Last update: 15.10.2020
Understanding Glass

Light, Energy and Heat

The physical definitions of light, energy and heat describe defined areas of the electromagnetic spectrum.

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The area relevant to architectural glass in connection with light and solar energy falls within a 300 - 2,500 nm (0.0003 mm - 0.0025 mm) wave-length range and is considered as short-wave radiation. Heat is long wave radiation and related to a wave length range between 5,000 and 50,000 nm (0,005 – 0,05 mm).

Longer wavelengths are referred to as radar, micro and radio waves, while shorter wavelengths are known as high energy X-ray and gamma radiation.

Electromagnetic spectrum


Solar Energy

The radiation emitted by the sun that strikes the Earth is called solar energy. This wavelength range has been defined through international standardisation (EN 410) as ranging from 300 to 2,500 nm and includes:

  • Ultra violet (UV) radiation    300 … 380 nm 
  • Visible light (VIS)    radiation    380 … 780 nm
  • Near infrared (IR) radiation    780 … 2,500 nm 

Solar energy is considered to be in the short-wave range.

Solar energy

The worldwide-accredited global radiation distribution curve (acc. to C.I.E., Publication No. 20) shows the intensity of total solar radiation in its respective wave ranges. 52 per cent of these wavelengths are visible and 48 per cent are invisible. 

The shorter the wave length, the more energy is transported. This means that there is a considerable quantity of energy in the visible portion of the radiation. Therefore, light and energy cannot be separated from each other. This is a critical aspect in using and improving architectural glass.

Important properties that are critical for characterising the nature of architectural glass such as solar energy transmission, reflection and absorption, and total energy transmittance, can be derived from the solar energy in the global radiation wavelength range (300 - 2,500 nm) and its interactions with glass.

Global radiation distribution curve
Visible Light

The small area of the solar spectrum that can be seen by the human eye is called (visible) light. The spectral range is defined by the European standard EN 410 as between 380 nm and 780 nm.

Unbroken (visible) light hitting the human eye is perceived as white light. It is, however, composed of a light spectrum where the various wavelengths – each representing a defined energy – flow into each other:

Visible Light

When light hits an object, the object absorbs part of the energy spectrum. Glass, however, transmits light, reflecting the rest of the energy. Depending on the nature of the object, certain wavelengths are reflected and others absorbed. The human eye perceives the reflected colour as being the colour of the object.

Artificial lighting can result in colour misinterpretation due to missing wavelength ranges. A well-known example is low-pressure sodium vapour lamps. Since they lack blue, green and red wavelengths, everything appears in monochromatic yellow tones.


Heat or heat radiation are a wavelength range that is not part of the solar spectrum. Heat radiation has far longer wavelengths and is to be found in the far infrared range. The spectral range is defined in the European standard EN 673 as between 5,000 and 50,000 nm.

Its interaction with heat defines the insulation characteristics of architectural glass and is influenced by heat radiation, heat conduction and convection.  The Ug value – heat transmission coefficient – is the fundamental characteristic for evaluating the glass construction material’s heat insulation capability.

UV radiation

If the UV impact is too strong, this radiation has not only a harmful impact on the skin but also on many other organic materials (paintings, furniture, sealants, etc.). 

UV is high-energy radiation and is separated into three ranges:
UV-C:    100 … 280 nm (blocked by the ozone layer of the atmosphere.
UV-B:    280 … 315 nm (blocked by float glass products).
UV-A:    315 … 380 nm (transmitted through glazing to a certain degree).

UV radiation

If the UV impact is too strong, this radiation has not only a harmful impact on the skin but also on many other organic materials (paintings, furniture, sealants, etc.). Normal insulating glass with two glass panes reduces the UV transmission by more than 50%, and when combined with laminated safety glass, the radiation is almost completely filtered out.

UV radiationUV radiation


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