How does solar protection work ?

Thermal and optical factors

Thermal and optical factors, defined by the European standard EN 14501 "Blinds and shutters, thermal and visual comfort, performance characteristics and classification", allow to measure the solar protection performance of the fabrics. This standard was drawn up by a European working group comprising the Fraunhofer ISE (Institute of Solar Energy).

The EN 14501 standard defines the gtot total solar factor (fabric + glazing) as being the major property for thermal comfort and the Tv value for visual comfort.

Solar protection and light control factors are laboratory-tested.

Thermal factors relating to the fabric alone

Solar transmittance (Ts)

This factor gives the proportion of solar energy transmitted through the fabric. A low percentage means the fabric performs well at reducing solar energy.

Solar reflectance (Rs)

This factor gives the proportion of solar radiation reflected by the fabric. A high percentage means the fabric performs well at reflecting solar energy.

Solar absorptance (As)

This factor gives the proportion of solar radiation absorbed by the fabric. A low percentage means the fabric absorbs little solar energy.

Solar radiation is always partially transmitted through, absorbed or reflected by the fabric. The sum of all 3 equals 100. Ts + Rs + As = 100% of solar energy.

The thermal and optical values are calculated using a reference glazing and according to the blind position (internal or external)

Gtot solar factor is determined for 4 standardized glazings as defined in Annex A of EN 14501 standard. The base glazing is C (thermal transmission factor of the glazing alone U = 1.2 W/m²K - solar factor of the glazing alone gv = 0.59).

Total solar factor or gtot factor
The percentage of solar energy which actually penetrates into a room through the blind and glazing. A low value means good thermal performance.

Optical factors

Openness factor (OF, Co)
Relative area of the openings in the fabric (hole). It is considered as independent of the colour. For fabrics with the same weave, it should be measured using the darkest colour in the range.

Visible light transmittance (Tv or TL)
This factor gives the total percentage of light radiated through the fabric over a wavelength of 380 to 780 nm (nanometers), called the visible spectrum (total illumination).

Visible light reflectance (Rv)

Proportion of light reflected by the fabric.

Diffuse transmission factor (Tdif)
This factor is obtained by correlating the two factors above : Tdif = Tv - OF.
It is indicated as Tvndif for the aspects of glare and shape recognition (outward visibility / night privacy). A low figure shows a better visual comfort.

Additionally when the fabric is hit by the sun, it becomes a natural light source which is defined as Tvdifh; it is used to ascertain a fabric's light diffusion capacity. A high figure means more natural light.

Ultraviolet transmittance factor (Tuv)

This factor gives the percentage of ultraviolet light radiated through the fabric over a wavelength of 280 to 380 nm (nanometres). UV radiation accelerates natural ageing. All means of solar protection ensure a certain amount of protection from UV light.

Various parameters enable a good match between the solar protection sought and a suitable fabric to be defined:

- The building's orientation

- Geographic location

- Seasons

- Type, shape and dimension of the glazing (clear, low emissivity, reflective)

All these elements cause the solar radiation's direct impact on the window to vary.