For window film optical & thermal property testing, the window film must be applied onto a glass substrate, and it is not practical to test the window film as a standalone film.
If the objective is to obtain the performance data of the window film on a specific type of glass, the film needs to be applied to the known glass type for testing. The glass substrate could be a single glazing glass (e.g. a laminated glass) or a double-glazing unit (DGU).
If the objective is to generate the general performance data of a window film, the following glass substrates are recommended in NFRC 304:
Applied films are measured with transparent glass with a solar transmittance greater than 0.820 (Tsol > 0.820) and a visible transmittance greater than 0.890 (Tvis > 0.890).
Glass that meets this criterion includes 3mm clear glass, 3 mm low-iron glass, 6 mm low-iron glass.
In practice, we recommend using 3-6 mm clear or low-iron glasses as the substrate.
Some tinted solar control window films are with very low visible light transmittance (e.g. less than 5%). The glass looks almost opaque. However, the SHGC and SC of such glasses are not close to 0 (indeed they are still quite high). This post aims to explain why the SHGC & SC of opaque glasses are still very high.
Two examples: opaque black glass and opaque white glass
Listed in the table below are the optical & thermal properties of two opaque glasses:
Opaque black glass: a perfect black glass with 0% transmittance and 0% reflectance
Opaque white glass: a perfect white glass with 0% transmittance and 100% reflectance
Bear in mind the relationship: transmittance + reflectance + absorptance = 100%
0% transmittance means that both glasses are opaque.
0% reflectance means the glass absorbs 100% incident radiation and reflects 0% back. The glass looks black.
100% reflectance means the glass absorbs 0% incident radiation and reflects 100% back. The glass looks white.
Opaque black glass
Opaque white glass
Visible light transmittance
0%
0%
Visible light reflectance, front
0%
100%
Solar energy transmittance
0%
0%
Solar energy reflectance, front
0%
100%
Solar energy absorptance, front
100%
0%
Solar heat gain coefficient (SHGC)
0.323
0.000
Shading coefficient (SC)
0.371
0.000
As presented above, the SHGC & SC of the opaque black glass are still very high, whereas the SHGC & SC of the opaque white glass are 0.
The reason is that the opaque black glass absorbs 100% of solar radiation. This portion of solar heat can still be transferred to the indoor space and it is counted as part of the SHGC & SC.
In practice, most tinted solar control window films are in dark colors (close to the opaque black glass). Therefore, their SHGC & SC are still high, despite of the small transmittance.
The theory: secondary solar heat gain
As shown in the sketch below, there are two components in the glass solar heat gain:
For the opaque black glass, its primary solar heat gain is 0, but the secondary solar heat gain is still high. Actually, the secondary solar heat gain of the opaque black glass is the highest, as its solar energy absorptance is 100%.
For the opaque whie glass, both its primary solar heat gain and secondary solar heat gain are 0. It is possible to archieve zero solar heat gain with the opaque white glass.
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.
