Working of a Sunscreen

Influence of colours

The choice of the colour has direct influence on the criteria which justify the use of sunscreen protection:
• Protection against visible light, expressed by the factor Tv.
• Protection against sun-energy, expressed by the G value.
• Protection against secondary heat, expressed by the factor Qi.
• Protection against UV- light, expressed by the factor Tuv.

The G-factor

Sunscreens are always used in combination with a glazing. These together will prevent a large quantity of energy, sent by
the sun to the earth, which is indicated by the: Total Solar Energy Transmittance, or G-factor.

The G value is the ratio between the total solar energy transmitted into a room through a window and the incident solar energy
on the window.
The Gtot is the solar factor of the combination of glazing and solar protection device.
The Gv is the solar factor of the glazing alone.

The shading coefficient is defined as the ratio of the solar factor of the combined glazing and solar protection device Gtot to that
of the glazing alone Gv.

The total solar energy transmitted through a window consists of two parts:
1) Radiation: measured by the solar transmittance: e,tot
2) Heat: measured by the secondary heat transfer: Qi

G = e,tot + Qi
The factor e,tot, is quantity of energy, which will pass the combination solar protection device and window.
The factor Qi is the quantity of heat which is released by the absorption of energy in the sunscreen protection system =
combination sunscreen + glazing.

The G-factor is the most important factor to explain the efficiency of a combination sunscreen + glazing, as protection
against the energy of the sun. The G-factor divided into his components explains the difference in efficiency between
exterior and interior sunscreen.

G = e,tot + Qi
The direct solar transmittance e,tot is the same for interior and exterior use of sunscreens.
The secondary heat factor Qi for interior sunscreen is bigger then for exterior sunscreen. For interior use, the heat, produced by the absorption of energy, will be transmitted to the room inside. By exterior use, the heat will be transmitted to the outside, without any inconvenience at the inside.

Also the colour of the sunscreen has an influence on the G-factor.
Dark colours will absorb a lot of sun energy and will transmit this to heat. If the screen is used for exterior, heat will have no influence inside the room, contrary to a screen used for interior. This is why a darker screen is ideal for exterior use and a lighter screen for interior use.