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# Total energy transmission value and luminous transmittance

## Sunlight and solar energy

From a physical perspective, wave radiation penetrates into the inside of a building through the window — visibly in the form of light waves and invisibly as energy and heat. The radiation transmission is measured as the luminous transmittance τv and the total energy transmission g-value. In addition to the U-value (thermal transmission coefficient), the latter in particular influences the energy assessment of the window. If the U-value and g-value are in the correct proportion, the window can even pay into the energy account, and for passive houses, for example, is an important component in the energy and heating concept.

# G-value

## Total energy transmission value

The total energy transmission value (g-value) is used to assess the level of energy input. This is that part of the solar energy that enters a room through glazing in the wavelength range λ = 0,3 to 2,5 µm in total. The lower the g-value, the lower the amount of solar energy that is allowed to enter the room. The total energy transmission value is made up of the direct solar energy transmission τe and the secondary heat dissipation to the inside qi.

The g-value that is advantageous for a specific building depends on the type of building, the usage and the position and orientation of the building: For houses, a high g-value is favourable for saving heating energy in order to increase the energy gains in winter; however, the risk of overheating in summer must then be taken into account. Note also that the U-value of the glazing should be small to keep the transmission heat losses low.

# τv

## Luminous transmittance

The luminous transmittance τv specifies the proportion of visible irradiation (wavelength range λ = 0,38 to 0,78 µm) that passes through a transparent component perpendicularly. The larger the value, the more light that passes through the component. A higher luminous transmittance is favourable on the one hand to improve the level of daylight lighting in a room; and on the other hand, the outlook to the outside is brighter and the distortion of colours is lower.

# Standard values for the total energy transmission value

 Glass type and designation Structure G Ƭv U (mm) [mm] [-] [W/m2 K)] Single glazing 4 0,87 0,90 5,8 Double glazing 4/ZR/4 0,78 0,82 2,7 Triple glazing 4/ZR/4/ZR/4 0,67 0,72 1,9 Double heat protection glass, coated 4/ZR-AR/b4 0,63 0,80 1,1 Triple heat protection glass, coated 4/ZR-AR/4/ZR-AR/b4 0,50 0,71 0,5 SOLAR + triple protection glass coating 4/ZR-AR/4/ZR-AR/b4 0,62 0,73 0,6