatial bin summary table at the top of the page. When reporting sDA (or any spatial metric), the DA threshold is written as a second subscript behind the illuminance target used for the DA calculations (ie. sDA_30fc_,50% would be the % of an analysis area that exceeds 30fc for 50% of the time). The term ‘sDA’ and the form adopted by LEED v4 does not use variable targets but includes specific details explained in the IESNA LM-83 document created by the Daylighting Metrics Committee. For example, it sets 300lux and 50% as hard thresholds, and defines the analysis period to always be 8am-6pm annually. SPOT calculates sDA to conform to the LM-83 definition and hence sDA is listed separately and may differ from the normal DA calculations. Note: When sDA is shown on a workplane plot, the plotted numbers are actually DA values calculated according to the sDA standards and not sDA values as there is a single sDA number for an entire space.
Annual Sunlight Exposure – a new metric developed in conjunction to the sDA to attempt to detect incidences of too much direct sunlight and high glare potential. This metric calculates the number of hours a year in which a point exceeds 1000lux from direct sunlight alone and with no window treatments on any windows. It is reported as a single ‘spatial’ number similar to the sDA metric and uses 250 hours a year as the time threshold. This metric can be a good indicator of how dependant a given daylighting design is on the window treatments. If ASE is low, then it may be a good ‘passive’ daylighting design, not reliant on active interior control elements for its performance. If ASE is high and the other metrics show good performance, than it may be good design but it is highly dependant on the proper functioning of the window treatments. High ASE might be a good argument for more automated shade controls so that the daylighting performance of the space is not dependant on user behavior. Note: When ASE is shown on a workplane plot, the plotted numbers are actually Annual Sunlight Fraction (ASF) values calculated according to the ASE standards and not final ASE values as there is a single ASE number for an entire space.
The workplane plot on the right of the page is color-coded according to different spatial binning of the points. A small Spatial Binning summary table sits above the workplane plot and reports the area fractions for each spatial bin. As discussed in the sDA section above, these values are termed “spatial” metrics and are refered to as spatial Daylight Saturation (sDS), spatial Daylight Autonomy (sDA) and so forth. The bin ranges used for the different daylight results are as follows:
- When one of the Design Day or Annual illuminance rows are selected, the spatial bins report the fraction of the workplane that partially meet or exceed the design illuminance using bins of <33%, 33%-66%, 66%-100%, and >100%. Portions of the workplane that are >66% of the design illuminance (color coded orange) are likely the best zones to control with a photosensor system.
- When one of the annual sufficiency daylight metrics (DS, DA, UDI) are selected, the spatial bins report the fraction of the workplane that exceed certain metric thresholds. The bins used are 0-40%, >40%, >60%, and >80%. The >40% bin will inform the fraction of space with minimally sufficient daylight contribution. The >60% bin represents the fraction of space with good daylight contribution and the >80% represents the fraction of the space with great daylight contribution.
- When one of the annual excessive daylight metrics (DE, UDIe) is selected the spatial bins report the fraction of the workplane that exceed much lower metric thresholds as these metric indicate excessively high and unwanted daylight contribution. The bins used are 0%, >0%, >5% and >10%. Different thresholds will be applicable to different space types. For spaces with very critical tasks, anything greater than 2% may be unacceptable. For other spaces with more leisure tasks and some tolerance to direct sunlight the >10% bin may be more applicable.
- When the spatial Daylight Autonomy (sDA) row is selected, the spatial bins used are 0-30%, >30%, >50%, and >70%. The 50% threshold represents ‘sufficient’ daylight while the 70% threshold represents ‘good’ daylight.
- When the Annual Sunlight Exposure (ASE) row is selected, the spatial bins used are 0%, >0%, >6.8% (250hrs of 8am-6pm schedule), >10%. The >6.8% threshold has been adopted by LEED v4 as indicating too much direct sunlight.
At any time, the View Zone Diagram button can be used to pull up the zoning diagram for the space. This is useful in analyzing the various zone averages in the table and where they occur in the space.
Daylight Glare Probability (DGP) – an upcoming glare metric that is beginning to be able to predict the probability of glare given a view angle and accurate luminance calculations in the view direction for specific office settings. However, DGP is still being heavily researched, the results are not yet repeatable even in laboratory settings, and it is not ready to be used for reliable glare predictions. Softwares claiming to provide accurate glare calculations or annual glare calculations using DGP are jumping the gun and may be giving mis-leading information. More field and laboratory research is needed to continue to develop the DGP approach making it more robust and a reliable predictor of glare probability in a wider range of scenarios. Having said that, DGP calculations are still possible in a SPOT project behind the scenes. All the renderings generated by SPOT are High-Dynamic Range (HDR) images that include accurate luminance information. A DGP calculation can be done on any of these images using a command prompt in your projects “/images/” directory or wherever else you saved your renderings. See that Radiance online documentation for information on how to perfrom this calculation using the Radiance tool Evalglare.