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Table 1 - Sky type
The light transmittance through Sun Tunnels or tubes depends on numerous parameters such as sky conditions (cloudy, sunny), direction of sun light, orientation of the Sun Tunnel or tube and its length. Based on measurements in the VELUX Photometer the following relation between sky type and lumen output at noon has been derived for VELUX Sun Tunnels. (Lumen is a term for the amount of light emitted from a light source). The results are for a 1 m TWR mounted in a south facing roof with 15° pitch including elbow etc.
View illustration
Examples of calculation
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 |  | Table 2 - Light source
The lumen output from the VELUX Sun Tunnel can be compared to the lumen output from typical well-known light sources (bulbs, fluorescent tubes etc.) mounted in a typical lamp.
View illustration
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Solar gains
As light is also energy, the solar energy gains through the VELUX Sun tunnel are closely linked to the amount of light passing though the VELUX Sun tunnel. The energy content of natural light is roughly 9W pr 1000 lumen. If for example the light output from the VELUX Sun tunnel is 2000 lumen the solar energy gain will be around 18W.
U-Value
Ut = 2.2 W/m2K
Method: NFRC 100 (NFRC = National Fenestration Rating Council - www.nfrc.org)
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