Total solar energy rejection? Visible light transmission? What do these values really mean? Let’s take a look at the technical parameters of window films.
Window film technical data can be categorized into three groups: values related to energy, to visible light and to other factors. When looking at energy values, it is crucial to know what type of window the film will be applied on, because the data will be completely different in the case of different glazing systems. Manufacturers generally publish their values for single-pane windows, or for dual-pane clear windows at most, so be careful. SkyFol is one of the few brands that publish technical data for several glazing structures, thus providing fair information to customers.
The most important energy value is Total Solar Energy Rejected (TSER), which shows the percentage of energy kept outside by the filmed glass. TSER is calculated from Total Solar Energy Transmission (TST), Total Solar Energy Absorption (TSA) and Total Solar Energy Reflection (TSR), so TSER is the indicator of energy efficiency. We should treat the TSER value specifically for different glazing systems – e.g. the TSER value of the SI15 film on single-pane windows is 79%, while 64% on dual-pane Low-e glasses. This difference comes from the additional energy absorption of developed glazing systems, where rahter exterior window films are recommended. For glasses, it is usually the „g” or SHGC value which is given: SHGC=1-TSER.
Values related to visible light indicate the light transmission and reflectivity of the window film. Visible Light Transmission (VLT) is the percentage of light entering the room through the filmed window. The number of glass panel layers and their metallization has little effect on VLT, but a dyed window does affect it considerably. Visible Light Reflection (VLR) shows the percentage of visible light reflected by the window film. We usually distinguish between VLR IN (reflectivity towards the inside) and VLR EX (reflectivity towards the outside), which are important especially if the given film has a reflective exterior, but a neutral dyed interior layer.
The third group of parameters is that of other technical values. The most important of these is Ultraviolet Rejection (UVR), which shows the percentage of UV radiation filtered by the filmed window. U-Value indicates the heat transfer of the film, which is an important factor related to heat loss in winter. Luminous Efficacy shows if the film is indeed spectrally selective or not: we can speak of a film being spectrally selective with a value above 1.