Online course in Window films, part 2: Exterior vs. interior

In the last article it was demonstrated that irradiated energy consists of three parts: reflected, transmitted and absorbed energy. The amount of energy absorbed plays a very important role in assessing the efficiency of the window film. Glass manufacturers started to produce dual layer glasses already decades ago, in order to avoid losses because of air movement and heat conduction – the question of absorbed energy is a more serious question with this type of windows. Soon fine, soft metallic coatings also appeared on glasses, whose primary aim is to reduce heat loss in winter by reflecting heat radiated towards the outside back into the room (more info will follow in part 10). Nowadays, triple pane windows with metallic coating are spreading in an unstoppable way. The window films industry has to follow these trends, therefore the appearance of interior and exterior films with low heat absorption. Let’s take a look at a few examples to illustrate the differences.

Single pane vs. dual pane low-e glass, interior SI35 film

  SkyFol SI35 reflective silver interior film on single pane window SkyFol SI35 reflective silver interior film on dual pane low-e window
Total Solar Energy Rejected 68% 55%

Single pane vs. dual pane low-e glass, interior DR35 film

  SkyFol DR35 neutral interior film on single pane window SkyFol DR35 neutral interior film on dual pane low-e window
Total Solar Energy Rejected 60% 38%

Dual pane low-e glass with P35, SI35 and DR35 films

  SkyFol P35 spectrally selective interior film on dual pane low-e window SkyFol SI35 reflective silver interior film on dual pane low-e window SkyFol DR35 neutral interior film on dual pane low-e window
Total Solar Energy Rejected 67% 55% 38%

Dual pane low-e glass with interior vs. exterior silver 35 films

  SkyFol SI35 reflective silver interior film on dual pane low-e window SkyFol XT33S reflective silver exterior film on dual pane low-e window
Total Solar Energy Rejected 55% 78%

Dual pane low-e glass with interior neutral vs. exterior neutral 35 films

  SkyFol DR35 neutral interior film on dual pane low-e window SkyFol XT36DR neutral exterior film on dual pane low-e window
Total Solar Energy Rejected 38% 74%

It can be clearly seen from the above that there might be considerable differences in the technical parameters of certain window films when applied on a developed glazing system. Unfortunately, a lot of distributors and installers offer window films based on data measured on single pane glasses, which is a serious professional mistake. There are, naturally, other aspects as well which must be taken into consideration, e.g. if the client asks for low reflectivity or low visible light filtering.The next part of this series will present the possibilities along individual film series.

Online course in Window films, part 1: Radiation and working principle

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.

How do window films work?

By its nature, the glass of windows transmits radiation between 300 and 2,500 nm. This range consists of a part of UV radiation (300-380 nm), visible light (380-780 nm) and a part of infrared radiation (750-2,500 nm). UV radiation is responsible, among other factors, for the fading of objects, and it also poses health risks – it can cause mutations of the skin, or in extreme cases, skin cancer. For biological reasons, visible light is indispensable, but almost half (47%) of irradiating energy also comes from this range. Besides other reasons, this is why it is difficult to manufacture a strongly heat protective, but at the same time light window film. The bigger portion (53%) of energy comes from the infrared range – for energy-protection reasons, the filtering of this part of the energy is our most important objective. Nevertheless, the way energy is filtered is also a crucial factor.

The energy falling on any glazing system is made up of three parts.

  1. Energy transmitted through the surface
  2. Energy reflected from the surface
  3. Energy absorbed, than emitted by the surface

The heat absorption capacity of the glazing system is highly important. For advanced glazings, it is essential to choose a window film with low heat absorption. As an example, take a look at the performance of an interior silver and of an interior neutral film.

  ablak_ateresztes_angol_7    ablak_ateresztes_angol_8

In the case of advanced glazing systems, if heat protection during the summer is a priority, exterior window films are recommended, because the emission of the absorbed heat is a lot more efficient using these materials. As an example, take the performance of an interior and an exterior silver film on a 4-16-4 low-e window.


UV radiation

We can often hear that window films of our days generally filter UV radiation efficiently (with the exception of a lot of DIY films), but what does UV radiation mean? What problems can it cause if the atmosphere filters its major portion? Why is it recommended to protect ourselves from UV radiation, and how does a window film provide this protection?

