Annual Illuminance - Climate-Based Daylight Modelling

Climate-based daylight modelling (CBDM) is the prediction of illuminance on the working plane using realistic sun and sky conditions based on standardised climate data. In DesignBuilder, CBDM evaluations are carried out for a full year at a time-step of an hour in order to capture the daily and seasonal dynamics of natural daylight. The Daysim daylighting engine is used in these simulations. You can read more about Daysim and the CBDM metrics output by DesignBuilder below. CBDM is also known as "dynamic daylighting".

Dayim

 

Daysim is a validated, Radiance-based daylighting analysis software that models the annual amount of daylight in and around buildings. Simulation outputs include climate-based daylighting metrics such as Daylight Autonomy (sDA), Annual Sunlight Exposure (ASE) and Useful Daylight Illuminance (UDI). Daysim combines a daylight coefficient approach with the Perez all weather sky model and the Radiance backward ray-tracer to generate a time series of illuminance values over a grid of sensors points to derive the climate-based daylighting metrics.

Climate-Based Daylighting Metrics

Over the past decade a new family of daylighting metrics have been developed to describe and evaluate daylight in spaces. These metrics summarise the daylight availability over the whole year and throughout a space. Three main daylighting metrics calculated by DesignBuilder Daysim are:

• Daylight Autonomy (sDA) is a measure of how often a minimum work plane illuminance threshold (often 300 lux) can be maintained by daylight alone. It is expressed as the percentage of occupied time during the year when a minimum work plane illuminance threshold of 300 lux can be maintained by daylight alone. It defines the extent to which a space has enough natural daylight to allow the prescribed activity to be carried out without the need for electric lighting. This is a recommend metric by the Illuminating Engineering Society of North America (IESNA).
• Annual Sunlight Exposure (ASE) is the number of hours per year a point on the working plane receives direct sunlight greater than a threshold value. This metric is often used alongside sDA to quantify the risk of visual discomfort due to glare.
• Useful Daylight Illuminance (UDI) is the annual occurrence of illuminance distribution across the working plane that are within a range considered “useful” by occupants. It excludes any times when there is inadequate natural daylight or excessive direct sunlight which would give rise to a risk of glare. UDI therefore not only provides information about useful daylight illuminance, but also on the propensity for excessive levels of daylight that are associated with glare, occupant discomfort and unwanted solar gains. In this way the single UDI metric can be used to summarise the overall daylight performance of a space.