Understand the complexities of daylight analysis
SDA 1: MEET THE METRIC
(Note: This series originally published in 2016. Please see a 2018 update to this series in an excerpt of a speaker session presented by LightStanza Founder – Daniel Glaser, PhD at Greenbuild 2018 here)
The sDA Metric: Is this a sufficiently daylit space?
Welcome to the reboot of our series on Spatial Daylight Autonomy (sDA), where we will be discussing sDA as a new, annual metric for a more accurate measure of daylight. In Part 1 of our series we will explain what an annual metric is, the difference between how much daylight a space is getting versus the portion of daylight that is usable, and why this distinction is important. An annual metric is a function of hourly simulation results across an entire year in conjunction with climate analysis. The climate analysis is based on typical meteorological year (TMY) climate data. This type of continuous (and intuitive) analysis of a space can remove the uncertainties found in analyses that only evaluate a single point in time by accounting for hourly and seasonal changes in daylight availability and sun angle. LEED v4 prioritizes annual metrics for daylight credits, which is an improvement over the 2 solar positions simulated for LEED 2009 and the single ratio used for Daylight Factor. Although, LEED v4 does still offer a second option of 2 solar positions, excluding blinds, if sDA scores are too low.
Daylight availability can change by the hour, making annual hourly measurements a more accurate indicator of how much daylight is available to a space compared to point in time metrics.
While there are multiple annual metrics, sDA is the first one to use hourly daylight measures in conjunction with manual blind operation use. As pictured below, blinds are used by building occupants to control glare and maintain visual comfort, so acknowledging occupants’ influence on available daylight is crucial for calculating an accurate score.
By not accounting for blind use, annual metrics other than sDA are overestimating the amount of usable light and underestimating energy consumption. The image below shows that even with available daylight, electricity is often used in place to avoid direct sunlight. This not only increases use of electricity, but it also negates window benefits by allowing heating and air conditioning to escape when we’re not using the windows for daylight.
If blinds, along with the fenestration, are not properly simulated, energy consumption can be underestimated and human benefits can be exaggerated in a building’s sDA score.
If your software doesn’t show blinds operating, your score is inaccurate. It is necessary to use a dynamic software simulation that accounts for how occupants may interact with their daylit environment: after all, what do lighting levels mean without considering the people experiencing them?
sDA 2: Overview of sDA Calculation
The sDA metric scores a space’s daylighting in conjunction with manual blind operation in a two-step simulation process, which we will briefly outline in this blog post and go over in more detail in following posts. The first determines the position of the blinds (whether they are open or closed) and the second measures daylight levels with the corresponding position of the blinds. Blind positions are determined by how much direct sun gets into the windows. When a space receives too much direct sun, blinds close in groups until the amount is sufficiently decreased (Step 1).
Step 1. Blind Operations: Blinds, either electronically or manually controlled, contribute greatly to the quality and quantity of light. In the figure above, we see that as the sun’s position changes, different groups of blinds are used to maintain visual comfort in the space. Facades that use dynamic glass or redirect film to control for direct sun do not need blinds.
Realistic illuminance for the space is then calculated using a simulation of the blinds in their determined position from step 1 for each hour of the day. The sDA score is calculated using a formula that takes these raw illuminance values as input (Step 2). The score is the percent of points on the grid that meet the minimum thresholds.
Step 2. Scoring: Annual metrics like sDA determine the level of illumination at each point and at each hour after the blinds are in position., these metrics take thousands of time points of illuminance data comprising potentially millions of light readings and thousands of blind positions and compact them into a single value.
In the next several parts of the sDA series, we will cover blind operations and scoring in detail.
sDA 3: Terminology
Workplane
A workplane is an imaginary plane where work is performed and illumination is specified. The standard height for the workplane is shown in the image below, but there are different methods for choosing how to define workplanes.
Schematic Section
LEED Definition: The USGBC defines the workplane at 30″ above the floor, which is a standard height for table tops. If you are seeking LEED credit you will need to simulate at workplane height, 30″ above the floor.
BREEAM Definition: The Building Research Establishment Environmental Assessment Methodology (BREEAM) uses CIBSE LG10 to define the workplane as the horizontal, vertical or inclined plane in which a visual task lies. The working plane is normally taken as 0.7m above the floor for offices and 0.85m for industry.
Flexible Definition: However, not all furniture has a height of 30″, .7m, or .85m. Before deciding on a workplane height, consider researching the height of the work surfaces (i.e. tables and desks) that will be used in the space for a more accurate analysis. For example, tables in science labs are often 32 to 36 inches tall.
Illuminance Grid
The workplane is a type of Illuminance Grid since it is made up of a grid of points and has the function of collecting light. The spacing of the points on the grid can be changed and will affect the precision and speed of the computation of the results. In other words, densely spaced grids will be more accurate yet more computationally expensive than a sparser point spacing. In the image below, we see how different spacing between points affects the daylight measurement. 2′-0″ spacing is standard for LEED, but choose an appropriate density that makes sense for your project and purpose.
Plan view of Illuminance Grids with different spacing between points. A denser plane of points is more accurate, but more costly to compute.
