Annual Daylighting Calculation Options

Annual daylighting calculations (aka Climate Base Daylight Modelling or CBDM) are based on a simulation of internal illuminance levels using actual hourly sky distribution data over a whole year. Simulations are controlled using a set of options on the Daylighting Calculation options dialog which is displayed before the calculations are started. It is important to make selections consistent with the purpose of the analysis and nature of the building model.

Calculation Options

Simulation type

Select the Simulation type. Options are:

1-Custom, where sDA, ASE and UDI results can be generated with options to override standard settings.
2-SDA/ASE/UDI where the main annual daylighting metric are generated based on standard settings.
3-LEED v4 Option 1, simulations for the LEED v4 daylight credit , option 1 through a combination of sDA and ASE metrics.
4-LEED v4.1 Option 1, simulations for the LEED v4.1 daylight credit , option 1 through a combination of sDA and ASE metrics.
5-BREEAM HEA 01 option 4b, to check that the relevant building areas meet good practice average and minimum point daylight illuminance criteria as defined by BREEAM Hea 01 Visual comfort requirements.

Detail template

Select a detail template from the drop list to load a set of ambient parameters to the dialog as shown in the table below.

You do not necessarily need to have an in-depth understanding of the meaning of each of these parameters to carry out an accurate assessment in DesignBuilder. However knowing a little bit about the way the calculations work and how the options affect the calculations will help you to understand how to get the best trade-off between accuracy and simulation times.

Detail template	Ambient bounces	Ambient accuracy	Ambient resolution	Ambient divisions	Ambient super-samples
Radiance code	-ab	-aa	-ar	-ad	-as
1-Fast Not recommended for project work	1	0.3	128	256	128
2-Standard Lowest usable quality - but will underestimate illuminance	3	0.25	256	512	256
3-Good (no interpolation)	4	0.00	512	1024	512
4-Good Default setting for balanced accuracy and simulation speed	4	0.22	512	1024	512
5-Accurate Best setting for highest practical accuracy	5	0.20	512	2048	1024
6-High quality Use when highest quality is required and a long wait for results is acceptable	7	0.18	1024	4096	2048

Tip: For a discussion on the relative merits of using some of the above settings for daylight credit calculations see below.

Working plane height

Enter the height of the working plane above floor level for each zone in the daylight simulation (in m or ft). The default value will depend on the Report type selected. The working plane height entered on this dialog corresponds with the building level setting on the Output model data tab.

The working plane height is normally set at the average height of the top surface of the desks above the floor. A typical value will be in the range 0.7-0.8m.

Note: The working plane height used in Radiance calculations is measured from the top of the floor surface level, not the base of block as was the case in v5.0 and earlier.

Tip: To define different working plane heights for different zones, you can override the building level setting on this dialog by entering values under the Daylighting outputs header on the Outputs model data tab at block or zone level.

Margin

Enter the margin (in m or ft) around the zone boundaries where illuminance data is not to be calculated or included in summary results. This option can be used to help avoid inclusion of potentially misleading illuminance data close to walls and windows. DesignBuilder sets the margin to 0 by default as a margin is not required for most CBDM daylighting calculations.

Ground plane extension

If you enter a value greater than zero in this cell, DesignBuilder automatically generates a ground plane below all site objects to model ground-reflected light. The ground plane is generated behind the scenes and its dimensions are calculated by setting up a rectangular box in the plan view around the building extents and adding the Ground plane extension at each edge.

The auto-generated ground plane uses the Ground reflectance defined at site level.

An alternative approach to model ground reflectance is to set the Ground plane extension to zero and to use ground component blocks instead. Or a combination of ground component blocks and the ground plane extension can be used.

Tip: While it may be tempting to enter a very large number here to cover a large area, that is not usually recommended as it would significantly change the scene dimensions used within Radiance. Larger scene dimensions require high detail settings which may require very long simulation times.

Shading

This setting allows any window blinds selected on the Openings tab to be applied for daylighting calculations as well, or LM-83-12 rules for dynamic blinds can be applied.

