Daylighting Glossary

The U-Value is a measure of the thermal conductivity of a window: The lower the U-value, the better a window is at limiting heat losses. A single-glazed window has a U-value of about 6 W/m 2-K, while triple-glazed windows have U-values between 1 and 2.

Coolness Factor Ratio of the visible transmittance to the shading coefficient. If the value is greater than 1.0, the glass is considered "selective" and will offer better performance in a hot climate than a similar glazing with a lower coolness factor.

The shading coefficient describes a window's ability to transmit solar heat gain, relative to the transmittance of a 3-mm, clear single pane of glass. The shading coefficient is given as a unitless value between 0 and 1; the lower the SC the less solar heat it transmits. (The shading coefficient is being replaced by the "solar heat gain coefficient" which gives the fraction of incident energy that enters the space (with a value between 0 and 1).

Daylight Autonomy Percentage of occupied time per year when target illuminance can be maintained by daylight alone. May be used to express the percentage of electrical lighting energy saved by a daylighting system.

Visible transmittance is the percentage of visible light striking the glass that penetrates to the interior, expressed as a number between 0 and 1.

Daylight Availability Daylight illuminance from sun and sky at a particular site that impacts a building exterior on a horizontal, vertical, or other light-admitting surface.

Spectral selectivity is a window's ability to transmit visible light while reducing solar heat gain, and is expressed by a number between 0 and 1.

Daylight Factor Interior horizontal daylight illuminance expressed as a percentage of the horizontal daylight illuminance available to an unobstructed site.

The coolness factor is the ratio of the visible transmittance to the shading coefficient. If the value is greater than 1.0, the glass is considered "selective" and will offer better performance in a hot climate than a similar glazing with a lower coolness factor.

Daylight Factor Dot Charts A series of circular formatted charts for overcast and clear sky conditions. Each chart contains a pattern of dots randomly distributed as a function of sky luminance and cosine corrected for the angle of incidence. When overlaid onto a site obstruction diagram and an interior room drawing, allows estimation of light available to a particular interior point as a fraction of the light available to an unobstructed site.

Effective Aperture (EA) Measure of the light-transmitting ability of a glazed window aperture. Product of glazing visible transmittance and the ratio of window-to-wall area.

Lighting Power Density Maximum lighting power per unit area of a building allowed by code according to the classification of space function.

Light-to-Solar-Gain (LSG) Ratio of visible light transmitted to solar heat gain. A value below one means the glazing transmits more heat than light. A value above one means the glazing transmits more light than heat. Spectrally selective glazing is characterized by having a high LSG value (=1.25).

Luminous Flux Rate Flow of radiant energy, measured in lumens (lm).

Nomogram Graphical assessment tool consisting of a three coplanar curves, each graduated for a different variable so that a straight line cutting all three curves intersects the related values of each variable.

No Sky Plot Plan Drawing that shows the extent of daylight penetration by dividing the work plane into two parts: the part from which the sky can be seen directly, and the part from which it can't.

Passive Solar Heating Strategy Use of the sun's energy for heating a building interior.

Shading Coefficient A window's ability to transmit solar heat gain, relative to the transmittance of a 3-mm, clear single pane of glass. The shading coefficient is given as a unitless value between 0 and 1; the lower the SC the less solar heat it transmits. (The shading coefficient is being replaced by the "solar heat gain coefficient" which gives the fraction of incident energy that enters the space (with a value between 0 and 1).

Spectral Selectivity A window's ability to transmit visible light while reducing solar heat gain. Expressed by a number between 0 and 1.

Sun Angle Calculator Solar shading design tool consisting of a series of sun path charts with a profile angle and incident angle overlay. Used for determining solar position at a specific latitude and time, and the associated profile angle relative to a particular facade orientation. (Originally produced by Libbey-Owens-Ford up until 1974, but now available from the Society of Building Science Educators. Visit www.sbse.org/resources/sac/index.htm.

Target Illuminance Established minimum illuminance based on space function, task, and occupants' age; measured in lux (lx) or footcandles (fc).

U-Value A measure of the thermal conductivity of a window: The lower the U-value, the better a window is at limiting heat losses. A single-glazed window has a U-value of about 6 W/m 2-K, while triple-glazed windows have U-values between 1 and 2.

Visible Transmittance Percentage of visible light striking the glass that penetrates to the interior, expressed as a number between 0 and 1.

Published April 1995. Reprinted with permission from the International Association for Energy-Efficient Lighting. All rights reserved.