An Illuminance Grid will typically be defined as a horizontal plane but can be placed to measure light on any plane inside the building. For example, they can be defined to measure light on a vertical surface such as a wall. This function can be useful in assessing situations like daylight hitting artifacts in a museum. Some users may also want to collect light on the ceiling, in which case the Illuminance Grid would be horizontal, and located just below the ceiling, with the active side facing down.
sDA 4: Step 1. Blinds Operations
The first step of the sDA calculation determines whether the blinds are open or closed, depending on how much direct sun gets into the space. Within this step, the windows must first be categorized into groups and then the position of the blinds is determined hourly.
Form Window Groups: Windows must be controlled in groups. A window group is defined as a “group of coplanar windows, with similar shadow patterns from exterior shading and obstructions, and with similar shading device type and operation, which are associated with the same analysis area” [Illuminance Grid] (IES LM-83-12 Section 2.2.6).
This means that all windows that are on the same facade with the same external shading devices (overhangs, awnings, lattices, etc.) will be in a group, and that the blinds on every window in this group will open and close together. The blinds operation of each window group is distinct from that of other groups, but every window in the same group will behave the same way.
The image below shows how windows are grouped. In pair A, all the windows are in the same group because they are 1. on the same plane, 2. have the same external shading strategy (which in this case is none), and they all correspond to the same analysis area. In pair B, each window is in its own group because they are all on different planes (facades). Their different orientations will allow light to enter the room in different ways so they will behave independently of each other. In pair C, the top windows make up Group 1 and the windows on the bottom make up Group 2. Even though all the windows are on the same plane, the top windows (Group 1) have no shading and the bottom windows (Group 2) have an awning over them.
Three sets of plan and elevation describing window groups.
Determine the position of the blinds or shades at each hour using the 2% rule: Once window groups are established, the position of their blinds is based on the 2% Rule, which is when “2% or more of the analysis points receive direct sunlight” (IES LM-83-12 Section 2.2.6). If more than 2% of the points on the Illuminance Grid receive direct sun, the blinds will close to bring it below the 2% threshold, and the sDA score will come from the illuminance values with closed blinds instead. Some blinds may stay open, and LightStanza calculates how to simulate with the optimal combination of open and closed blinds to maximize daylight without exceeding the 2% threshold.
The following three images are plan views of an Illuminance Grid with 100 points. In the first image, only two points on the grid are hit by direct sunlight. In other words, there is only 2% direct sunlight in the space, so the blinds don’t have to be drawn. In the second image, 28%of the Illuminance Grid is hit by direct sunlight. Since this is greater than the 2% that the sDA metric allows, this simulation will assume the blinds are closed and blocking the sun. The last image shows the same day and time as the second, now with the blinds closed. This is the data that will be used to score the sDA metric.
Direct Sun Exposure: Plan view of a space at three instances illustrating the two percent rule and blind use for the sDA metric.
These steps are specific to the sDA simulation. In the next post, we will provide examples of operable blinds in action.
sDA 5: Blinds Operations Examples
Let’s look at examples of blinds operations. The below image shows a space that has windows with daylight (upper) and view (lower) components. The daylight and view windows are separated into different window groups since the view window has an overhang above it, while the daylight window is exposed. This image shows, though, that the groups act independently, namely that the daylight window remains open while the view window is closed. As a result, some direct sunlight is cast back into the room, but not an excessive amount.
Model from Clanton & Associates
Let’s look at this in more detail, in accordance to measuring grids as prescribed in LM-83. Below is an Illuminance Grid of this same space with one point that is red, signifying that it is illuminated to more than 1,000 lux (ie it is receiving direct sunlight). This indicates at least one blind group is open. If all blind groups were closed, it would eliminate the red point. But since the sDA metric allows for up 2% of the grid to be hit by direct sun, this one point is permissible and the blinds can stay open.
A rendering of the space at the same time reveals that the clerestory blinds are open.
Now let’s see how the Illuminance Grid changes over the course of a day, with and without operable blinds. The image below shows how operable blinds affect illumination levels in a space on a simulation of December 21st. Row A shows a series of renderings of a building without operable blinds. Row B shows a plan view of the Illuminance Grid in a space without operable blinds. Row C shows a series of renderings of operable blinds opening and closing according the sDA metric. Row D shows the same plan view, but this time the space has operable blinds.
Model from Clanton & Associates
An animated simulation of operable blinds in the workspace can be seen below. As the area of direct light increases to over 2% of the total space, blinds close to decrease the area and create a more visually comfortable environment.
Inside and outside views of the space are illustrated for the analysis period on March 21st.
How is exposure to direct light affected by solar angle? The position of the sun in the sky will affect how much light gets into your space. For example, let’s say it’s June 21st and your building is on the equator. The room that you are measuring light in has one window on the southern exposure. The blinds will stay open all day even if it’s sunny, because the sun will pass directly over your building from east to west and never enter the space.
Now take that same building and move it to Boulder, CO (40 degrees north). In the same climate conditions (sunny all day) the blinds will close for a significant portion of the day, because the sun will be at a lower position in the sky, meaning its rays are more horizontal and will penetrate into the space.
Next up, we will discuss how to score the sDA metric.
sDA 6: Step 2. Scoring
The first step of the simulation determined whether the blinds are open or closed. The second step of the simulation determines the level of illumination at each point and at each hour after the blinds are in position. For LEED V4, a point must meet a minimum illuminance of 300 lux (28 footcandles) for at least 50% of the year: sDA (300, 50%).
