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The analysis grid defines a grid of calculation points; this is typically the workplane on which we would like to check illuminance values. In this case, we are interested in setting up a horizontal workplane, 900mm above FFL.
To define the analysis grid, first select the surfaces that would be used to define the shape of the grid; in this case the 2 floor surfaces of the 2 zones. In the Analysis Grid tab, click on "Fit to selected objects -> Fit grid in current axis (2D)" under the Grid Data & Scale section. Notice that we are defining a 2D plane in the XY axis. The default offset is 600mm. Change it to 900mm.
Notice that the default resolution of the analysis grid is quite low; there are 16 divisions along the entire length and breadth. The spacing is defined by the number of divisions along each axis. Click on Grid Management in the Grid Settings section to change the settings. Assuming we'd like to have a grid spacing of about 500mm, the Analysis Grid Management dialog box shows that the grid size (rectangular bounds of the 2 surfaces we previously selected)is 9375mm. This implies we'd need 19 divisions. Notice that after entering the desired number of divisions and selecting OK, the analysis grid defaults to the entire rectangular bounds. Since the 2 floor surfaces are still selected, simply click on "Fit to selected objects -> Fit grid in current axis (2D)" again.
Note that it is good practice to clear any previous calculation data. The "reset" button is in the same Analysis Grid Management dialog.
 
With the analysis grid defined, click on the "Perform Calculation" button at the bottom of the Analysis Grid tab. Since we have not defined lamps yet, select Natural Light Levels -> Over the Analysis Grid -> Medium (Since the geometry is very simple) -> 7742 lux (Calculate for me->Tregenza), Overcast -> Average (x 0.9) Window Cleanliness -> OK.
Analysis results (Model with skylight on left, no skylight on right)
Beacause a simplified split-flux method is used, the calculations take just a while. We are interested in illuminance values, so select "Overall Light Levels" in the Grid Data & Scale section in the tab. Remember to adjust the scale as well (hitting enter after entering a value will refresh the display). You can also display numerical values or plot styles in the Grid Settings section in the tab.
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As noted from the results above, limits of the simplified method/low resolution become obvious, such as on the southeast wall of the small office (line of high illuminance on left side). Even when the grid size is reduced and more sample rays are used, there is still visible variance in the results (below left). To achieve more accurate results, we use the Radiance rendering engine instead.
Comparing analysis results (Ecotect on left, Radiance on right)
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Updated 19 August 2008, YC Huang.
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