Typically, window film optical & thermal properties are tested with 3 – 6 mm clear or low-iron glasses as the substrate. Window film properties obtained with such high transparency glass substrates are more appropriate for product performance rating purposes.
In real buildings, window film products can be attached to all possible glass substrate types, such as tinted glasses, low-e coated glasses, laminated glasses, and double glazing units (DGUs). There are two methods to get window film optical & thermal properties with different glass substrates, as described below.
Option 1: direct physical test method
With the direct physical test method, the window film shall be attached to the actual glass substrate to be used. The whole glass system with window film is tested as usual.
This method is recommended for most applications, with a small number of glass substrate types.
Option 2: physical test + calculation method
With the physical test + calculation method, the following glasses need to be tested (based on the NFRC 304 method):
- Window film on a reference glass substrate (typically a 3 – 6 mm clear or low iron glass)
- The reference glass substrate (without window film)
- Other glass substrates
With the test results of glasses 1 & 2, the window film only optical data can be calculated. The window film only optical data can then be added to all glass substrates tested in step 3 to get the combined glass with window film optical & thermal properties.
This method is recommended for product development applications, with a large number of glass substrate types.