Daylight Illuminance

Average annual daylight illuminance is calculated in footcandles at a point nine feet from the window in the center of the room.

The design of a window and choice of glazing can dramatically affect the quantity and quality of daylight in a space and how it is experienced. Most visual tasks require from 30 to 70 footcandles. Larger windows with low-transmission glass can have the same average daylight illuminance as small windows with high-transmission glass. South-facing windows generally have higher daylight levels than north-, east-, and west-facing windows because of direct sun. Higher latitudes have greater average daylight levels on the south than lower latitude locations.

The daylight illuminance set point is currently set at 50 footcandles.

• Zone Depth: A primary daylight zone depth is calculated as the minimum of a) the room depth, b) 1.5 times the facade wall height, and c) 15 feet.
• Sensor height: The daylight sensors are located 2.5 feet above the floor of each daylight zone.
• Sensor #1: Daylight sensor #1 is positioned 2/3 of the primary daylight zone depth from facade wall, centered in the width of the facade zone. Sensor #1 controls a fraction of the facade zone lights equal to the primary daylight zone depth divided by the facade zone depth.
• Sensor #2: Any remaining depth in the facade zone is considered a secondary daylight zone. Reference Point #2 is positioned halfway between the primary daylight zone depth and the "back wall". Sensor #2, if used, controls the remaining fraction of lights.