Obtain illuminance levels at each hour using a climate-based simulation. This step is common to all annual metrics using a 10 hour per day “analysis time period extending from 8 am to 6 pm according to local clock time” (IES LM-83-12 Section 2.1.2) . The only difference with sDA is that it measures illuminance for the space with the blinds in their determined position for each hour of the day.
Process illuminance data according to the sDA metric. Each annual metric has a unique way of processing the illuminance data. The sDA metric checks each point on an Illuminance Grid to see if its illuminance is at or above 300 lux for at least 50% of the year during the analysis period with the blinds in operation. With the daily 10 hour analysis period as stated above, “50% represents 1,825 hours per year” (IES LM-83-12 Section 2.2.6).
Below, Figure 1 shows the measured illuminance for one point, and every point on the Illuminance Grid is measured this way.
Figure 1. The orange dots show the hours that the point meets the minimum illuminance threshold of 300 lux. The gray dots show hours that the point does not meet the illuminance threshold.
All of the time points that meet the 300 lux threshold are summed together and are compared to the number of time points that do not meet this illumination threshold (Figure 2).
Figure 2. Total hours of illumination above 300 lux.
The point must meet the minimum threshold for at least 50% (1,825 hours) of the annual analysis period to be counted in the final score (Figure 3).
Figure 3. Since the point meets 300 lux for 53% of the time, it is acceptable.
The final score is the percent of points on the grid that meet the minimum illumination threshold for 50% of the time (Figure 4). An sDA score greater than 55% achieves 2 points, and a score greater than 75% achieves 3 points. So in the example below (Figure 4), an sDA score of 68% will achieve 2 points.
Figure 4. Percentage of points that meet the 50% threshold.
LEED v4 Scorecard Update: The USGBC recently made some clarifications regarding the LEED v4 daylight rating system and alternative strategies for Option 1 credit. Here are the major changes to note:
1. Spaces with an automated dynamic facade system OR under 250 square feet will now be exempt from ASE.
2. You can now achieve daylight credit for spaces with an ASE score between 10 and 20%, and an extra point for spaces below 10%!
You can check out the full rundown here. LightStanza has been updated to reflect these changes.
DAYLIGHT TERMS
Here are some key terms to help you participate in conversations about daylighting and metrics. This list will grow and change as we do, so come back if you ever need a reference.
Daylighting
skylights, tubular daylight devices, light shelves, overhangs, redirect films, other advanced daylight products. Good daylighting can reduce a building’s demand on energy either by eliminating or minimizing the need for artificial (electric) lighting. In commercial buildings, daylighting strategies can produce savings of 15-40% on electrical energy. It also contributes to occupant health and comfort, which is why daylighting can help a building achieve LEED and WELL Certification through the Indoor Environmental Quality (IEQ) metric. A growing body of research has shown that people operating in full-spectrum, daylit conditions perform better work and are more satisfied than those who operate under electric lighting. Good daylighting is better for people and our planet.
LEED Daylight Credit (Green Building Certification)
LEED (Leadership in Energy and Environmental Design) is a set of rating systems for the design, construction, operation and maintenance of green buildings developed by the U.S. Green Building Council (USGBC).
LightStanza helps you simulate, analyze, and document for the LEED Daylight credit.
Occupied Spaces
LEED defines Occupied Spaces as enclosed spaces that can accommodate human activity. Occupied Spaces are further defined as regularly occupied or non-regularly occupied spaces based on the duration of the occupancy, individual or multi-occupant based on the quantity of occupants, and densely or non-densely occupied spaces based upon the concentration of occupants in the space. In other words, Occupied Spaces are rooms that will see regular use, like offices, conference rooms and kitchens, but excludes spaces like bathrooms and storage. There are rare exceptions that allow occupied spaces to not utilize daylight when it impairs its function, such as a sound recording studio or a specialized laboratory. LightStanza can measure and analyze daylight anywhere in your project, but if you are pursuing LEED, you only need to measure and score light in Occupied Spaces.
SCHEMATIC FLOOR PLAN
Workplane and Furniture
A workplane is an imaginary plane where work is performed and illumination is specified. There are different methods for choosing how to define workplanes.
LEED Definition: The USGBC defines the workplane at 30″ above the floor, which is a standard height for table tops. If you are seeking LEED credit you will need to simulate at workplane height, 30″ above the floor.
BREEAM Definition: The Building Research Establishment Environmental Assessment Methodology (BREEAM) uses CIBSE LG10 to define the workplane as the horizontal, vertical or inclined plane in which a visual task lies. The working plane is normally taken as 0.7m above the floor for offices and 0.85m for industry.
Flexible Definition: However, not all furniture has a height of 30″, .7m, or .85m. Before deciding on a workplane height, consider researching the height of the work surfaces (i.e. tables and desks) that will be used in the space for a more accurate analysis. For example, tables in science labs are often 32 to 36 inches tall. If you won’t be simulating for LEED, you can model your furniture and Illuminance Grid at more accurate heights. Additionally, your performance scores will usually increase when you simulate without furniture since your room will “lighten up.” Therefore it can be good modeling practice to put furniture on its own layer.
