Glass solar radiation

Solar radiation, comprising about 50% visible light and 50% energy in the UV and infrared ranges, is at the origin of life on Earth. When it strikes a glazed surface, three things can happen:

• part of it is reflected by the surface of the glass (energy reflectance, ρe);
• part of it is absorbed by the glass and re-radiated to the inside and the outside (energy absorptance, αe);
• part of it is transmitted through the glass (energy transmittance, τe).

ρe + αe + τe = 1 (= 100% of the incident solar energy)

Solar factor (g)

The solar factor (g) of a glazing is the ratio of the total energy entering the room through the glazing to the incident solar energy. The total energy entering the room is the sum of the energy entering by direct transmission and the part of the energy absorbed which is then re-radiated and transferred by convection towards the inside.

The solar factor is calculated in accordance with BS EN 410. The lower the solar factor, the lower the heat transferred into the building.

Shading coefficient (SC)

This is the ratio of the solar factor of the glass to the solar factor of a single sheet of 3 mm clear float glass.

Light transmittance (LT)

This is the percentage of visible light (380 to 780 nm) transmitted through the glass. The higher the light transmittance, the greater the amount of natural light entering the building.

Light reflectance (LR)

This is the percentage of visible light (380 to 780 nm) reflected by the glass.

Selectivity (S)

Selectivity is calculated as the ratio of light transmittance to the solar factor (S = LT / g). The higher this ratio, the better the glass transmits light while restricting the entry of solar energy. Selectivity is an important consideration when specifying glass for environments where natural light is desirable but solar gain must be limited.

Selecting solar control glass

Solar control glass reduces the amount of solar heat entering a building, by reflecting and absorbing a part of the incident solar radiation. It can also retain a high level of light transmittance.

Solar control glass is used to:

• limit overheating in summer
• maintain a high level of natural light
• reduce or eliminate the need for air conditioning
• provide pleasing aesthetics, with a variety of colours and reflective levels available.