Sky Factor Extension

Introduction

The Sky Factor is the ratio of the illuminance received directly through openings (unglazed), to the illuminance on a horizontal plane due to an unobstructed hemisphere under a sky of uniform luminance distribution. The Sky Factor does not include reflected light from outdoor or indoor surfaces.

• Image of a Sky Factor rendering with 0.2% limit:
  

The Sky Factor indicator is mostly used in UK policies, both for new constructions assesment and to work out whether a proposed new building will affect the 'Rights of Light' of an existing building. The impact of the new building on the 0.2% limit position allows to evaluate the loss of light.

• Image of a 'Rights of Light' rendering, a new building results in loss of light (red area) compared to the previous situation:
  

Calculation method

This implementation of the Sky Factor indicator uses a ray-tracing simulation of direct illumination without interior or exterior reflexions. This extension does not use the Waldram diagram method.

Consequently, this extension does not claim to replace the method of Waldram if use of that method is required.

• Sky Factor

  The most common use is to determine the portion of an area with an indicator value greater than 0.2%. The measurement is carried out on a virtual plane located 0.85m above the floor.

  The distance of 0.85m is the default setting of the virtual surfaces for this extension (value configurable in the preferences). For the use of virtual surfaces, see page: Create virtual surfaces.

  The traditional method of evaluating the 0.2% delimited area uses "Waldram diagrams". This extension uses a different method to determine the areas passing the criterion: the computation is made on a significant number of sensors, then the area is computed by assigning to each sensor the portion of surface it covers (coverage of Voronoï polygons).

  See below under How to use Sky Factor Extension.

• Rights of Light

  In a 'Rights of Light' context, the objective is to establish a comparison between 2 situations: before and after a new construction. The extension integrates a functionality of comparison between 2 calculations carried out beforehand and provides the gains and losses of lighting, in the form of a table and false colors textures on the SketchUp model.

  See below under How to use Sky Factor to make a "Rights of Light" comparison.

How to use Sky Factor Extension

Important: layers corresponding to glazing must be hidden. You can set the glazing layers invisible within SketchUp, or use the DL-Light transparency feature (see page Make layers "transparent").

1. Open SketchUp model. Follow recommendations to prepare your model for DL-Light calculations.

2. Open Sky Factor dialog from toolbar or menu Extensions -> De Luminae -> Sky Factor -> Activate extension and enter the parameters of the calculation.

   

   Fields description:

   • Project name - Project name is used during export and for output filenames generation. The SketchUp model name is used by default.

   • Results - Directory where model will be exported for calculation. It should be an existing or a new directory where user has appropriate write/read privileges. When calculation is started model geometry and resulting files will be saved in skyfactor_output subdirectory.
     For SketchUp projects stored on network disk (Windows), see Work on a copy of the model.

   • Mode - Calculation mode selection:
     Both modes uses the same computation, but the graphical representation differs:

     • The '0.2% threshold' uses a bicolor palette. The default threshold of the palette is the usual 0.2% value. Using such a bicolor palette is required for a later comparison stage.
     • The 'Levels' mode proposes a palette with more thresholds, intended to allow a more detailed analysis of the distribution of light on the surface.

   • Details - By default, the calculation uses a dedicated grid 'SF grid' with 16 sensors per square meter. Any other grid can be used, however Sky Factor calculation requires a dense grid to achieve sufficient precision.

     DL-Light allows to choose:

     • the sensor grid applicable by default to all calculated surfaces,
     • different sensor grids for some surfaces ("exceptions" button).
     

     See sensors selection for more information on the choices of proposed sensors.

3. The '0.2% threshold' mode uses a bicolor palette. On this palette only the threshold and colors can be modified. The 'Levels' mode uses a palette whose all parameters can be modified.

   

4. After entering the parameters, select surfaces and press Render or button.

   

   The model shown contains openings on 3 sides and complex obstructions. Notice at the top left a block marked as transparent (see page Make layers "transparent"):

   

   Calculation time depends on number of selected surfaces and model complexity.

5. When results are imported, selected surface will have appropriate false colors and Palette and Report dialogs will be opened.

   

   Use the icon to display only false color textures:

   

6. Results analysis.

   • The Report dialog displays the distribution of results over current palette and calculation input parameters.

     The '0.2%' threshold defines the 'lighting' limit of the surface. For each surface, the total area of the surface (m²) and the lit area (both m² and percentage) are displayed.

     

7. For further calculations in the same model:

   • To run calculation on any surface simply select it and Render .
   • It is possible to modify the parameters of the calculation. Whatever the changes, use the Render button. Previously calculated surfaces will be updated with the new settings.

   DL-Light uses a calculation cache algorithm. It automatically detects if a surface requires recalculation or if a previous result can be reused. By default, the cache keeps the information of the last 10 calculations (value configurable in the preferences).

8. Archiving results:

   DL-Light allows to archive all the results of a calculation: reports, textures and calculation steps (exported Radiance model ...). This archive can be reloaded on the model, subject to using exactly the same model as during the calculation.

   See the page Archiving results for more informations about this feature.

   • To archive the calculation results, click on the button Save archive.
   • To load the results of a previous calculation, click on the button Load archive

   See the page Comparison of results for more informations about comparison of archives.

9. Snapshots:

   Current SketchUp model view with imported results can be exported with palette or without palette to png format. Open the folder of images with .
   This option is not supported in trial version of the extension (see licence information).

   

How to use Sky Factor to make a "Rights of Light" comparison

In a few words: calculate and archive 2 versions of the model, then use the "comparison" tab.

1. Calculate the Sky Factor indicator for the model as described in the previous section.

2. Archive the result, see page Archiving results.

3. Display a difference on the SketchUp model to evaluate in terms of "Rights of light".
   For an efficient organization, it is recommended to place the geometry changes (new constructions) in layers, to hide them for the first simulation, then to make them visible for the second.

   In this example, the layer transparent in the previous calculation has been restored:

   

4. Calculate and archive the Sky Factor indicator for the same surfaces, with the same "0.2%" palette and the same graphic parameters (type of interpolation, texture resolution).

   

5. Open the "Comparison" tab, it allows you to select two archives A and B to compare. The comparison module will calculate the ratios for the difference "B - A".

   

6. Select the archives of previous computation.

   

7. Press Render, the result of comparison should be displayed quickly. Areas with loss of illumination are shown in red.

   

   The red zone corresponds to the difference between the results "before" and "after":

   

   To change the colors, just edit the comparison palette in the last tab and press Render again.

   

8. The report shows distribution of results and ratios for before / after comparison. On the right side of the report are specific columns for "Rights of light" analysis called "EFZ" (Equivalent First Zone). Display of EFZ columns is an option in the preferences.