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.

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.

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.


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 Sky Factor icon or menu Extensions -> De Luminae -> Sky Factor -> Sky Factor calculation and enter the parameters of the calculation.

    DL-Light extension sketchup Sky Factor General tab

    Fields description:

  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.

    DL-Light extension sketchup Sky Factor Panel tab
  4. After entering the parameters, select surfaces and press Render or run_selected button.

    DL-Light extension sketchup Sky Factor selection

    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"):

    DL-Light extension sketchup Sky Factor result 1

    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.

    DL-Light extension sketchup Sky Factor result 1

    Use the run_selected icon to display only false color textures:

    DL-Light extension sketchup Sky Factor result 2
  6. Results analysis.

  7. For further calculations in the same model:

    Since version 8.0, 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.

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

  9. Snapshots:

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

    DL-Light extension sketchup Sky Factor snapshot image

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:

    DL-Light extension sketchup Sky Factor global after
  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).

    DL-Light extension sketchup Sky Factor results after
  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".

    DL-Light extension sketchup Sky Factor comparison
  6. Select the archives of previous computation.

    DL-Light extension sketchup Sky Factor comparison AB
  7. Press Render, the result of comparison should be displayed quickly. Areas with loss of illumination are shown in red.

    DL-Light extension sketchup Sky Factor Rights of light comparison

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

    DL-Light extension sketchup Sky Factor Rights of light before after

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

    DL-Light extension sketchup Sky comparison palette
  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.

    DL-Light extension sketchup Sky Factor Report dialog