SCHEMATIC SECTION
Illuminance Grid
The Illuminance Grid is a plane like the workplane, but it is made up of a grid of points and has the function of collecting light. The spacing of the points on the grid can be changed and will affect the precision and speed of computation of the results. In other words, densely spaced grids will be more accurate yet more computationally expensive than a sparse measurement grid.
The Illuminance Grid is one of two methods used in LightStanza to measure and graphically represent daylight in a space. An Illuminance Grid will typically be defined as a horizontal plane but can be placed to measure light on any plane inside the building. For example, they can be defined to measure light on a vertical surface (wall). This function would be useful to assess daylight hitting the walls in a museum. Some users may want to collect light on the ceiling, in which case the Illuminance Grid would be horizontal, and located just below the ceiling, with the active side facing down.
There are specific requirements for Illuminance Grids if you are pursuing LEED credit. Grids must be placed at a height of 30″ above the floor, in all occupied spaces, and should have a point spacing of two feet.
Plan view of Illuminance Grids with different spacing between points. A denser plane of points is more accurate, but more costly to compute. 2′-0″ spacing is a standard for LEED.
This plan shows how the points on an Illuminance Grid measure where daylight goes. You can see how denser data portrays where the light is in a space more accurately. However, it takes more resources to run a simulation if there are a lot of points to analyze, so choose a density that makes sense for your project and purpose.
Renderings (Viewpoints)
The rendering is one of two methods used in LightStanza to measure and graphically represent daylight in a space. In SketchUp, renderings are referred to as “Scenes.” A rendering is an image of how a space would look with daylight in it. It’s like taking a picture of the space. There are different lenses you can use to produce the rendering. LightStanza currently offers a standard perspective lens and a hemispheric (fisheye) lens. The perspective lens will produce a normal looking image as though you took a picture with a regular camera. The hemispheric lens is an ultra wide-angle lens that produces strong visual distortion intended to create a wide panoramic or hemispherical image and is most useful for analyzing glare.
Choosing to view simulation output in renderings is a good way to check if the model is set up correctly. For example, if the camera is located in the appropriate place, you may see defects in the model construction, like a gap between the roof and wall, which would cause a light leak and contribute to faulty results.
Renderings are the most useful way to get experiential or qualitative data, but they can offer quantitative data as well. LightStanza’s false color, glare, and exposure analysis are interactive ways to get more information out of your simulations.
| Standard Perspective Rendering | Pseudocolor Analysis of Rendering |
|---|---|
 Tone mapping a rendering is a familiar way of looking at information from a camera lens. |  Standard renderings can be quantitatively analyzed, typically for glare calculations. |
2% Rule
Here’s what the authorities behind the LM-83 have to say about it: “Blinds shall close whenever more than 2% of analysis points receive direct sun as defined below. Blinds for window groups can close in any combination, until the criterion value for each hour is achieved. This type of analysis shall be conducted for each hour of the year” (5).
The 2% Rule applies to the sDA metric and says that when more than 2% of the analysis area (Illuminance Grid, in LightStanza’s terms) is lit with direct sun, window blinds or shades must be deployed until less than 2% of the analysis area has direct sun on it. The first step of the sDA simulation is determining the position of the window blinds using the 2% rule, followed by a step that determines illumination values once the blinds are in position. This second step is what gives you your final score.
Annual Metrics
Annual metrics are a way of evaluating daylight in a space across an entire year. The results are a function of hourly simulation results in conjunction with location specific climate data. There are several different metrics, all of which use the same set of data, but each have a different way of interpreting daylight and tell a different piece of the daylight story. LightStanza offers Average Illuminance (AI), Daylight Autonomy (DA), Continuous Daylight Autonomy (cDA), Annual Sunlight Exposure (ASE), Spatial Daylight Autonomy (sDA), and Useful Daylight Illuminance (UDI).
Direct Sun
Here’s what the authorities behind the LM-83 have to say about it: “Direct sun is defined as an interior horizontal measurement of 1,000 lux or more of direct beam sunlight that accounts for window transmittance and excludes the effect of any blinds, with no contribution from reflected light (i.e., a zero bounce analysis) and no contribution from the diffuse sky component” (5).
In other words, direct sun is light on the Illuminance Grid that is greater than or equal to 1,000 lux from a simulation that does not include reflected light or coverings like blinds, shades or other daylight products.
Operable Blinds
You know what window blinds are. They are included in this list because they are ubiquitous in the “real world,” but almost always overlooked in daylight simulations. To get a truly accurate understanding of daylight in your space, use operable blinds.
Window Group
“A window group is defined as a group of coplanar windows, with similar shadow patterns from exterior shading and obstructions, and with similar shading device type operation, which are associated with the same analysis area” (5).
In other words, all the windows in a group should be on the same face of the building, have the same shading strategy, and be in the same room.
Lighting Products
Lighting products like electric lights and some types of skylights can be modeled and simulated by placing IES files. IES files are data files that describe the lighting distribution and characteristics of specific lighting products. These types of files are a standard in the electric lighting industry for describing the light characteristics of a static product.
