Daylight Terms

Here are some key terms to help you participate in conversations about daylighting and metrics. This list will grow and change as we do, so come back if you ever need a reference.

2% Rule

Here’s what the authorities behind the LM-83 have to say about it: “Blinds shall close whenever more than 2% of analysis points receive direct sun as defined below. Blinds for window groups can close in any combination, until the criterion value for each hour is achieved. This type of analysis shall be conducted for each hour of the year” (5).

The 2% Rule applies to the sDA metric and says that when more than 2% of the analysis area (Illuminance Grid, in LightStanza’s terms) is lit with direct sun, window blinds or shades must be deployed until less than 2% of the analysis area has direct sun on it. The first step of the sDA simulation is determining the position of the window blinds using the 2% rule, followed by a step that determines illumination values once the blinds are in position. This second step is what gives you your final score.

Annual Metrics

Annual metrics are a way of evaluating daylight in a space across an entire year. The results are a function of hourly simulation results in conjunction with location specific climate data. There are several different metrics, all of which use the same set of data, but each have a different way of interpreting daylight and tell a different piece of the daylight story. LightStanza offers Average Illuminance (AI), Daylight Autonomy (DA), Continuous Daylight Autonomy (cDA), Annual Sunlight Exposure (ASE), Spatial Daylight Autonomy (sDA), and Useful Daylight Illuminance (UDI).

Direct sun 

Here’s what the authorities behind the LM-83 have to say about it: “Direct sun is defined as an interior horizontal measurement of 1,000 lux or more of direct beam sunlight that accounts for window transmittance and excludes the effect of any blinds, with no contribution from reflected light (i.e., a zero bounce analysis) and no contribution from the diffuse sky component” (5).

In other words, direct sun is light on the Illuminance Grid that is greater than or equal to 1,000 lux from a simulation that does not include reflected light or coverings like blinds, shades or other daylight products.

Operable Blinds

You know what window blinds are. They are included in this list because they are ubiquitous in the “real world,” but almost always overlooked in daylight simulations. To get a truly accurate understanding of daylight in your space, use operable blinds.

Window group

“A window group is defined as a group of coplanar windows, with similar shadow patterns from exterior shading and obstructions, and with similar shading device type operation, which are associated with the same analysis area” (5).

In other words, all the windows in a group should be on the same face of the building, have the same shading strategy, and be in the same room.






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