Film datasheets are full of numbers, and most buyers compare them as if they were properties of the product alone. They are not. Optical and solar data are measured on a defined substrate under a defined method, and both travel with the figure whether or not they are printed next to it.
The substrate problem
A film measured on 3 mm clear float will show a different visible transmission and a different solar factor when applied to 6 mm tinted glass or to a sealed unit. The film's optical properties are constant; the assembly's performance is not. Two suppliers quoting the "same" TSER may have measured on different glass.
So the first question about any figure is: on what, and by which method. A datasheet that states both can be compared. One that states neither cannot, and the buyer who compares anyway is comparing marketing.
The figures that predict performance
Solar heat gain coefficient describes the fraction of incident solar energy that ends up as heat inside. It is the number building engineers use, and it is the one that predicts cooling load. Total solar energy rejected is its complement expressed as a percentage, and it is the number sales material prefers because it is larger.
Visible light transmission describes daylight and glare. Ultraviolet rejection describes the band below about 380 nm. Infrared rejection describes a slice of the spectrum and is useful for comparison between films, but it is not a load figure and should never be presented as one.
What to do with an IR percentage
Infrared rejection figures are the most abused number in this industry, because they can be measured selectively. A film can show a high IR rejection at one wavelength band and a modest total solar rejection, and the high number will be printed in large type. When a buyer asks why two films with the same IR figure behave differently, the answer is that IR is not the whole spectrum.
Ask for TSER or SHGC alongside it. If only IR is offered, the supplier is choosing the flattering number.
Safety and security testing
Mechanical performance is tested differently from optical performance, and this is where the claims get loosest. Fragment retention, impact resistance and forced-entry delay are properties of an assembly — glass, film, frame and fixing — not of a film in isolation. A report on a film sample does not describe what happens to a filmed window in a wall.
Where a project requires a performance class, the route is to have the assembly tested. A supplier who says otherwise is either confused or selling.
Certificates versus reports
A certificate usually states that a quality system or a product conforms to a standard. A test report states what was measured, on what, by which method, and what the result was. For a buyer, the report is more useful than the certificate, and the batch record is more useful than either — it connects the film in the carton to the run that was tested.
The short version
- Every figure carries a substrate and a method; ask for both.
- Use SHGC or TSER to predict cooling load, not IR rejection.
- Mechanical claims belong to the assembly, not to the film.
- Prefer a batch record over a generic certificate.
- Two datasheets are comparable only when they were measured the same way.
