Sun protection key figures
The effectiveness of sun protection glazing is described with a whole series of key figures. The most important ones are:
Light transmission (LT in %):
It indicates the proportion of sun irradiation in the range of visible light (380 - 780 nm) that passes directly through the glazing.
Total energy transmittance (g in %)
It indicates what proportion of the total solar irradiation (300 - 2500 nm) is energetically usable behind glazing. It is the sum of direct and indirect radiation transmission.
Heat transfer coefficient (U value in W/m²K):
It is a measure of the heat loss through glazing. The lower the U value, the better the heat insulation.
Selectivity index (S):
It indicates how well a glazing can separate visible light
from infrared radiation, S = LT / g.
Shading coefficient (b-factor, b):
It is an arithmetic variable needed for the calculation of cooling loads according to VDI Guideline 2078, b = g / 0.8.
Light reflectance (LR in %):
It indicates the proportion of sun irradiation in the range of visible light (380 - 780 nm) that is reflected from outside with incidence of light. The larger the LR, the more reflective the effect of the glass is.
Colour rendering index Ra
The colour rendering characteristics of the daylight transmitted are described by the general colour rendering index Ra. The reference illuminant is D65 or the radiation of the most similar colour temperature.
Radiation transmittance is the ratio of the total radiant energy transmitted by the glazing (perpendicular to the surface) to the incident radiant energy.