Material Optics
Optical properties of materials affect how light is directed around a space. A bright wall paint will cause the same space to look very different than if it was painted with a dark or muted paint color. Similarly, not all glazing transmits the same amount of light (see figures below). Visible transmittance (VT) is the fraction of light in the visible portion of the spectrum that passes through a glazing material. A higher VT means that more light will be transmitted into a space. Accuracy in defining your materials is critical to get accurate simulation results.
| 40% Window Transmittance (VT) | 80% Window Transmittance (VT) |
|---|---|
 |  |
| 40% Window Transmittance (VT) | 80% Window Transmittance (VT) |
|---|---|
 |  |
Complex Glazing (BSDF Materials)
A specific application of material optics is with BSDF (Bi-directional Scattering Distribution Function) files. Using BSDF files is how you will achieve the right material optics for complex daylight products. These files are mathematical models that spread light dynamically according to material, solar angles and sky intensities.
Standard windows and skylights can be modeled in your 3D modeling tool, uploaded to LightStanza and easily simulated within the tool. More complex fenestration, like Daylight Redirecting Films or Dynamic Glass, should be characterized as complex glazing in LightStanza.
This section shows how material affects the behavior of light. Section A shows sunlight reflecting off and transmitting through normal glass. Section B shows sunlight reflecting off and transmitting through glass with a BSDF material applied to it, like daylight film.
Glare
Glare is caused by a significant ratio of luminance between the task (that which is being looked at) and the glare source. It is the result of too much direct sun entering a space. Glare should be minimized in a space as it causes occupant discomfort—think of how uncomfortable is it to drive into direct sunlight. LightStanza offers a few options for glare analysis, including pseudocolor renderings, daylight glare probability, and the ASE metric.
Psuedocolor renderings map color to black and white image based on intensity values, in this case intensity of illuminance. This type of image is useful to get a quantitative understanding of illuminance values in your space. Illuminance values are shown in candelas per square meter (cd/m²) The candela per square metre (cd/m²) is the derived SI unit of luminance. The unit is based on the candela, the SI unit of luminous intensity, and the square metre, the SI unit of area. As a measure of light emitted per unit area, this unit is frequently used to specify the brightness of a display device.
This pseudocolor rendering displays illuminance values in cd/m². Yellow values represent the most intense illuminance and blue values as the least intense.
Daylight Glare Probability Guidelines:
Imperceptible 0.0-.35
Perceptible .35-.4
Disturbing 0.4-.45
Intolerable .45-1.0
Operable Blinds
In daylit spaces, blinds can open or close to mitigate illumination for occupant comfort. Some annual metrics (i.e. Spatial Daylight Autonomy) simulate for the use of operable blinds on windows to block sunlight when direct sun hits the Illuminance Grid. This function mimics a person opening or closing the blinds to control the amount of direct sun in the room.
A series of renderings showing the opening and closing of blinds to control the amount of direct sun inside the space. Model from Clanton & Associates.
Sunlight is cast back in a room and does not hit the workplane Illuminance Grid. Model from Clanton & Associates.
Ground Plane Reflectance
Light that enters a building doesn’t enter only as direct sun. It also reflects off of trees, buildings, and other elements of the outdoor setting, including the ground plane. We recommend geolocating when you build a digital model because of this reflectance. It helps our software predict how much light will be reflected from surfaces in the setting back into your building.
Direct and Indirect Illumination
Indirect Illumination Simulation: Most metrics, including Average Illuminance (AI), Continuous Daylight Autonomy (cDA), (Classic) Daylight Autonomy (DA), Useful Daylight Illuminance (UDI), and Spatial Daylight Autonomy (sDA) use the Indirect Illumination Simulation type. In this simulation, the Illuminance Grid senses light as it would realistically exist in a space. This includes sun from direct beams of light as well as light reflected off the ground, ceiling, walls and other surfaces.
Lighting Metrics Guide
1. Point in Time Metrics
“Point in Time” means that lighting is only sampled at a few defined times. Point in time metrics are limited in their expressiveness, since daylight is dynamic and changes through the day, seasons, and climate.
1B. Daylight Factor
Daylight Factor is a daylight metric that expresses the ratio of illumination inside to the level outside. This metric only uses an overcast sky.
Daylight Factor has helped to standardize daylight scores across different geographies since it only uses an overcast sky, but can be less useful in very sunny climates. It is used in some international green building certification systems, like BREEAM.
It is calculated by dividing the illumination at the workplane by the illumination values outside the building and multiplying by 100. For example, if the value at a point on the Illuminance Grid is 500 lux and illumination outside is 10,000 lux, you would calculate (500/10,000)100=5. The higher the value, the brighter the space.
2. Annual Metrics
The remainder of this guide describes annual metrics.
In the past decade, there has been a push to evaluate daylighting in terms of how it performs across an entire year. An annual metric is a function of hourly simulation results in conjunction with climate analysis. The climate analysis is based on typical meteorological year (TMY) climate data. This type of continuous (rigorous) analysis of a space can remove the uncertainties found in analyses that only evaluate a single point in time.
The climate analysis is based on Typical Meteorological Year (TMY) data. This data is also encapsulated by Energy Plus Weather Data files.
The Annual Sunlight Exposure (ASE) and Spatial Daylight Autonomy (sDA) metrics simulate 10 hours a day between 8:00 am and 6:00 pm. These times comes from the requirements for the LEED v4 certification compliance path set forth by the USGBC and are inflexible.