Ultraviolet (UV) light is a type of electromagnetic radiation, which has a shorter wavelength than visible light, but longer than X-rays. The Sun emits UV light in the UVA, UVB and UVC bands, but because of the absorption happening in the ozone layer of the atmosphere, 98,7% of UV radiation reaching the surface of the Earth is of the UVA band. (A part of the UVB and UVC bands are responsible for the formation of the ozone layer.)

Traditional glass is partially transparent for UVA, but it is opaque for shorter wavelengths. UV radiation entering through the window is dangerous for two reasons: it can risk our health and it is responsible for the fading of our valuables.


Harmful health effects

Being excessively exposed to UV radiation can cause sunburn and certain types of skin cancer, and also acute or chronic damages of the skin, eyes and the immune system. The deadliest mutation (malignant melanoma) is mainly the result of direct DNA damage. Ultraviolet photons are harmful to living organisms’ DNA in different ways.

Both UVA, UVB and UVC can damage collagen tissues, thus fastening the aging of the skin. UVA and UVB destroys vitamin A in the skin, which might result in more skin damage. High intensity UV light is dangerous for the eyes – it can cause photokeratitis, cataract, pterygium and pingeucula.

SkyFol films provide efficient filtering in the whole UV range, thanks to their special Ultraviolet Absorbers (UVA). As these absorbers are found within the polyester structure, their efficiency does not decrease during the warranty period.

Beside all of the above, let’s not forget about the beneficial effects of UV radiation. Stimulating the formation of vitamin D in the skin is a positive effect of UV exposure. It is estimated that several tens of thousands of premature deaths in the USA are the result of different cancerous diseases caused by the lack of vitamin D. People prefer food with added vitamin D and also food supplements as opposed to UVB exposure, because of the higher risk of skin cancer caused by UV radiation. UV light has other medical applications as well.



If you have a window opening towards the outside, sooner or later you will have to deal with the harmful effects of the Sun, related to carpets, furniture, linoleum flooring and floorboards, antiques.

Fading is a result of three main factors: UV radiation, heat visible light. A fourth factor includes the effects of humidity, the type of fabric, the strength of colour fixing agent and other conditions.

From among the above, UV radiation is the most important factor with a weight of 40%. Don’t be misled by the fact that the Sun does not shine directly into the room – this does not mean that objects in your home or office are not exposed to this harmful section of the electromagnetic spectrum. The best-performing window films provide a UV rejection of 99%, which means that UV radiation is almost completely blocked from entering through the window.

Beware of claims stating that the window film completely eliminates the harmful effects of the Sun. This is not true. The only way to do so is to do away with the windows themselves, which is not an option. Window films provide a perfect possibility to reduce damages done by the Sun, which means you can save a lot of replacement costs.

SkyFol offers optimal anti-fading films with high visible light transmission, such as the ThermoProtect P70, P50, E50 and EL70 films. All of these materials filter 99% of the UV radiation, and they also strongly do so with infrared radiation (primarily by reflection and not by absorption).

How disturbing is it to darken the windows?

A lot of customers ask us how much the window film affects the quantity of light inside a room. Most often what the customer sees is the visible light transmission value. If we explain it to them that a 15 film filters 85% of visible light, they can easily develop doubts about the given film.

It is a fact that workplaces receive little external light, if at all. Offices and workshops are lit very well, so additional daylight is not necessary – in the majority of the cases, even one third of the artificial light is a lot stronger.

Psychologically speaking, being able to look outside is a lot more important than natural daylight. From this point of view, solar window films provide a lot more comfort than other protecting devices (shutters, blinds, etc.)

Besides the above-mentioned reasons, a decisive factor is that perceived light is a lot stronger than the actual light conditions. The explanation for this is that the eye automatically adapts to changing light by the dilation of the pupils. For example:

Visible light transmission of the window   80%      60%     40%     15%
Perceived light                                                  90%      82%     74%     50%

It can be clearly seen that films with a visible light transmission of 50% darken incoming light in a way which is barely noticeable, so it is no wonder these products are the most popular in the case of offices and residential buildings.