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Building Design

Revit: Illuminance Simulations

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Autodesk Revit now features fully configurable illuminance simulations via its "Rendering as a Service" feature (RaaS). This playlist will teach you how to use the tool.

You may want to download the following Revit file to use while learning the tool: Advanced_Model - DaylightingAnalysis.zip

Index and Anchor Links

 

Quick Start: Illuminance Rendering in Revit

What is illuminance and why is it useful?

Illuminance is a measure of how much light falls on a surface. It is useful for determining whether or not there is enough light to perform different activities (like reading, office work, or drafting). Illuminance is measured in lux or footcandles (1 footcandle = 10.7 lux). 

You need approximately 50 to 1000 lux, or 5 to 100 foot-candles for activities inside a building depending on the activity. An illuminance rendering shows you whether your lighting design meets the requirements of the space, and it also helps you understand how much of this light you’ll be able to get from daylighting.  

For a table of illuminance values for common tasks, see the Autodesk Sustainability Workshop.

Daylighting performance depends on the design and the day, time, and sky conditions.  Illuminance renderings for representative times offer a way to understand the performance of a space for daylighting.  For example, the LEED daylighting credit (8.1) requires 75% of regularly occupied spaces to have daylight illuminance values between 10 and 500 foot-candles in a clear sky condition on September 21 at 9am and 3pm. Other metrics like Daylight Autonomy aggregate illuminance values for the entire year.  For more information on measuring light, see the Autodesk Sustainability Workshop.

 

Workflow Summary: Illuminance Rendering in Revit.

1. From the View ribbon, select Render in Cloud

 

2. You will be prompted with the following screen. It may take a few seconds for the dialog box to appear. Select a 3D View for the illuminance rendering. You can select one or multiple views to render at the same time.  The 3D views in the dialog box correspond to the 3D views you created in your project (See Setting Up Your Revit Model for Illuminance Rendering later in this playlist).

 

3. For Output Type, select Illuminance

 

4. After selecting Illuminance, you will be prompted to select the Location/Date, Sky Model, and Legend Settings. Work through these tabs to set the simulation settings for the rendering. 

a. Location/Date

The location for the illuminance rendering is always taken from the Revit Model Location. You can learn how to set the location for your project on the Specifying the Project Location page.

The date and time to be used in the rendering can be set in the dialog box. Alternatively, all 3d views contain a setting for the date and time, and this setting can be used automatically.  This may be a setting used from a previously conducted Solar Study, or a date and time used simply to visualize shadows in the Revit 3d model view.  You can choose to use those settings for the rendering by checking the box below the date field. These settings, along with the location, will be used to dictate the position of the sun in the rendering.

Be sure that your time is set using the 24 hour clock (i.e 0:00 denotes midnight and 15:00 denotes 3pm). 

b. Sky Model 

The Sky Model tab features 6 different sky models for the illumination rendering, and also allows you to specify the sun’s intensity (irradiance). Use this tab to select the sky model that best suits your needs and enter irradiance values appropriate to your location.

Good sources for weather data with irradiance values are Green Building Studio, where you can download CSV format weather data for your Revit project location, or the DOE weather data site

See more in: Sky Models & Irradiance Values later in this playlist

c. Legend 

In the Legend Setting tab you can set:

  • Units (Footcandles or Lux) – the measure of luminous flux per unit area
    • Lux is the metric or SI unit equal to one lumen per square meter
    • Foot-candles is the English or IP unit equal to the illuminance cast on a surface by a one-candela source one foot away
  • Scale Range:
    • You can choose to specify the minimum and maximum values 
    • Alternatively, you can select “Automatic” to have the scale’s range set based on the minimum value and maximum value (95th percentile) calculated in the simulation with 10 proportional subdivisions.
  • Scale Increment:
    • By default, a linear scale is used with 10 proportional subdivisions based on the range chosen.
    • If you choose Logarithmic, the scale will be logarithmic so that you can more easily visually distinguish between values in the lower portion of the range while still capturing very high values.
      • This scale is particularly useful when you select an “Automatic” range because it’s important to be able to distinguish between zero and 2,000 lux for interior lighting design. With a linear scale, the “Automatic” range often renders these critical values within the same color increment.
      • The human eye can function over a huge range of illuminance values – and a logarithmic scale better represents how we perceive levels of brightness. 

