There is a certain correlation between the thermal insulation performance and light transmittance of automotive film, but it is not an absolute linear relationship. The balance between the two is one of the core considerations in the design and selection of automotive film. The specific relationship can be understood from the following three aspects:

1. Basic correlation: the "natural contradiction" determined by the principle of thermal insulation

The thermal insulation performance of automotive film mainly depends on blocking infrared radiation (the main source of heat) and reflecting visible/ultraviolet light.

To pursue higher thermal insulation (such as blocking more infrared radiation), the coating of the film (such as metal sputtering layer, nano ceramic layer) may be thicker or denser, which will to some extent block visible light and lead to a decrease in transmittance;

If you want to pursue high light transmittance (such as a windshield that meets regulatory requirements of ≥ 70% transmittance), you need to control the thickness and density of the insulation coating to avoid excessive obstruction of light, so the insulation performance may be limited to some extent.

In short, at the same level of technology, there is a basic trend of "one goes against the other" between the two - the higher the insulation rate, the lower the light transmittance may be; The higher the transmittance, the more limited the thermal insulation may be.

2. Technological breakthrough: breaking the possibility of "absolute contradiction"

With the development of membrane technology, such as nano ceramic films and multi-layer optical sputtering films, some products can achieve good thermal insulation effects while ensuring high light transmittance through precise control of coating composition and structure. For example:

Nano ceramic film utilizes ceramic particles to selectively reflect infrared radiation, with less obstruction of visible light. It can achieve an infrared blocking rate of over 80% even when the transmittance is above 70%;

Multilayer optical films achieve "selective filtering" by stacking different materials, prioritizing the blocking of heat (infrared) while allowing visible light to pass through more.

Therefore, not all films follow the absolute rule of "the higher the insulation, the lower the light transmission", and technological upgrades can narrow the contradiction between the two.

3. Regulations and Scenarios: Hard Limitations on Transmittance

There are clear regulatory requirements for the transmittance of automotive glass in different parts (such as the Chinese GB9656-2021 standard: the transmittance of the front windshield is ≥ 70%, and the transmittance of the side and rear windows is ≥ 50%). This means:

The front windshield film must first meet the requirements of light transmittance, and the thermal insulation performance needs to be optimized within the compliance range;

The side rear window film can be selected with lower light transmittance (such as privacy film) according to the needs, in order to match higher insulation rate.

In summary, the thermal insulation performance of automotive film is related to light transmittance but not absolutely bound: the basic principle leads to a natural balance between the two, but technological progress can achieve a balance between "high light transmittance+good thermal insulation". The final choice needs to be comprehensively judged based on regulatory requirements, usage scenarios (such as front/side rear gears), and personal needs (priority for thermal insulation/light transmittance).