Climate-based
Daylight Modelling

Dynamic Daylight Simulations

CBDM

Climate-Based Daylight Modelling (CBDM) is the process of simulating and analysing the annual daylight profile within architectural 3D computer models.

Unlike traditional daylight analysis, CBDM uses local climate weather data to analyse the light received under varying sky conditions, throughout a typical year.

The dynamic nature of daylight makes annual 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 every effort is made to ensure the accuracy of CBDM results, in reality, the resulting data will never be wholly accurate – it will only be representative of a typical year. TMY weather files are intended to represent a ‘Typical Meteorological Year', designed to be used for making estimates of predicted energy consumption. TMY files are composite of average months from a database of historical weather data.

In response to the dynamic nature of daylight, 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 expected to be achieved.

This provides a more holistic understanding of the varying illuminance levels throughout a typical year, in direct response to the local climate, and therefore does not just focus on 'worst-case' overcast sky conditions.

This approach to daylight analysis helps to ensure shading is considered as part of the design strategy, to help in reducing potential unwanted visual comfort issues, such as glare and veiling luminance, as well as enabling the energy-saving potential of a daylight-linked electric lighting system to be more accurately and realistically assessed.

CBDM Metrics

Spatial Daylight Autonomy (sDA)

The Spatial Daylight Autonomy (sDA) should be ≥50% across at least 50% of the working plane, during occupied hours.

Useful Daylight Illuminance UDI-a (100-3000lux)

The UDI-a (100-3000lux) should be >80% (ave) across the working plane, of each space, during occupied hours.

Useful Daylight Illuminance UDI-e (>3000lux)

The combined occurrence of illuminance levels below 100lux (UDI-s) and above 3000lux (UDI-e) should occur for no more than 20% of the time (ave).

Useful Daylight Illuminance UDI-s (<100lux)

The combined occurrence of illuminance levels below 100lux (UDI-s) and above 3000lux (UDI-e) should occur for no more than 20% of the time (ave).

Services

Lighting Analysts are pioneers in the use and application of Climate-based Daylight Modelling (CBDM), with over 20-years of experience in daylight design, simulation and analysis.

CBDM provides an in-depth analysis of the daylight within a space and can be used to better understand the dynamic range of light levels throughout a typical year and to better inform the electric lighting design.

CBDM is currently used in the design and development of new schools in England, but it can also be applied to other commercial applications, such as offices, hospitals, and retail.

Cumulative exposure can also be assessed, which is important when predicting the daylight exposure to light-sensitive artworks and artifacts, in museums and galleries. The same technique can be applied to the design of sports stadia, optimising the structure to maximise light exposure to the playing surface, a technique we developed when assisting in the design of Wembley Stadium over 18-years ago.