The short answer: The window film the blocks the most heat is the one with the highest Infrared Rejection Rates (Commonly noted as IR). This specification should be clearly stated or provided by the supplier of the film that you are considering for your project. Modern Nano-Ceramic and Multi-Layered Optical films can boast IR Rejection rates at over 90%, which translates to heat rejections often over 50%. The long answer: The heat that we feel from the sun is due to a wide spectrum of radiant energy. Some of this energy is actually visible, since it is part of the visible light spectrum. We receive from the sun UV Rays, Visible Light and Infrared Rays from the sun. This is spectrum is illustrated in the following image:
Window film is like a bouncer in a club. It wants to stop all UV rays and Infrared Rays, since those are the harmful ones no one wants, but still allow the visible light to enter the club. A "perfect" window film, which does not exist, would reject all UV and Infrared rays but allow all visible light to penetrate. In this sense it would be "spectrally selective" since it would select some radiation to block and some to allow.
This is why it's a bit of a loaded question, a window film that blocks the most heat would be a complete whiteout film, since this would block UV, Visible Light and Infrared, but then you can't see through the window. Here is what a whiteout looks like:
But we want to keep visibility, so we need to sacrifice some performance for that view-through factor. Because of this, we typically find that even within the same film line-up, darker films will typically have better heat rejection numbers than lighter films.
Most modern films, even the clear ones used for security, boast 99% UV rays. Scientists made that breakthrough a while back. Infrared ray rejection is where technology is advancing, currently the highest-performing films in their categories are the ones with exceptionally high Infrared (IR) Rejection. This being the case, the films that can keep great visibility but reject very high Infrared rejection are often coveted by clients and sold at a premium because of their cutting-edge technology.
Ceramic films will often have IR rejection over 80%. Newer Nano-Ceramic films will have IR rejections over 90%. 3M's Flagship film, Prestige, is a new generation multi-layered optical film that boats 97% IR Rejection. Typically even in these high performing films, the darker the film, the higher the performance it will boast.
This does have a possible downside as well however, heat rejection is a combination of reflection and absorbtion, and all the heat that is rejected must now pass through the glass several times instead of just once, which causes the glass to get hotter. Usually this a good sign, it's all heat that is not able to enter your space. But some glass is not designed to handle this heat load and can cause thermal fracture issues if a proper glass checklist is not completed. This is why we recommend hiring a professional company that can consult you on the correct film for your situation, and with this knowledge you can make an informed decision, so that you too can live Sea Cool!