Calculation of solar and light transmittance

The standard CSN EN 13363-1+A1 Solar protection devices combined with glazing - Calculation of solar and light transmittance defines a simplified method of calculation of a sun protection device combined with glazing based on thermal transmittance and total solar transmittance through glazing, and on light transmittance and reflectivity of a sun protection device for the evaluation of total solar transmittance.

The method is applicable to any type of sun protection devices in parallel with glazing, such as shutters or blinds. Sun protection devices can be installed inside a protected room, outside or in a gap between double glazing. The method is applicable in cases when total solar transmittance through glazing ranges between 0.15 and 0.85. It is anticipated that roller shutters have to be fastened to prevent direct solar radiation. It is presumed that in exterior sun protection devices and in-built sun protection installations the space between sun protection devices and the glazing is not ventilated, whilst it is ventilated in interior sun protection installations.

Basic concepts used

Transmittance - radiation that is transmitted through window into an interior; radiation transmittance factor τe achieves values between 0 and 100; i.e. 0 to 1.

Reflection (take-off) - shine that is returned to the external area by window, coefficient of returned shine ρe acquires the values from 0 to 100 % that is 0 to 1.

Absorption - radiation that is absorbed by window and increases its temperature; the radiation factor αe achieves values between 0 and 100; i.e. 0 to 1.

Emissivity - ability to radiate heat εe; it applies that ε = α
As result, the following equation applies: τe + ρe + αe = 100 %; or 1.


Value of factor g for the calculation is usually defined by the manufacturer of window panes or windows. 
gtot - total solar transmittance factor with solar protection. 
Fc - reduction coefficient. The value of the coefficient ranges between 0 (theoretically best protection from solar radiation) and 1 (zero protection from solar radiation; in that case it applies that: g = gtot).

Solar and light transmittance according to CSN EN 13363-1+A1 for selected products of ISOTRA a.s.