See the Autodesk Sustainability Workshop for more on measuring and perceiving light. 

 

5. Set the image size to set the resolution of the rendering.  Note that the resolution will influence how many cloud credits the rendering will cost you. You can find more information here

6. When you have completed your selections, select Start Rendering

7. When your renderings are competed you will be notified with a pop up window in the lower corner of the Revit screen, and by email if you selected that option. You can view the renderings by selecting Render Gallery in the View ribbon from Revit. You will need to login to Autodesk 360 in order to access the images. Your Autodesk 360 account is the same as the account you use for Revit.  

8. Note that you can re-render the same scene using different illuminance settings directly from the Render Gallery, without using Revit (it will use the same geometry, materials, and camera view from the original Revit render). See “Rendering from the Render Gallery” later in this playlist.

 

Sky Models & Irradiance Values

Sky Model Explanations

The Perez sky model is the most commonly used model in daylighting simulation applications and the model most analysts use in Radiance. The Perez model yields accurate results for all sky types from clear to fully overcast.  It is also the model LEED requires be used for daylighting predictions.  

The CIE models are essentially simplified and standardized instances of the Perez model. They provide standard conditions for daylighting applications (Overcast sky, Intermediate sky, Clear sky, and Uniform sky). If you know what kind of sky you will have, or you’re trying to get values for a particular situation (clear or overcast) then it is a good idea to use a CIE model.  

When you choose the Daylight Factor Sky, your results will be expressed as a percentage. This is the percentage of natural light falling on surfaces compared to that which would have fallen on a completely unobstructed horizontal surface under same sky conditions.  The location or time of day does not matter if you are using a Daylight Factor Sky.

It is important to NOT include electric lighting in Daylight Factor renderings.

The Daylight Factor Sky is actually based on the CIE overcast sky model, with a GHI (global horizontal irradiance) value pre-set to the equivalent of 100 foot-candles. With this pre-set, illuminance measurements effectively represent the daylight factor, or the percentage) of daylight falling on a surface.

If you are unsure of which sky model to use, a good place to start is the CIE Overcast Sky model. This model will not have the unique characteristics of direct sun, and represents an overcast condition that could happen at any time of day and is a good way to put your design to the test.  It is often standard practice to also render using the Perez model at an equinox and the solstices at noon, 9am and 3pm to visualize a range of standard conditions. 

 

Irradiance Values

With the exception of the Daylight Factor Sky model, you will need to specify the sun’s intensity by entering irradiance values:  DNI (Direct Normal Irradiance) and DHI (Diffuse Horizontal Irradiance). For all DNI and DHI values, make sure you use units of W/m2.  These values can be found in most typical climate or weather files meant for analysis work as described below.

DNI = Direct Normal Irradiance [Input] - The terrestrial solar irradiance received per unit area of a surface that is normal to the sun’s position.

DHI = Diffuse Horizontal Irradiance [Input] - The terrestrial solar irradiance received by a horizontal surface which has been scattered or diffused by the atmosphere. It is the component of global horizontal irradiance which does not come from the beam of the sun.

GHI = Global Horizontal Irradiance [Calculated] - The total amount of terrestrial solar irradiance falling on a surface horizontal to the surface of the earth. Calculated as: GHI = DHI + DNI * cos (solar zenith angle)

Technically GHI also includes ground-reflected radiation. In practice, this value is often so small it’s negligible.

