Film datasheets lead with three numbers. Buyers compare them across suppliers as if they were prices. They are not interchangeable, and one of them is routinely misused.
VLT — visible light transmission
The share of visible light that passes through. It sets how dark the room looks and how much daylight you keep. A 70% VLT film is nearly invisible at night; a 20% film reads as tinted glass. VLT is also the number that decides whether occupants accept the installation, so it is worth treating as a design constraint rather than a performance score.
TSER — total solar energy rejected
The share of the whole solar spectrum (ultraviolet, visible, near-infrared) that the glazing assembly does not admit. Convenient, comparable, and limited: it counts energy that is absorbed by the glass and later re-radiated inward as "rejected", which overstates the benefit for dark films that heat up and release that heat into the room.
SHGC — solar heat gain coefficient
The fraction of incident solar energy that ends up as heat inside the building, expressed 0–1. This is what building energy models use, what codes limit, and what actually predicts cooling load. SHGC accounts for inward re-radiation; TSER does not. The two are related but not inverse: a film with TSER 60% might land at SHGC 0.38, while another with TSER 55% might reach SHGC 0.33 if it absorbs less and reflects more.
Why the same film behaves differently on different glass
Film performance is measured on a reference glass build, usually 6 mm clear monolithic. Put the same film on 6 mm bronze, on laminated glass, or on a low-E sealed unit, and the numbers move — sometimes by a lot. The film's absorption adds to the glass's own; the cavity in a sealed unit traps heat; the coating changes what reaches the film at all.
If a supplier quotes performance without naming the glass build, the number is not a prediction. It is a label.
What to ask for instead
- Performance data for your glass build, not the reference build.
- SHGC alongside TSER, so the figure can go into a model.
- IR rejection with the wavelength band stated — "99% IR" without a range is not a measurement.
- Glare metrics if the complaint is screens, not heat: VLT alone does not describe glare.
None of this makes film harder to sell. It makes the specification defensible when a facilities manager asks, six months later, why the fourth floor is still warm.
