Calculation of light transmittance after filming
The reason why I specifically talk about this problem is because the statements of all kinds of film or anti-film experts on the Internet are basically wrong. They can be summarized into the following two types of errors.
1. Paste a film with 75% light transmittance, and the light transmittance is 75% after pasting. This statement does not take into account the light transmittance of the window glass itself.
2. The light transmittance of the window glass is 80%, and after the film of 75% is pasted, the light transmittance is equal to the multiplication of the two, which is 60%. This statement doesn't make it clear how 75% came from.
The model of car window glass is very different, and the light transmittance of different models is different, which is greatly affected by factors such as composition, thermal insulation, and whether it is laminated. Judging from the data found on the Internet, the light transmittance of the front windshield laminated glass is mostly between 75% and 82%, and some of them reach 86%. The light transmittance of the non-laminated glass used for the side window and the ordinary white tempered glass that is not insulated is generally more than 85%, and the heat insulation is generally 70% to 75%.
There is a standard and unified measurement method for the light transmittance of the car film, which is the light transmittance measured by pasting the car film on the 3mm thick ordinary white float glass. From the general information queried on the Internet, it can be seen that the light transmittance of ordinary white float glass is generally 90%. Assuming that the car film manufacturers use 90% glass to measure, then the film with a light transmittance index of 75% reduces the light transmitted by the 3mm float glass by one-sixth, that is, the light transmittance of the film itself is 83.3%. This is only a rough calculation, and the actual film effect is also affected by the medium properties of the contact surface, and the effect of the same film on different types of glass may also change slightly. But the difference is negligible.
As for the window glass, first of all, it is not white float glass, and the process requirements such as lamination, heat insulation, explosion-proof, and bending make its composition different, reduce the light transmittance, and the physical properties are also different from float glass. Secondly, there is almost no 3mm thickness of automotive glass, and it is basically 5mm.
Therefore, if the light transmittance parameter of the car film paste is 70%, then the overall light transmittance after the film is basically lower than 70% (new car inspection qualification standard), because the light transmittance of the front windshield of the car window cannot reach the level of 3mm white float glass. If the light transmittance of the original car laminated heat insulation glass is 80%, then if the film with a light transmittance of 75% is pasted, the light transmittance after the film is roughly estimated as follows: the light transmittance of the glass is 80%, multiplied by the light transmittance of the film is 83.3%, which is equal to 66.7%. It can be seen that it is difficult to meet the national mandatory 70% transmittance standard for a film with a transmittance index of 75%.