NOTE: There are a parallel set of measures often found in climate files that are based on illuminance (measured in lux), instead of irradiance (measured in Watts). They are: DNL (Direct Normal Illuminance), DHL (Diffuse Horizontal Illuminance), and GHL (Global Horizontal Illuminance). Again, be sure to enter irradiance values into the tool… NOT these illuminance values.

On a clear day, most of the solar radiation received by a horizontal surface will be DNI, while on a cloudy day most will be DHI.

The easiest way to find good values for DNI and DHI using Autodesk tools is to download a Weather file for your Revit model location using Green Building Studio.  More information on this can be found under Using GBS Weather Files to get DNI & DHI settings later in this playlist. 

 

Rendering and Re-Rendering from the Render Gallery

You or any collaborator who you have granted access to your account can also create or modify illuminance renderings from the Render Gallery through your Autodesk 360 account. This service is available for renderings you have already completed through Revit. You cannot change views, but you can render an existing image as a visual or an illuminance rendering, modify legend scales, times and dates, or sky models, for example. Note that this option works from a version of the model geometry stored in the cloud, and thus will not take into account any changes you have made to the model since the first rendering was done. 

1. Access the Render Gallery from the Graphics panel in the View tab within Revit or by signing in to Rendering.360.autodesk.com.

 

2. From the Render Gallery, select the rendered view you want to change. Drop down the arrow and select Re-render using new settings

 

3. You will then be prompted with a dialog box similar to the one you went through to create illuminance renderings through Revit. Review the settings at the Quick Start page. 

 

4. Once you have completed your selections, select Start Rendering. The rendering with the new settings will save as a separate image from the original. Be sure to refresh your browser and show All Renderings to view the new rendering. The new rendering will not be titled anything different or have any information about the rendering settings so be sure to keep track of your changes and the associated images (a good tip is to download and save the image with a name that corresponds to the settings after it is completed—the date and time in the file name when you download are for when the rendering was completed, not your rendering settings). 

Revit Settings for Illuminance Simulations

Setting up a Plan View for Illuminance Simulations

Illuminance simulations in Revit use standard Revit 3D views as the basis for the analysis. To get a useful view for analysis, you need to be thoughtful about how you place your cameras in Revit. The process below will tell you how to set up the proper cameras to get a good floor plan view.

 

Why to use floor plans for analysis

Illuminance analysis is most commonly done on a floor plan view of the room or space. With good data overlaid on top of a floor plan, it is straightforward to calculate the percentage of floor area that is within minimum and maximum illuminance requirements for a space or to understand light sufficiency on a work surface such as a desk or table.

Perspective views are usually not used for illuminance simulations – but could be useful if measuring how much light falls on a whiteboard in a classroom or a piece of art in a museum, for example.

Note that perspective views ARE commonly used for LUMINANCE simulations (light coming FROM a source or surface), used to analyze glare from the perspective of building occupants. This workflow is not currently supported in Revit.

Placing Cameras for Illuminance Renderings

Here’s some guidance on how to use and place a 3D camera to approximate a 2D floor plan view for illuminance simulations.

Do NOT do a section cut of the building! The method using a section cut doesn’t work for daylighting scenes, because it actually removes the top part of the building from the camera view. This would mean your illuminance render assumes there is no roof!

What you need to do to get a good plan view for Revit illuminance simulations is create a non-perspective 3D camera looking straight down, and located between the ceiling and the floor of the level of interest.  Typically you should put the camera at the floor plan cut plane about 48” above the floor.

1. Select a built in Floor Plan view of the floor you want to have your illuminance rendering of. 

 

2. From the View ribbon, select 3D View, then Camera

 

3. Before placing the camera, it is very important you deselect Perspective to create an orthographic 3D view. This means that the camera is actually a PLANE that measures illuminance values parallel to the direction it’s pointing, NOT a POINT that perceives the scene like the human eye (or camera). 

 

 

4. Place the camera anywhere in the view. Don’t worry about placement or where you point it. We will change this setting in following steps.

