International Window Film Association

Infrared Radiation and Solar Heat Gain

Solar properties for fenestration systems are calculated following recommended industry standard procedures. The recommended procedures and solar properties for the window film industry are stated in AIMCAL documents.

These solar properties are calculated in a specific wavelength range in the electromagnetic spectrum. This range is referred to as the Solar Spectrum and includes radiation from the 300nm to the 2500nm wavelengths. The Solar Spectrum is divided into three spectral regions: Ultraviolet, or UV (300-380nm), Visible, or VIS (380-780nm) and Infrared, or IR (780-2500nm). This IR range is also referred as Near Infrared or NIR. Far Infrared, or FIR, is not part of the Solar Spectrum.

Heat produced by the Sun’s rays, which passes through a glazing system, is called Solar Heat Gain (SHG). SHG has two components: directly transmitted solar radiation and absorbed solar radiation (ASHRAE 2005 Handbook Fundamentals). In order to understand the total amount of heat that may pass through a glazing system, one must understand the energy (flux distribution) that each spectral region (UV, VIS and NIR) contributes to the SHG in the Solar Spectrum.

The figure below shows the direct solar energy distribution. This shows the contribution to the SHG of each spectral region to any given glazing system.

Figure Solar Energy Distribution from the 300nm to the 2500nm wavelength (ASHRAE 2005 Handbook Fundamentals– 31.14).

Total heat control performance comparisons between glazing systems can be done using the Solar Heat Gain Coefficient (SHGC). The US Department of Energy, the Energy Star Program, the International Code Council and the National Fenestration Rating Council require the use of the SHGC. The SHGC provides the consumer with a tool to compare the total heat gain of different glazing systems. The lower the number (less heat gain), the better the solar control performance. The SHGC includes the contribution of the UV, VIS and IR spectral regions and can be used to determine the solar control performance over the complete Solar Spectrum. The use of performance data from any single region should not be considered representative of the total solar heat gain performance of any product.