Most of the annual metrics, including Continuous Daylight Autonomy (cDA),(Classic) Daylight Autonomy (DA), and Useful Daylight Illuminance (UDI), simulate illuminance values hourly during daylight hours (from sunrise to sunset).
The Annual Sunlight Exposure (ASE) and Spatial Daylight Autonomy (sDA) metrics measure 3,650 data points: 10 hours a day between 8:00 a.m. to 6:00 p.m., every day of the year.
Annual Metrics can help identify daylight related issues in your model. Standard metrics have been proposed by the scientific community to characterize lighting for an entire year. These are:
2A. Average Illuminance
Average Illuminance is the simplest annual metric. First, it measures illuminance levels at each point in your space during daylight hours over the course of one year.
Then it averages the hourly values, which are represented as a percent on each point of the Illuminance Grid. This is the first step of every simulation except for ASE.
The average illuminance value of this point over the course of one year is 312 lux.
2B. Continuous Daylight Autonomy (cDA)
This measures how much of the time a room’s lighting needs can be met by daylight alone. It provides partial credit when the minimum threshold is not met. For example, if a point receives 15 fc (~150 lux) of light and its target illumination is 30 fc (~300 lux) it would get partial credit. 300 lux (cDA300) is a common threshold. This metric is useful for exploring potentials of electric lighting with dimming systems.
First, it measures illuminance values of each point at every daylight hour in the year. Once we know the point’s illuminance at each hour, we assess whether or not it meets the illuminance threshold. It gets full credit for each hour it meets the threshold, and partial credit based on how close the illuminance value is to the threshold.
Partial credit is offered for illuminance that does not meet the minimum threshold in the cDA metric.
This point scores 71%. This score represents the percent of time the point the minimum illuminance threshold, with partial credit given for values below the threshold.
That is the process for one point. We do it for every point on the grid, then give you your simulation results, which include your overall score shown as a percent and a grid detailing illumination levels.
2C. Daylight Autonomy (DA)
This metric is similar to Continuous Daylight Autonomy (cDA), except no partial credit is given. For example, if a point receives 15 fc (~150 lux) and its target illumination is 30 fc (~300 lux) it would receive no credit. A common threshold for the lighting requirement is 300 lux which would be written as DA300. This metric is useful when looking at potentials for on/off electric switching systems with no dimming options.
Once we know the point’s illuminance at each hour, we assess whether or not it meets the illuminance threshold. It gets full credit for each hour it meets the threshold. In this metric, it does not receive any credit for values under the minimum threshold.
After the illuminance values are measured hourly, we check to see if the values meet the minimum illuminance threshold.
The point we looked at gets a final score, which represents the percent of time it meets or exceeds the minimum illuminance threshold.
That is the process for one point. We do it for every point on the grid, then give you your simulation results, which include your overall score shown as a percent and a grid detailing illumination levels.
Detail of final Daylight Autonomy results.
2D. Useful Daylight Illuminance (UDI)
This metric describes the percentage of the hours in a year that a point in space is within a range of acceptable illuminance values. Values that are too high or too low are not counted. Common thresholds are 10 footcandles as a minimum, and 250 footcandles as a maximum (UDI 10,250).
First, it measures illuminance values of each point at every daylight hour in the year. Then it evaluates each value to see if it is within the range of acceptable values. If it is too high or too low, it is not counted.
The point we looked at gets a final score, which represents the percent of time it meets or exceeds the minimum illuminance threshold.
This point is within the useful range of daylight for 44% of one year.
The rest of the time it is either too bright or too dark.
That is the process for one point. We do it for every point on the grid, then give you your simulation results, which include your overall score shown as a percent and a grid detailing illumination levels.
Detail of final Useful Daylight Illuminance results.
LEED v4 Metrics:
LEED v4 uses the next two metrics to evaluate projects and assign credit.
2E. Annual Sunlight Exposure (ASE) (LEED v4 Option 1)
The ASE metric is a simplified daylight simulation that only looks for direct sun with operable blinds left open throughout the year. It measures illuminance values at each point based on a direct sun simulation, which does not include ambient or redirected light.
Illuminance values in an ASE simulation will typically be either 0 (no direct sun) or in the thousands (direct sun). They are never a middle value because this simulation doesn’t measure ambient or redirected light. The point is either in direct sun or not.
Once we know the illuminance values at this point, we assess whether or not it is in direct sun and count the number of hours in a year that it is. The ASE metric allows each point to be in direct sun for no more than 250 hours of the year.
This graphic shows one point on an Illuminance Grid at several hours in the year. When it is in direct sun, it is yellow; when it is not, it is gray.
This point is in direct sun for 212 hours in the year, which is under the 250 hour limit. It will be represented as green on the Illuminance Grid.
Total hours of direct sun.
That is the process for one point. We do it for every point on the grid, then give you your simulation results, which include your overall score shown as a percent and a grid detailing where the direct sun is.
5% of the points on this grid are in direct sun for more than 250 hours in the year.
Legend for ASE scoring.
