Radiation and working principle
Luckily, only a part of the electromagnetic radiation that comes from the Sun to the Earth actually reaches our windows. The range which is transmitted by the atmosphere is called solar spectrum. The glass of a window transmits approximately the 300-2500 nm part of this range, which consists of three parts: UV radiation (300-380 nm), visible light (380-750 nm) and a portion of infrared radiation (750-2500 nm). From a health perspective, filtering UV radiation is extremely important, because when it reaches deeper layers of the skin, UV radiation can cause mutations and the greying of the eye lenses. Visible light, as its name indicates, is the only visible range to us; 44% of energy comes from this section. Infrared radiation is invisible to us, but we can feel it as heat; this range is responsible for 53% of the irradiated energy.
Window films were developed to filter incoming radiation in the UV, visible light and infrared ranges, when applied on windows. To understand their working principle, let’s take a look at forms of heat transmission. The transmission of heat can happen by conduction (e.g. if I’m heating the top of a spoon, the bottom becomes hotter and hotter as well), by convection (e.g. heat transmission by the movement of air) and by radiation (e.g. irradiated heat). Window films help reduce incoming energy by filtering these ranges completely or selectively.
Incoming energy consists of three parts: the parts reflected, transmitted and absorbed by the window. The amount of energy which is reflected by the filmed window into the environment is called Total Solar Energy Reflected (TSR). Total Solar Energy Transmitted (TST) indicates part of the energy that enters the room through the window. The third part is Total Solar Absorbtion (TSA), which is absorbed by the glazing system and then radiated into the environment at a completely different wavelength. Total Solar Energy Rejected (TSER) contains all of these factors, and this is the best indicator of the heat protective function of a window film.
In the next part of our series, we will look at the importance of absorbed energy, and in part 11, when discussing manufacturing technologies, it will be explained what makes a film capable of filtering the above ranges.