CBDM

Climate-Based Daylight Modelling (CBDM) is the process of simulating and analysing daylight within architectural 3D computer models. Unlike traditional daylight analysis, which focuses on the Daylight Factor (DF%), CBDM uses local climate weather data (TRY ) to analyse the light received under varying sky conditions, throughout a typical year.

The dynamic nature of daylight makes simulation a complex and computationally expensive process. Whilst the position and movement of the sun can be precisely calculated and modelled, it is virtually impossible to precisely predict future sky (weather) conditions. Whilst ever effort is made to ensure the accuracy of CBDM, the reality is that results will never be wholly accurate, partly due to the difficulties in predicting future sky conditions and partly due to the complexity of accurately modelling the annual daylight profile within 3D models.

CBDM metrics such as Daylight Autonomy or Useful Daylight Illuminance, do not focus on predicting absolute instantaneous light levels, but instead analyse the area of the space [where] and period of the year [when] a specific illuminance target, or illuminance range can be achieved from daylight alone.

This provides a more holistic understanding of the expected illuminance levels, throughout a typical year, as a result of local climate and therefore does not just focus on 'worst-case' overcast sky conditions. This approach also helps to ensure shading is considered as part of the design strategy, aiding to reduce potential visual comfort issues such as glare and veiling luminance, as well as enabling the energy saving potential of the electric lighting system to be predicted, when used in conjunction with daylight-linked controls. 

  • Daylight Factor (DF%)
    The ratio of internal to external illuminance under an overcast sky.
  • Daylight Autonomy (DA%)
    The percentage of time that an illuminance threshold is achieve through daylight alone.
  • Spatial Daylight Autonomy (sDA%)
    The percentage of floor area achieving the target illuminance for a set period of time.
  • Useful Daylight Illuminance (UDI)
    The annual occurance of illuminances, within a fixed range, across the working plane.
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