There are two sets of criteria for interpreting ASE results. Both sets of values refer to the percent of points on the Illuminance Grid that are in direct sun for more than 250 hours. The first is using the criteria set out by the Illuminating Engineering Society (IES) in the LM83-12 document. It sets out results as follows:
0-2.99% – clearly acceptable
3-6.99% – nominally acceptable
7-9.99% – undefined per LM83
10-100% – unsatisfactory
The second set of criteria is set out by LEED v4:
0-10% – acceptable
10.01-100% – unacceptable (you will receive no daylight credit, regardless of your sDA score)
The first compliance path (up to 3 credits and 1 exemplary credit possible) for LEED v4, requires your ASE score to be 20% or less. If it is 10% or less, it will go toward an exemplary LEED v4 Daylight Credit Option 1 point. If your space is over 20% you get zero daylight credits no matter what your sDA metric score is. If it passes this criteria, it goes on to be evaluated using the sDA metric, which determines whether the space gets zero, two or three credits.
The USGBC recently made some clarifications regarding the LEED v4 daylight rating system and alternative strategies for Option 1 credit. Here are the major changes to note:
1. Spaces with an automated dynamic facade system OR under 250 square feet will now be exempt from ASE.
2. You can now achieve daylight credit for spaces with an ASE score between 10 and 20%, and an extra point for spaces below 10%! You can check out the full rundown here. LightStanza has been updated to reflect these changes.
If the space gets zero credits with its ASE/sDA calculations, Option 2 is another less stringent LEED v4 compliance path that is similar to LEED 2009 and can lead to two credits.
The Bedford Building, Winnipeg
Above is an example of a situation where illuminance grid points within the orange graphic would be in direct sunlight, scoring illuminance values in the thousands. If a work plane at 30 inches above the floor was in the space above, too much of this throughout the year would be problematic for inhabitants.
2F. Spatial Daylight Autonomy (sDA) (LEED v4 Option 1)
* The sDA metric requires operable blinds.
The sDA metric scores a space’s daylighting in conjunction with manual blind operation in a two-step simulation process. The first determines the position of the blinds (whether they are open or closed) and the second measures daylight levels with the corresponding position of the blinds.
Blind positions are determined by how much direct sun gets into the windows. If more than 2% of the area inside receives direct sun, blinds close in groups until the percent drops to below 2%. Illuminance for the space is then calculated with the blinds on their determined position for each hour of the day. The final sDA score is a formula taking these raw illuminance values as input.
Step 1: Determine Blind Position
The first step of the sDA calculation determines whether the blinds are open or closed, depending on the direct sunlight in the space and a 2% allowance.
Conceptual illustration of blind control at 10 am and 5 pm. When too much direct sun gets into the space, the blinds on the window that it shines through are closed.
Window Groups
The sDA metric requires that occupants control blinds in terms of groups. A window group is defined as a group of coplanar windows, with similar shadow patterns from exterior shading and obstructions, and with similar shading device type and operation, which are associated with the same analysis area (Illuminance Grid). This means that all windows that are on the same facade with the same external shading devices (overhangs, awnings, lattices, etc.) will be in a group, and that the blinds on every window in this group will open and close together.
The blinds operation of each window group is distinct from that of other groups, but every window in the same group will behave the same way. These pairs of plans and elevations show how windows are grouped.
In pair A, all the windows are in the same group because they are 1. on the same plane, 2. have the same external shading strategy (which in this case is none), and they all correspond to the same analysis area.
In pair B, each window is in its own group because they are all on different planes (facades). Their different orientations will allow light to enter the room in different ways so they will behave independently of each other.
In pair C, the top windows make up Group 1 and the windows on the bottom make up Group 2. Even though all the windows are on the same plane, the top windows (Group 1) have no shading and the bottom windows (Group 2) have an awning over them.
Three sets of plan and elevation describing window groups.
Direct Sun Allowance (The 2% Rule) for Blind Operation
Operable blinds will open and close during a simulation depending on the percentage of points on the Illuminance Grid that are hit by direct sun. For example, the sDA metric allows for 2% direct sun in a space, meaning that if more than 2% of the points on an Illuminance Grid receive direct sun, it will “close” the blinds and take the illuminance values with closed blinds instead.
Note: For LEED v4, there is a loophole regarding the sDA metric that exempts you from having to use operable blinds if your ASE score is below 7%. This is due to the belief that a low Annual Sunlight Exposure (ASE) score indicates effective hourly solar control—though in reality, a good annual score does not guarantee that every hour will be comfortable.
The following three images are plan views of an Illuminance Grid with 100 points. In the first image, only two points on the grid are hit by direct sunlight. In other words, there is only 2% direct sunlight in the space, so the blinds don’t have to be drawn.
Since the second image (noon, Blinds Open) is greater than the 2% that the sDA metric allows, this simulation will close the blinds (third image, noon, Blinds close) and no direct sun will enter. The third image is the data that will be scored by the sDA metric.
Direct Sun Exposure: Plan view of a space at three instances illustrating the two percent rule and and blind use for the sDA metric.
Sunlight is cast back in a room and does not hit the workplane Illuminance Grid. Model from Clanton & Associates.
Let’s look at this concept in a model. Here is an Illuminance Grid with one point that is red, signifying that it is illuminated to more than 1,000 lux.
A rendering of the space at the same time reveals that the clerestory blinds are open. If they were closed, it would eliminate the red point. But since the sDA metric allows for up 2% of the grid to be hit by direct sun, this one point is permissible and the blinds can stay open.
Climate and Solar Angle
Exposure to direct sunlight, and therefore operation of blinds, also depends on climate and solar angle.
