Cool daylighting
is an integrated approach that uses natural light to reduce the need for electric lighting, while also reducing solar heat gain and glare. Successfully daylit buildings use the following four principles:
Window placement.
Too much light directly in the field of view is uncomfortable. Ideal window design uses a clerestory to let in light high (where it can bounce off the ceiling) plus lower view windows to provide a view. The amount of glass increases with height, bringing more useable light into the room while cutting glare.
Brightness control.
The sun, clouds, sky, and reflected light can overwhelm the eyes. These bright sources must be controlled through the use of overhangs and window blinds. This is especially important for view windows.
Limited light transmission.
Even when direct light is controlled, the sky can supply an overwhelming amount of light. This leads to glare—one of the chief reasons that daylighting fails. To control glare, darker glass is used. Visible light transmittance is limited to 0.38-0.60 for clerestories and 0.18-0.25 for view windows (depending on design conditions).
Even light distribution.
The human eye does not like large visual contrasts. Several daylighting techniques help distribute light evenly:
Direct-indirect lighting.
This type of fluorescent lighting provides direct downlighting and indirect light bounced off the ceiling.
Daylighting sensors.
Sensors in the ceiling detect luminance levels and turn off lights as needed to keep light levels constant. Lights near the windows are dimmed, while lights near the back of the room are on more often. This prevents the "cave effect," where the back of the room appears poorly lit because the contrast between the back and front of the room is too great.
Wall treatments.
Lighter colored paints are used for the ceiling and for the wall near the ceiling. Darker paints are used below. This helps create a bright canopy of light out of the field of view.
