Designing and simulating daylight

Daylight metrics
There are several metrics for quantifying the amount of daylight in a space.

Illuminance
Measured in lux (foot-candles), illuminance indicates the amount of light falling on a surface. The illuminance value does not depend on surface properties, though surface properties are important to understand how much light is reflected and seen by the eye.

The amount of suitable illuminance varies greatly depending on factors such as age of the viewer and task. For example, an art gallery with a light-sensitive collection may keep light to around 50 lux (4 fc) to reduce degradation, while a workstation in a laboratory requiring precision tasks might require 1500 lux (140 fc). Typical office environments target light levels around 300 lux (27 fc).

Daylight factor
The daylight factor indicates the percentage ratio of indoor daylight illuminance to the outside illuminance. This is calculated at a single point location, and can then be calculated for a room as an average of daylight factors for multiple points in the room. Actual illuminance varies depending on cloud cover and position of the sun—therefore, daylight factor provides an indicator of the amount of natural light in a space, irrespective of weather.

Spatial daylight autonomy
sDA is the percentage area meeting a minimum lux level throughout the year. This metric is calculated using local weather data, and it compares daylight illuminance at a work-plane to a minimum requirement. Annual sunlight exposure, combined with daylight autonomy as defined in IES LM 83-12, Approved Method: IES Spatial Daylight Autonomy and Annual Sunlight Exposure, is a measure of visual discomfort and is the percentage area where the direct sunlight exceeds a maximum illuminance.

Daylight glare probability
The daylight glare probability indicates if the light levels get too high and could cause uncomfortable contrast for occupants within the space.

Simulation tools
At the core of simulation software are the algorithms that calculate light levels and render images. Radiance, developed by the U.S. Lawrence Berkeley National Laboratories (LBNL), is one such software; it is used as part of a number of other front-end software packages to do lighting calculations.

Radiance uses a combination of direct path ray tracing as well as Monte Carlo sampling of other directions where light might come from. The calculations start at a measurement point and trace light back to the emitting sources. Several light paths are considered:

  • direct light;
  • specular (i.e. directional transmit/reflect from surfaces and glass); and
  • diffuse (i.e. scattered light off surfaces and glass with no directional preference).

In preparing a daylight model, there are several key steps to the workflow:

1. The model geometry is built using 3D drawing software.

2. Material and surface properties are assigned, including glass visible light transmittance (VLT) and surface reflectance properties (e.g. wall, ceiling, floor, ground, roof).

3. Simulation parameters are set, for example, based on simulation standards such as IES LM 83-12.Considerations include:

  • number of light bounces;
  • grid size;
  • sky condition (e.g. overcast/clear); and
  • time of day.

Daylight models can be used to provide a number of analytical results and images that are useful to a design team. Some commonly used outputs from daylight simulation software are described below.

Illuminance perspective images
An example illuminance perspective is shown in Figure 1. Renderings such as these show lux illuminance values within a space. Perspective images are useful to visualize how the light will be seen by occupants, and to see what surfaces will be in light or shadow.

Illuminance plan
An illuminance plan is typically used to study illuminance at either a working plane or the floor level. The results shown in Figure 2 are lux levels at two different times of day. The results would be helpful in quantifying what area of the floorplate is above a lux threshold such as for a LEED target, for understanding what workstations are receiving light, and for understanding how deep light travels 
into the room.

Glare probability analysis
Daylight models can be used to estimate likelihood of glare and allow testing of different glare-control devices such as internal shades. In Figure 3, a sample view is shown from the point of view of a person seated at a workstation with an adjacent window. 
The simulation was run to test the likelihood of glare within this field of view. The results without shades (top image) indicate there are times with intolerable glare (red and yellow), particularly from low sun angles in winter. By allowing shades to be closed whenever there is bright light (bottom image), the amount of intolerable glare can be significantly reduced.

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