 

5. After placing the camera, the camera view will appear. Use the view cube to rotate the camera to the TOP view. This ensures that the camera is looking straight down.

 

6. In the camera Properties, change the Eye Elevation so it is below the elevation of the ceiling, but above the elevation of the floor for the level you are interested in.

The Eye Elevation is the height of the camera. Best practice is to place the Eye Elevation about 4 feet above the floor plan elevation you are interested in.

The Target Elevation is how far the camera will be able to “see” into the model. Change the Target Elevation so it is below the elevation of the floor (any value less than the floor plan elevation you are interested in --typically a value of 0, or the ground plane, will be sufficient).  

   

You can use a section elevation to help determine what these values should be. Note the elevations of the floors and ceilings, especially if there are drop ceilings. In the image below, an Eye Elevation of 9000 and Target Elevation of 0 are good settings to view the 3rd floor plan.  

 

7. Finally, be sure the Crop Region is visible in the Extents section of the camera Properties window, and adjust the camera crop region extents in the model view so you can see the entire floor plan or whatever portion you want to focus on for your illuminance rendering.

Rendering Settings for Illuminance

The Rendering Settings dialog box is where you can select the quality and light sources for your illumination rendering. You can access it through the Properties of each of your 3D views.

 

The Lighting determines the light sources for your rendering. The QualityBackground, and Image settings are not valid for illuminance renderings. 

Within the Scheme dropdown, the Exterior and Interior descriptions don’t apply to illumination renderings (“Exterior: Sun only” will produce the same results as “Interior: Sun Only”). However, you can choose to include the Sun, Artificial lights, or both. It is worth noting that artificial lights are only included in the rendering if you select them here and have included electric (“artificial”) lights in your model and scheduled them to be on. If you want to measure daylight-only illuminance values (for LEED, for example), make sure to select “Sun Only” from this dialogue box.

The Sun Settings defined here can be used in your illuminance rendering, or can be overwritten in the “Render in Cloud” dialogue box.

The Background settings do not affect illuminance analysis results – they are for aesthetic purposes only. The sky model used in the illuminance rendering is, instead, determined in the “Render in Cloud” dialogue box.

 

Surface and Glazing Material Settings for Illuminance

You can easily create an illuminance rendering using default material properties within Revit families. However, to make your results more accurate, you’ll want to assign the appropriate material properties to your window glass and interior surfaces.  

Glazing Properties for Illuminance Renderings

Accessing the Material “Appearance” Tab

Select the glass pane to which you want to assign material properties. From the Properties editor, select Edit Type. The Type Properties dialog box will appear. You’ll want to change the properties associated with the Material.

Note that the Analytical Properties of the glass is only used in energy analysis and does NOT affect illuminance analysis results (even though visible light transmittance appears like it might be used in the rendering – it is NOT).

Also note that, depending on how the family is built, this material parameter might have a different title or be in a different parameter category.

Click on the “” in the Value column for Material. The Material Browser dialog box will appear. Go straight to the Appearance tab. This tab is used to determine the physical properties of the materials used in the rendering.  Any changes made in the Graphics tab will ONLY affect the appearance of the object on the design canvas; it will not affect the rendering. 

From the Appearance tab, the only settings used in the illuminance renderings are the Color field under Glazing (accessible when you choose a Custom color – and explained below in “Setting the T-Vis of the glass”).

The Reflectance is NOT used for the illuminance rendering. Instead 360 Rendering uses a pre-defined value of ~4%. At this time Revit is unable to support other values.

Sheets of Glass is also not used for the rendering as 360 Rendering uses the actual model geometry. So, for example, if the layered construction of the window includes two panes of glass, the illuminance rendering will consider the combined effect of both panels (based on the “color” defined). 

Setting the T-vis of the glass

The visible transmittance (T-Vis) of glass in the rendering is currently calculated from the Color and Tint values ONLY. However, it is best practice NOT to use Tint, meaning T-Vis should be set by changing ONLY the Custom Color parameter.

