Daylight #

Introduction#

Daylight analyses are a group of metrics that relate to the daylight received at a point from the sky. The daylight factor measures the potential for lighting a space with natural daylight. Point-in-time illuminance measures the exact amount of daylight received from a localized sky at a specific point in time. Daylight autonomy and useful daylight illuminance evaluates the amount of daylight received over time in relation to a threshold or an interval.

All these analyses can aid in the effort to create well-lit, comfortable environments that enhance occupant well-being, reduce reliance on artificial lighting, and improve energy efficiency in buildings.

Set Up#

Similar inputs are used to calculate the different daylight metrics, such as shading masks, a sky and a threshold or an interval.

Shading Masks#

Shading masks are used in all daylight metrics as they describe the relationship between the sky and the positions that should be evaluated. A shading mask is created by mapping the light paths from an analysis point to the sky patches described in a hemisphere, taking geometry and materials into account in the process.

Warning

The hemisphere for the shading masks has to match the hemisphere in any potential sky used to evaluate the same metric.

A typical interior setup when evaluating daylight metrics. The model consists of reflective interior geometry and transparent window geometry and is used in the shading mask analysis together with the analysis points to create shading masks for each position in the analysis point grid. The shading masks are then multiplied with the illuminance data from the sky to calculate the final metric.

Sky Instance#

The sky instance is used for point in time illuminance and represents localised sky conditions based on weather data at a specific point in time. The sky subdivision is defined using a hemisphere.

Sky Series#

The sky series is used to calculate useful daylight illuminance and daylight autonomy and represents localised sky conditions based on weather data over a time period. The time period and the frequency at which it is sampled is defined using a time pattern. The sky subdivision is defined using a hemisphere.

Point In Time Illuminance#

The point in time illuminance analysis measures the amount of illuminance that is received from the sky, at a position, at a specific point in time. It can be useful when evaluating daylight levels using localised sky conditions to review lighting design or glare.

To calculate the illuminance each shading mask is multiplied by the sky instance for the specific location and point in time. This produces a daylight illuminance value in lux for each position in the analysis.

A sample scene for point in time illuminance analysis where each cell on the floor has been coloured by the received illuminance in lux. Point 1 is in an obstructed area resulting in 250 lux while Point 2 is in front of a window with about 3250 lux of daylight.

Daylight Factor#

The daylight factor (DF) analysis measures the ratio of illuminance from an overcast sky between a point on an unobstructed horizontal surface and a shading mask created at a specific position. It does not depend on a specific location, can be determined purely using a shading mask and is a good measure of the potential to light a space using daylight.

BREEAM certifications have specific daylight factor requirements depending on building type.

A sample scene for daylight factor analysis where each cell on the floor has been coloured by the proportion of light it receives from an overcast sky compared to an unobstructed point on a horizontal surface. Point 1 is in an obstructed area resulting in a daylight factor of 0.03 while Point 2 is in front of a window with a daylight factor of 0.16.

Daylight Metrics#

The daylight metrics analysis measures the average, minimum and maximum illuminance that is received from the sky, at a position, during the time specified in a sky series.

To calculate the metrics the shading mask is multiplied with the sky conditions for each time step in the sky series.

A sample scene for daylight metrics analysis where each cell on the floor has been coloured based on the minimum, average and maximum illuminance received from the sky. The illuminance is displayed on a logarithmic scale ranging from 10 to 50,000 Lux. Point 1 (Min) represents the minimum illuminance during the analysed period, Point 1 (Avg) the average illuminance over the period and and Point 1 (Max) the maximum illuminance.

Daylight Autonomy#

The daylight autonomy analysis measures the percentage of time at which a point receives daylight above a specified illuminance threshold. The time and localised sky conditions are specified using a sky series and the result is in relation to the amount of time in the sky series.

To calculate the portion of time above the illuminance threshold the shading mask for each point is multiplied with the sky conditions for each time step in the sky series. There are two measurements in the analysis, the daylight autonomy (DA) which is the percentage of time above the threshold and continuos daylight autonomy (cDA) which in addition to the DA awards partial scores for values below the threshold in a linear fashion.

A sample scene for daylight autonomy analysis where each cell on the floor has been coloured by the DA and cDA score. Point 1 is in an obstructed area resulting in a DA of 0.22 and a cDA of 0.52 while Point 2 is in front of a window with a DA of 0.76 and a cDA of 0.82.

Useful Daylight Illuminance#

The useful daylight illuminance analysis measures the percentage of time at which a point receives daylight within a specified illuminance interval. The time and localised sky conditions are specified using a sky series and the result is in relation to the amount of time in the sky series.

To calculate the portion of time within the illuminance interval the shading mask for each point is multiplied with the sky conditions for each time step in the sky series. There are three measurements in the analysis, the useful daylight illuminance (UDI) which is the percentage of time within the interval, the low useful daylight illuminance (UDI Low) which is the percentage of time below the interval and high useful daylight illuminance (UDI High) which is the percentage of time above the interval.

A sample scene for useful daylight illuminance analysis where each cell on the floor has been coloured by the UDI Low, the UDI and the UDI High score. Point 1 is in an obstructed area resulting in a UDI Low of of 0.22, a UDI of 0.73 and a UDI High of 0.05.

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