1-No shading - don't apply any Window shading for the daylighting simulation, even if it is selected on the Openings model data tab.
2-Fixed blinds - apply any Window shading model data settings from the Openings tab. Any blinds selected will be applied in their activated (closed) state.
3-Dynamic blinds (LM-83-12) - use the rules defined in Section 2.2.7 of LM-83-12 to dynamically control blinds to maximise daylight availability while avoiding glare. Note that when this option is selected, windows have dynamically controlled blinds applied regardless of whether the Window shading model data is selected. When dynamic blinds are selected and internally positioned slatted or diffusing blinds are specified on the Openings tab then the blind properties set in model data are used in the dynamic blinds. Otherwise, if Window shading is not set, or if blinds are not internal (they are external or mid-pane), or they are neither slatted nor diffusing blinds then a diffusing blind with reflectance of 0.2, transmittance of 0.05 and specularity of 0 are used for the dynamic blinds. This option is selected behind the scenes when the Simulation type is 2-SDA/ASE/UDI, 3-LEED v4 Option 1 or 4-LEED v4.1 Option 1.

If you need to control the Shading setting for your annual daylighting simulations then you must use the 1-Custom Simulation type.

Note: Local shading model data settings are always applied to Radiance simulations.

Override zone occupancy schedule

By default the occupancy schedule used for each zone in the Annual simulations are based on the zone Occupancy schedule defined on the Activity tab, however you can override this to use the same occupancy schedule for each zone by selecting this option and selecting the schedule in the browse control below.

Occupancy schedule

When the above Override zone occupancy schedule option is selected, you can select the occupancy schedule to be used to define when zones are occupied for all zones in the simulation. Values in the schedule greater than 0 indicate occupancy.

The default occupancy schedule provided by DesignBuilder does not include daylight saving time (DST). DST has to be added manually to the schedule. For example for the Northern Hemisphere you can use a schedule like the following:

Schedule:Compact,

8:00 - 18:00 Every day (DST Northern Hemisphere),

Fraction,

Through: 03/30,

For: AllDays,

Until: 08:00, 0,

Until: 18:00, 1,

Until: 24:00, 0,

Through: 10/26,

For: AllDays,

Until: 07:00, 0,

Until: 17:00, 1,

Until: 24:00, 0,

Through: 12/31,

For: AllDays,

Until: 08:00, 0,

Until: 18:00, 1,

Until: 24:00, 0;

Exclude unlit hours

You can exclude hours where no light is received in a zone from results by checking this checkbox. This option is only available when the 1-Custom Simulation type is selected. For all other Simulation type settings the calculations are carried out as if this option is unchecked.

Outputs and Thresholds

When the Simulation type is set to 1-General you can choose which specific outputs are to be generated and stored and define some key illuminance thresholds.

Spacial Daylight Autonomy (sDA)

Check this checkbox to generate sDA results, in which case a distribution map of Daylight Autonomy values on the working plane of the current object can be displayed.

Override zone illuminance target

When sDA results are being generated you can choose to either use the Target illuminance defined on the Activity tab for each zone (by leaving this option unchecked) or enter a constant illuminance target for all zones in the cell below.

sDA illuminance lower threshold

When sDA results are being generated and the Override zone illuminance target option (above) is selected you can enter the lower illuminance threshold, the minimum level of illuminance (in lux or fc) below which lighting levels are considered to be inadequate. A typical lower threshold value for sDA calculations is 300 lux.

Annual Sunlight Exposure (ASE)

Check this checkbox to generate ASE results, in which case a distribution map of Annual Sunlight Exposure values on the working plane of the current object can be displayed.

ASE illuminance threshold

When ASE results are being generated you can enter the maximum level of illuminance (in lux or fc) above which lighting levels are considered to be likely to lead to visual discomfort, i.e. glare. A typical upper threshold for ASE calculations is 1000 lux.

Useful Daylight Illuminance (UDI)

Check this checkbox to generate Useful Daylight Illuminance results, in which case a distribution map of UDI values on the working plane of the current object can be displayed.

UDI lower illuminance threshold

When UDI results are being generated you can enter the minimum level of illuminance (in lux or fc) below which lighting levels are considered to be inadequate. A typical lower threshold value for UDI calculations is 300 lux.