In the Color field, select Custom.

To set the T-Vis of your glass for illuminance renderings in Revit, Autodesk recommends setting RGB values to always be EQUAL. This is for simplicity’s sake and should not affect analytical results given that illuminance is measuring the QUANTITY of light, not the QUALITY or color of that light.

The T-Vis of the glass will depend not only on the color you define, but also on the THICKNESS of that glass and how many panes there are. You can use the table below to approximate a T-Vis appropriate to the actual glazing geometry modeled in Revit.

For example, if you have modeled a double pane window with geometry that is 3mm thick for each pane, and you want the T-Vis of the window assembly to be 70%, you would specify R150, G50, B50.

A “perfectly clear” piece of glass will have a T-Vis of about 92%. 

To use this chart:

1) determine the thickness of the window pane geometry in your Revit model and the number of panes modeled (single, dual, etc.)– this is the ROW;

2) determine the T-Vis value you’d like to simulate – this is the COLUMN;

3) set the “color” of the glass material in your Revit family to have RGB values that are each equal to the number in the CELL you’ve identified (high numbers are more transparent; low numbers are more opaque). 

Note: The values in this table may change with RaaS updates as we improve these workflows. Be sure to check back so you have the most accurate T-Vis values for your glazing. 

 

The actual T-Vis derivation for a single pane glass is calculated as:

Tvis = 0.9216 * 10^( thickness_in * log_10( (color / 255)^2 ) )

where 

thickness_in = the thickness of the pane in inches (note: there is currently a bug in Revit that overestimates the thickness of glass.  This bug is taken into account in the table above)

color = value in the Revit dialog [0-255] based

For multiple panes the leading constant changes.

The table above calculates the inverse of this function. The equation is derived from the physics of light transport in glass. It combines the Beer-Lambert law of transmission and the Fresnel equations (which are used to calculate the leading constant: 1-R). The renderer implements these physical equations because they automatically account for things like directional variation in reflectivity and transmissivity and greater absorption in thicker panes of glass. Further it ensures that glass is always handled in a physically correct and consistent manner.

 

Reflectivity of Opaque Materials for Illuminance

It is also important to consider the material properties of the other objects so that 360 Rendering accurately calculates the light bouncing off surfaces in the model to accurately visualize light intensity falling on a surface. You can change opaque material properties using a similar process to that of glazing.

Select the object to which you want to assign material properties. From the Properties editor, select Edit Type. The Type Properties dialog box will appear. You’ll want to change the properties associated with the Material (object) or Construction (wall, floors, roofs, and ceilings). If you are changing the properties for an object, the process of accessing the material editor is the same as for accessing glazing materials. The rest of this section will explain the process for walls, floors, roofs, and ceilings. Again, ignore the Analytical Properties.

For a wall, select the interior surface material of the construction by clicking on the last material on the interior side of the construction. This will take you to the Material Browser.  

As with glazing properties, the Appearance tab is the only one that affects the results of illuminance simulations. Tabs NOT USED used are mentioned below…

Information on the Identity tab is not used for rendering or appearance.  Use something descriptive in the Class for clarity only.

Information on the Graphics tab is only for how the material appears on the computer screen.  It is a good idea to select Use Render Appearance at the top so you can validate your model on screen.

The Physical and Thermal tabs also have no effect on rendering or illuminance performance.

On the Appearance Tab, the RGB Color settings within the Generic panel determine the reflectivity (%) of the material, based on the following formula:  (0.2126 R + 0.7152 G + 0.0722 B) / 255

If all other attributes in the Appearance tab are unchecked, the material will have a matte finish, with only diffuse reflectivity (not shiny/ specular). It is also possible to simulate specular materials by selecting “Reflectivity” and changing the settings there (described below). 

