The light transmittance of the VELUX Sun tunnel depends on numerous parameters such as sky conditions (cloudy, sunny), direction of sun light, orientation of the Sun Tunnel and length of the VELUX Sun tunnel.

Based on measurements in the VELUX Photometer the following relation between sky type and lumen output at noon has been derived. (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.

The calculation of lumen output is based on the following installation situation