UDI upper illuminance threshold

When UDI results are being generated you can enter the maximum level of illuminance (in lux or fc) above which lighting levels are considered to be excessive and lead to visual discomfort, i.e. glare. A typical upper value for UDI calculations is 3000 lux.

Min illuminance threshold

For BREEAM HEA 01 Option 4b only, allows you to enter a minimum illuminance threshold value from Table 12 based on the area type of the zones being analysed.

Avg illuminance threshold

For BREEAM HEA 01 Option 4b only, allows you to enter an average illuminance threshold value from Table 12 based on the area type of the zones being analysed.

Grid Size

Maximum grid size

The main grid size that can be used to divide up the working plane when calculating illuminance. Larger values will speed up calculations but give lower resolution in daylighting outputs.

You should consider the size and number of zones in the building when selecting the grid size. A very fine grid will of course cause Radiance calculations to take a lot longer to complete. On the other hand a very coarse grid in a small zone may not give a good enough distribution.

You should use a maximum grid size no greater than 5ft or 1.5m for LEED EQ8.1 calculations.

Minimum grid size

This is the smallest grid size that can be used to fill in gaps in the working plane in cases where the maximum grid size is too large, especially at the edges of the plane. Use smaller values (e.g. 0.01m) to fill in most gaps between the main grid cells and the borders of the working plane for a more continuous output. It is common to use a Minimum grid size that is a factor of between 0.5 and 0.1 times the Maximum grid size.

To maintain a uniform grid across the working plane set the Minimum grid size the same value as the Maximum grid size.

Guidance for Calculating Daylighting Credits for LEED, BREEAM, Green Star etc

DesignBuilder Radiance is ideal for calculating daylighting credits for certification schemes such as LEED, BREEAM and Green Star. In these cases there is often pressure to provide highest possible illuminance reports and, because some settings can cause illuminance values to be lower than expected, this section provides advice on the most appropriate settings to use.

Many of the LEED, BREEAM and Green Star daylighting credit calculations revolve around a "% of floor area having daylight illuminance over a threshold" report for each zone and for the whole building. The table below describes how to get the most accurate results from these calculations.

Setting	Recommended Setting	Advice
Detail template	5-Accurate	You should use the most detailed setting you can afford to wait for. The 1-Fast setting is not intended for project output and should be avoided for all credit calculations as it tends to lead to severe underestimates of illuminance levels. The accuracy increases as the level of detail in the calculation increases up to and including the 5-Accurate setting. The 6-High quality setting is not recommended for these sort of calculations either as it does not add much in terms of accuracy but takes a lot longer than the 5-Accurate setting.
Ambient Bounces	3-8	The 5 built in templates are ideal for “simple daylight scenes that have shallow spaces with relatively large window openings”. However in deep plan models or models with small windows or greater external/internal complexity of the model you may want to increase the number of bounces from the default set by the Detail template. Models which have complex exterior geometry may require up to 8 bounces to ensure that the light reflection from exterior building surfaces is correctly treated. Likewise to obtain the correct impact of variations in the floor reflectance you will need to use 5 or more bounces. Tip: If you aren't sure, try experimenting with different values for this setting to test sensitivity and learn impact on results.
Grid size	Max: 0.3m Min: 0.3m	Of course, the finer the overall grid the more accurate the results. Perhaps less obvious however is that using lower values for Minimum grid size can lead to more accurate and higher zone average illuminance results. This is because the small grid cells generated with the low minimum grid size can fill in around the edges of the working plane where often the highest levels of daylight are to be found.
Margin	0	Use a zero margin if you need the entire working plane to be included in the results. This will often lead to higher % areas over daylight thresholds because the edges of the working plane are usually closest to where many of the windows are located.

Advanced Options

Ambient bounces (-ab)

The number of ambient bounces is the maximum number of diffuse bounces computed by the indirect calculation. The number of ambient bounces that Radiance should apply varies depending on the type of building and daylighting system you are analysing. It can be set based on the number of reflections typically required by the light to reach the task plus one or two extra for inter reflection within the space.

The number of bounces should normally be set a minimum of 3 for most accurate calculations. Values lower than this are inadequate for most daylighting calculations.