If an Image is selected, the average colors of the picture determine the basic material reflectivity due to Color in percent, by the formula:  (0.2126 R + 0.7152 G + 0.0722 B) / 255 for each color in the picture.

The other Generic attributes only have an effect if you’re trying to create a specular material (Reflectivity is checked and the values entered there are non-zero). 

 

Properties used to create specular materials.

 

The term ‘Reflectivity’ here refers to Specular Reflectivity, which means non-diffuse, mirror-like reflectivity, not overall surface reflectivity like that determined by the Color attribute in the Generic section.

The other sections in the Appearance Tab (transparency, self-illumination, tint, etc.) define other surface appearance properties, and therefore affect light behavior.  They will all affect overall light reflectivity in about the way you would visually expect.

 

How Illumination Rendering Works in Revit

How is it so fast?

First of all, the calculation happens on the cloud – not on your local machine. But, as importantly, the ray tracing algorithm is faster.

Autodesk 360 Rendering uses bidirectional ray tracing but with an additional intelligent algorithm to determine the ray order such that the most important rays are generated first. The result is a much faster convergence to final results. Revit’s RaaS engine calculates all the bounces that are important for making conclusions. Other tools follow the bounces of a photon even past the point where it practically affects the rendering. The algorithm Revit uses is called Multidimensional Lightcuts (see Siggraph 2006), with trade secret/patented extensions.

Has it been validated?

The engine has been initially validated with the help of one of the top daylighting firms in the country. They used a consistent model to compare Revit’s360 Rendering illuminance simulation output to both Radiance (the industry standard lighting simulation tool) and to the actual space measured in the real world.

Results of the validation tests are strikingly similar in all ways but one…the time it takes to create the rendering. When we asked about the level of quality used in the Radiance rendering and how much time it took, the answer was: 5 bounces and it took about 4 hours to render. When we asked the cloud rendering technician how many bounces, the answer was ‘all of them’, and the rendering took about 10 minutes.

 

Electric Lighting Resources

In addition to rendering the illuminance based on daylight, you can include electric lighting in your renderings. Use the articles below to learn how to include electric lights in your model. The most important thing to remember for illuminance renderings is to include Artificial (i.e. Electric) lights in the Rendering Settings for each 3D view.  

Once you have included Artificial lights, you will have the option to turn fixtures on and off. 

 

Getting DNI & DHI settings from GBS Weather Files

You can download weather files from Green Building Studio for the weather specified in your Revit model and use them to determine your hourly DNI and DHI values. 

1. To access weather files from Revit you’ll first need to set your location in Revit on the Manage tab under Location, and then establish a “Green Building Studio Project.”  Green Building Studio, like 360 Rendering, is a cloud service.  To create a project from Revit, you simply need to conduct an energy simulation in the cloud by clicking Run Energy Simulation from the Analysis tab. For more information see the directions here for a basic start. 

Note that if you login to Green Building Studio directly, you can define a new project, set its location, and access the weather data without running an energy simulation.)

 

2. After your energy analysis has completed, open the results in Results & Compare screen by selecting the link in the Analysis tab. From Results & Compare, select Open> Green Building Studio to open the GBS web browser. 

 

3. Once in Green Building Studio, navigate to the Weather Station tab. Select Download Weather Data. Download a CSV file. 

 

Open your weather file. Locate the DirNormRad (DNI) and DiffHorizRad (DHI) columns. These are the DNI and DHI values you will enter in the illuminance rendering settings for the time and date you are using in your 360 Rendering dialog. Use the Month, Day, and Hour columns to find the DNI and DHI values you will need for your rendering times.

 

Note that some hours have cloudy weather, denoted by the value in the TotalSkyCover column of the weather file.  If you want to model a clear sky condition, be sure to choose a day that has ‘0’ for TotalSkyCover at the hour you are interested in.  If the day you have chosen in your rendering settings is cloudy, it is valid to choose a day within a week or two of that date with a clear sky to get the clear sky values for the day you are interested in.

 
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