DAYLIGHTING Magazine issue 29 July/August 2021

Page 14


Climate-Based Daylight Modelling – A Real World Approach Cieran Towse of MBS Software Ltd asks: In a world with growing concerns about climate, why not take a real world approach to assessing the quality and quantity of natural daylight and sunlight rooms within a building will receive?

As we carry out daylight and sunlight analysis, it is time to embrace climatebased daylight modelling (CBDM). The results provide a more detailed picture of how light interacts with the built environment which we inhabit and provides crucial information to aid designers. Whilst not a new concept, the industry has been slow to adopt this methodology, preferring instead to use ‘static’ metrics which, whilst not inaccurate, can perhaps be best described as sufficient. More importantly, they can be bettered, particularly as the tools to do so are readily available. This article attempts to breakdown and simplify CBDM.

What is Climate-Based Daylight Modelling? CBDM is the prediction of various radiant or luminous quantities (e.g. radiance, irradiance, luminance and illuminance) using realistic sun and sky conditions captured annually from standardised climate data. These data sets therefore create a realistic representation for an analysis environment, reflecting location and orientation. A climate file contains data for a specific geographic location for many weather variables, such as Illuminance,


July/August 2021

radiation, temperature and wind direction. Data is collected at every hour of every day for the year. A ‘typical year ‘is a climate file which contains data selected over a number of years (normally 10 years or more). For each month, the data is selected from the year considered most ‘typical’. For example, June might be from 2007 and July from 2012.

“.Whilst not a new concept, the industry has been slow to adopt this methodology, preferring instead to use ‘static’ metrics...”

Alternatively, a ‘historic year’ is data collected through a single continuous year rather than a composite one made up from average months. Standardised climate data is a readily available online resource, however not all files contain the relevant information required and can be biased towards particular variables.

How does Climate-Based Daylight Modelling work? The method used when carrying out CBDM is ray tracing, in particular backwards ray tracing, since it uses rays moving in the opposite direction to which photons actually travel. Backwards ray tracing is the process of shooting rays from the individual room grid points to the light source and is used for reasons of efficiency, eliminating any superfluous rays. Ray tracing allows for indirect light as