If the number of ambient bounces is set to 1, light is considered to reach the interior surfaces from the sun's direct rays, from the diffuse sky, and from first bounce reflections of direct sunlight rays from both interior and exterior surfaces. Reflections of sky light off interior or exterior surfaces to other interior surfaces will not be considered. Additional bounces can be added to consider additional flux paths.

Doubling ambient bounces can double rendering time.

Ambient accuracy (-aa)

Ambient accuracy (-aa) is the maximum error (expressed as a fraction) permitted in the indirect irradiance interpolation.

You should normally use a value between 1 and 0.1, with lower values giving the best accuracy. A value of zero gives no interpolation.

Halving Ambient accuracy approximately quadruples rendering time.

Ambient resolution (-ar)

The Ambient resolution sets the distance between ambient calculations by determining the maximum density of ambient values used in interpolation. Factors that influence the scale over which interpolation may occur are:

The ambient accuracy.
The ambient resolution.
The maximum scene dimension.

The minimum possible spacing between hemispherical indirect irradiance sampling points is the maximum scene dimension multiplied by the ambient accuracy divided by the ambient resolution. In other words, for distances less than this minimum, the calculation will always resort to interpolation, rather than initiate more sampling, regardless of the error estimate associated with that interpolation. This prevents the calculation from spending unnecessary time resolving irradiance gradients over negligible scales. This distance gives the scale at which the interpolation accuracy begins to deteriorate from the ambient accuracy setting. The minimum separation for calculated irradiances, Smin, is:

Smin= Dmax × Ambient accuracy / Ambient resolution

Where the scene dimension, Dmax, is a measure of the maximum dimension of the zone being treated.

The effect of increasing Ambient resolution depends on the scene but it can quadruple calculation times for double the value.

Ambient divisions (-ad)

Ambient divisions sets the number of initial sampling rays sent from each ambient point into the hemisphere to determine the indirect incident light. The error in the Monte Carlo calculation of indirect illuminance will be inversely proportional to the square root of this number. A value of zero implies no indirect calculation.

The Ambient divisions and super-samples parameters can be used to help reduce "noise" in a calculation. By setting these options higher more rays will be tested when calculating an ambient value for a point.

High values of this parameter (4096 is perhaps the highest value that would be used under normal circumstances) will minimise "patchiness" of daylighting outputs but slow calculations. Doubling Ambient divisions will approximately quadruple calculation time.

Ambient super-samples (-as)

The number of extra rays that will be used to sample areas in the divided hemisphere that appear to have high variance. Ambient super sampling should usually be set to about one half or one quarter of the Ambient divisions parameter. Super-samples are applied only to the ambient divisions which show a significant change.

The effect of increasing Ambient super-samples is to reduce "patchiness" in regions where indirect illuminance is changing rapidly, but adding to the Ambient divisions and calculation times.

Sky patches

Sun patches

Monte Carlo random sampling

When this option is selected Radiance uses Monte Carlo random sampling during the simulations which causes results for each simulation to have small variations. In other words you will see small differences in results each time you run the same simulation. To ensure that results are consistent between simulations you should uncheck this option.

Note: With this option checked Radiance uses a statistical Monte Carlo approach which means that you will not get exactly the same results if you repeat a calculation using the same model and options. You can reduce this effect by using higher detail settings but you can only eliminate it by unchecking this option.

Max usable cores

Enter the maximum number of cores that DesignBuilder should use for the Radiance simulations. Enter 0 to use all cores. Note that if you allocate all cores to Radiance, your machine may not be sufficiently responsive to allow you to work on other tasks while the simulations run.

Setting Ambient Parameters

Some general advice on the approach is:

Set Ambient divisions high enough to capture the visible luminous features at the first bounce.
Give sufficient Ambient bounces to redistribute the light.
Set the remaining ambient parameters to sufficiently high resolution to deliver acceptably smooth results.

Other Buildings

Include all buildings

If you would like other buildings to be included in the daylighting study as shading/reflecting surfaces then check the Include all buildings checkbox. Note that daylighting results will only ever to generated for the current building.