# Performance of Solar Control Films on Building Glazing: A Literature Review
## 2 Window Films
### Main Properties of Window Films  
The performance characteristics and properties of window films are described in relation to their class and type of film. The class of window films corresponds to which glass surface they can be applied to. The following classes are listed in EN 15752-1 [20]:  
* Class A: the window film is designed to be applied to the external glass surface in contact with the outdoor environment. These films are usually described as external films. * Class B: the window film is designed to be applied to the internal glass surface in contact with the indoor environment. These films are usually described as internal films. * Class C: the window film is designed to be applied as an external film to the glass surface within the cavity of an insulating glass unit facing the outdoor environment. * Class D: the window film is designed to be applied as an internal film to the glass surface within the cavity of an insulating glass unit facing the indoor environment. Figure 3 illustrates the different installation positions of each class of window films. Films with external application on the glass surface present lower durability than those with internal application because they are exposed to weather elements and possible damage induced by people or objects for glazing surfaces at the ground level. In addition, inappropriate cleaning tasks or insufficient maintenance routines have proven to decrease the life span of the films. The determination and knowledge of the thermal and solar optical properties of glazing systems are essential to ensure a good thermal and energy performance of buildings, particularly in building envelopes with high ratios of glazed to opaque areas. A suitable choice of glazing system at the design stage of a building should enhance occupants' thermal and visual comfort and, consequently, the energy efficiency of the building. According to EN 15752-1 [20], window films are defined based on their reflectance (interior and exterior) and their transmittance properties for the different radiation ranges: UV, solar, visible and thermal ranges. The following solar optical properties are listed in EN 15752-1 [20] to characterize a glass and film system:  
* Solar transmittance (\(\tau_{c}\)): fraction of solar radiation passing through the glass and film system. * Solar reflectance (\(\rho_{c}\)): fraction of solar radiation reflected by the glass and film system. * Solar absorptance (\(\alpha_{c}\)): fraction of solar radiation absorbed by the glass and film system. * Visible light transmittance (\(\tau_{V}\)): fraction of visible radiation passing through the glass and film system. * Exterior visible light reflectance (\(\rho_{V,c}\)): fraction of visible radiation reflected by the exterior surface of the glass and film system. * Interior visible light reflectance (\(\rho_{V,j}\)): fraction of visible radiation reflected by the interior surface of the glass and film system. Figure 3: Classes (installation position) of window films: (**a**) Single glazing; (**b**) Double glazing; (**c**) Triple glazing. * Total solar energy transmittance factor or solar factor (_g_): ratio between the total amount of radiation (transmitted directly and after being absorbed) flowing into the indoor environment and the global incident solar radiation on the glass and film system. * Total shading coefficient (SC): ratio between the gain of solar heat through a glass and film system and the gain of solar heat through a double-resistant transparent glass system. * UV transmittance (\(\tau_{UV}\)): fraction of UV radiation passing through the glass and film system. * General color rendering index (\(R_{a}\)): degree of reliability of the light source that permeates the film revealing the colors of the illuminated objects, compared to their appearance when illuminated by natural daylight.