Climate Based Daylight Modelling (CBDM)

Climate based daylight modelling (CBDM) is the dynamic analysis (over time) of daylight levels using a combination of sky types which are then modified by cloud cover or solar data from a selected simulation weather file. Climate based daylight modelling delivers predictions of illuminance or daylight factor over time which are dependent on the site location, building orientation, glazing and configuration.

CBDM is used for assessments for many voluntary rating systems. For all the assessments, the calculation uses a grid to represent the working plane, which is then used to determine the daylight performance of the space.

Four types of CBDM can be undertaken using RadianceIES:

Useful Daylight Illuminance (UDI)
Annual Sunlight Exposure (ASE)
Spatial Daylight Autonomy (sDA)
BRE 209 illuminance (Appendix G Trees & hedges) – see section BRE 209 Appendix G

These are all under the Advanced option in the Simulate tab.

CBDM is used to:

Obtain time-based daylight levels and statistics based on real climate data
Prove the ability of a space to use daylight for the benefit of its occupants.
Identify areas requiring additional daylight or artificial lighting.
Show how well a space is daylit and analyse the potential of daylight harvesting to reduce artificial lighting energy use.

All CBDM simulations are time-based dynamic calculations and use the Radiance rcontrib simulation methodology that performs a single higher quality ray-tracing calculation to create daylight coefficients for grid points. The coefficients are then used to calculate illuminance at each time step.