Glass & Solar Radiation

Spectrophotometric characteristics

Radiation

When solar radiation strikes glass, it is partly reflected, partly absorbed in the thickness of the glass and partly transmitted.

The ratio of each of these 3 parts to the incident solar radiation defines the reflectance factor, the absorptance factor and the transmittance factor of the glazing. If these ratios are plotted for the electromagnetic spectrum, they form the spectral curve of the glazing. Factors which will affect these ratios for a given incidence, are the tint of the glass, its thickness and, in the case of a coated glass, the nature of the coating.

Transmittance, reflectance and absorptance factors

The transmittance, reflectance and absorptance factors are the ratios of the transmitted, reflected or absorbed radiant flux to the incident radiant flux.

They are shown for wavelengths between 0.3 and 2.5 µm.

Light transmittance and light reflectance factors

The light transmittance and light reflectance factors are the ratios of the transmitted or reflected light flux to the incident light flux These factors are given for comparison purposes, since slight variations may occur during manufacture. Glass has an inherent green tint, which may be apparent in certain very thick or multiple glass constructions (in double-glazed and laminated form). This will vary with the overall thickness of the glazing or its constituents.

Solar factor or total transmittance

The solar factor (TT/SF/g) of glazing is the percentage of the total solar radiant heat energy entering the room through the glass.

It is the sum of the solar radiant heat energy entering by direct transmittance and the proportion of the energy absorbed and re-emitted by the glazing to the interior space.

See “Glass and thermal insulation”

Natural light

Daylight factor

Knowing the light transmittance factor of a particular glass makes it possible to assess the level of available light inside a room, when the exterior light level is also known. The ratio of the internal light level at a given point in a room to the exterior light level measured on a horizontal plane is constant, regardless of the time of day.

The ratio of internal light levels to external light levels is referred to as the “daylight factor” and is usually expressed as a percentage.

For example, a room with a daylight factor of 0.10 close to the glazing and 0.01 towards the back of the room (average figures for a typical room), an exterior light level of 5000 lux (overcast, thick clouds) provides an interior light level of 500 lux near the window and 50 lux at the back.

For the same room, a light level of 20000 lux (open sky, white clouds) produces light levels of 2000 and 200 lux respectively.

Comfortable light levels

The overall light level in a room is a major determinant to the feeling of well-being by ensuring optimum conditions and comfort for the eyes.

This is affected by the amount and distribution of light, the presence of glare and of any strong shadows.

The degree of comfort attained by natural light levels is determined by the light transmittance of the glass and is dependent on the overall light distribution, the orientation of the building and the size of the glazed area.

The fading effect

In certain circumstances, the colours of some materials, when exposed to direct sunlight, can fade.

As previously discussed, energy from the sun consists of three types of radiation: -ultra-violet rays (UV), ranging from 0.28 to 0.38 µm. It is this part of the electromagnetic spectrum that causes sunburn,

-visible radiation or daylight, composed of a narrow band of the electromagnetic spectrum with wavelengths in the range 0.38 µm (violet) to 0.78 µm (red). The combination of these wavelengths produces white light,

-infra-red radiation (IR), which we feel as heat, is in the range 0.78 to 2.5 µm Materials can change colour when exposed to solar radiation because the molecular bonds in the colouring agents are gradually weakened by high energy photons. This kind of photochemical reaction is mainly caused by ultra-violet radiation, though to a lesser extent it can also be due to short wavelength visible light (violet, blue).

When materials absorb solar radiation their temperatures rise, and this can also start chemical reactions that damage colours.

Generally colour fading is associated more commonly with organic colouring agents, in which the chemical bonds can be less stable than in mineral-based pigments.

Since all forms of radiation carry energy, objects cannot be completely protected from fading. Precautions can be taken, however, to minimise the problem, such as keeping them out of direct light, at low temperature and shielded from the atmosphere, especially from corrosive gases.

However, there are glass products which can provide effective solutions.

The most efficient way of preventing fading is to exclude ultra-violet radiation, since despite its low proportion within the electromagnetic spectrum, it is the main contributory factor to the process.

UV radiation can be virtually eliminated by the use of PVB laminated glass. Laminated glasses can transmit only 0.4 % of UV, compared to 44 % for 10mm clear float glass.

A second option is to use a body-tinted glass, which will filter light selectively : for example, yellow glass absorbs mainly violet and blue light. Thirdly, glass with a low solar factor could be used to reduce the thermal effect of the radiation.

It should be noted however, that no glass product can guarantee total protection from fading.

Optimising the performance of glazing must always involve a compromise between overall performance parameters and making a choice on grounds of aesthetics and economy.