Climate: Since using the operable blinds tool will open or close blinds based on how much direct sun enters your space, it makes sense that sky conditions will affect their motion. When it is cloudy, blinds can stay open because the daylight that enters is not as intense as direct sun. When it is sunny, blinds will usually have to close, because the daylight that enters is too intense.
Solar angle: The position of the sun in the sky will affect how much light gets into your space. For example, let’s say it’s June 21st and your building is on the equator. The room that you are measuring light in has one window on the southern exposure. The blinds will stay open all day even if it’s sunny, because the sun will pass directly over your building from east to west and never enter the space.
Now take that same building and move it to Boulder, CO (40 degrees north). In the same climate conditions (sunny all day) the blinds will close for a significant portion of the day, because the sun will be at a lower position in the sky, meaning its rays are more horizontal and will penetrate into the space.
The following images show how operable blinds affect illumination levels in a space. This is shown for June 21st and December 21st.
Row A shows a series of renderings of a building without operable blinds.
Row B shows a plan view of the an Illuminance Grid in a space without operable blinds.
Row C shows a series of renderings of operable blinds opening and closing according the sDA metric.
Row D shows the same plan view, but this time the space has operable blinds.
Model from Clanton & Associates.
Row A shows a series of renderings of a building without operable blinds.
Row B shows a plan view of the an Illuminance Grid in a space without operable blinds.
Row C shows a series of renderings of operable blinds opening and closing according the sDA metric.
Row D shows the same plan view, but this time the space has operable blinds.
Model from Clanton & Associates.
Limitations of Using Horizontal Illuminance Grids for Triggering Blind Operation
No metric is perfect, and there are ways there could be more than 2% direct sun in the space without triggering the blinds to close. The image below shows the light from a clerestory window sliding in between points on an Illuminance Grid, which means it won’t register. If you are concerned there is more than 2% direct sunlight in the space, consider using a denser point spacing on your Illuminance Grid.
There is also a somewhat controversial loophole for the sDA metric in its applications to LEED v4 noted in the section on the 2% Rule. If your ASE score is below 7%, you don’t have to simulate blinds for the sDA metric. A low ASE score indicates effective solar shading, but can’t guarantee that every hour will be lit to comfortable levels.
Model from Clanton & Associates.
Another limitation of this metric is that it doesn’t look at direct sun on the walls, because it doesn’t make contact with the Illuminance Grids.
This space is very bright, but the workplane Illuminance Grid doesn’t show any direct sun (red squares) because the light is cast deep into the room instead of down onto the floor. Model from Clanton & Associates.
Step 2: Run Simulation with Blinds in Position and Score
The first step of the simulation determined whether the blinds are open or closed. The second step of the simulation determines the level of illumination at each point and at each hour after the blinds are in position. For LEED V4, a point must meet a minimum illuminance of 300 lux (28 footcandles) for at least 50% (1825 hours) of the year: sDA (300, 50%).
Illumination values of a single point at several hours.
Once lighting levels at the point are recorded for each hour, they are evaluated according to the minimum threshold (300 lux.) This is why the formula is written sDA 300, 50%.
The orange dots here show the hours that the point meets the minimum illuminance threshold of 300 lux. The gray dots show hours that the point does not meet the illuminance threshold at that hour.
Total hours of illumination above 300 lux.
The point must meet the minimum threshold for at least 1,825 hours (or 50% of the time) to be counted in the final evaluation. This point meets the threshold for 1,937 hours, so it passes.
Since the point meets 300lux for 53% of the time, it is acceptable.
That is the process for one point, and every point on the Illuminance Grid is measured this way. Your final simulation results are the percent of points on the workplane with daylight illuminance above the minimum threshold for half the time.
Percentage of points that meet the 50% threshold.
Your final score is the percent of points on the grid that meet the minimum illumination threshold for 50% of the time.
LEED v4 Daylight Credits:
0-55% – unacceptable (no credit)
55-75% – nominally acceptable (up to 2 credits)
75-100% – preferred (up to 3 credits)
3. Illuminance Calculations (LEED v4 Option 2)
Another way to get LEED v4 Daylight Credit is through Option 2. This option doesn’t allow as many points as the Option 1 ASE and sDA annual calculations, but remains available if your Option 1 results do not get enough points. Here is how LightStanza determines Option 2 credit:
Step 1: Calculate illuminance intensity for sun (direct component) and sky (diffuse component) for clear-sky conditions.
a. Select one day within 15 days of September 21st and one day within 15 days of March 21st with the clearest sky conditions using average sky cover. Sky cover is the amount of the sky dome in tenths (i.e. 1 is 1/10 coverage) covered by clouds of obscuring phenomena at the hour indicated. These values are from the EnergyPlus database.
b. Find the average of the hourly value for the two selected days.
Step 2: Measure illuminance at 9am and 3pm with previously-determined clear-sky conditions. Illuminance values must be between 300 and 3,000 lux to achieve credit.
Once LightStanza has gone through each of these steps using your model’s location, it will generate output that shows how much of your space is between 300 and 3,000 lux at the 9am and 3pm time points. Then it will tell you how many LEED v4 Daylight Credit points can be achieved through Option 2.
For more information on LEED v4 credits, click here.
