The percentage of total visible light that passes through the glazing system
High VLT = lighter, less glare reduction, less privacy
Low VLT = darker, more glare reduction, more privacy
The percentage of total visible light that is reflected by a glazing system.
The percent of incident solar radiation that directly passes through a glazing system
The percent of incident solar radiation that is reflected by a glazing system
The percentage of incident solar radiation that is absorbed by a glazing system.
High absorption can expose poorly manufactured glazing systems – leading to glass breakage.
The percentage of total ultraviolet light that is prevented from passing through a glazing system.
The percent of total solar energy (heat) rejected by a glazing system. (it is equal to the solar reflectance plus the part of solar absorption which is re-radiated outward)
The ratio of solar heat gain passing through a glazing system to the solar heat gain that occurs under the same conditions if the window were made of clear, unshaded double strength window glass.
Solar heat gain with film : Solar heat gain without film (1)
Used to measure a films ability to combat solar heat gain.
The lower the number, the better the shading qualities of the glazing system
The amount of conductive heat energy (BTU’s) transferred through one square foot of a glazing system for each 1oF temperature difference between the indoor and outdoor air.
Used to measure how good a glazing system is at retaining heat.
The lower the U Value, the better the insulating qualities of the glazing system.
The ratio of daylight transmission to solar heat transmission that passes through a glazing system.
Visible Light Transmission : Shading Coefficient (1)
It is determined by dividing the visible light transmission by the shading coefficient. For example, if a film has a visible light transmission of 70% and a shading coefficient of 0.5, it will have a luminous efficacy of 1.40.
Used to measure how efficient a glazing system is at blocking heat, without sacrificing visible light.
The higher the number, the larger the portion of transmitted solar energy is of visible light – rather than heat.
Any product with a Luminous Efficacy above 1.0 is considered to be “Spectrally Selective”, and filters out high levels of heat without sacrificing the usual amount of visible light.
Emissivity is a measure of a surfaces ability to absorb or reflect far-infrared radiation. The lower the emissivity the higher the far-infrared reflection. Infrared radiation is effectively heat.
The suns rays contain UV, Visible and near-infrared radiation. Low E Films transmit almost all of this radiation from the sun, but reflect longer, far infrared wavelengths.
Far infrared radiation is re-radiated from objects (both inside and outside the home) which have been heated by the original radiation from the sun or in other ways.
Used to measure the heat retention qualities of the glazing system.
The lower the Emissivity, the better the insulating qualities of the glazing system.
A measure of the amount of solar energy that goes to the inside of the building by reflection and by absorption/re-radiation compared to the performance of 3 mm clear glass; it is being superseded by the g value.
The total amount of solar energy that goes to the inside of the building by reflection and by absorption / re-radiation; window film can reduce g values to as low as 0,17. Occasionally, the g value is given as a percentage but this is not strictly in accordance with EN standards. See also EN standards.