Window film spec sheets are full of acronyms and percentages that can be confusing if you have never seen one before. This guide explains every key metric — what it means, how it is measured, and what number to look for when comparing films.
The Six Key Metrics
The percentage of visible light (380–780nm wavelength) that passes through the film. A VLT of 35% means 35% of visible light gets through — your room will be noticeably darker than without film. A VLT of 70% means the film is nearly clear.
The percentage of total solar energy — visible light, infrared, and ultraviolet combined — that the film blocks from entering your space. This is the most important number for heat reduction. A TSER of 70% means 70% of all solar heat is blocked before it enters your room.
A number between 0 and 1 that represents how much solar heat passes through the film. SHGC and TSER are directly related: TSER = 1 − SHGC. A film with SHGC of 0.30 has a TSER of 70%. SHGC is the metric used in building codes and energy modeling software.
The percentage of ultraviolet radiation (280–380nm) that the film blocks. UV is the primary cause of fading in furniture, flooring, artwork, and fabrics. Most quality window films block 99% of UV. This is the one metric where nearly all films perform equally — 99% is the standard.
The percentage reduction in visible light compared to unfilmed glass. A glare reduction of 63% means the film reduces visible light entering the room by 63%. This is directly related to VLT — a film with 35% VLT has approximately 63% glare reduction (since clear glass transmits ~90% visible light).
The percentage of infrared radiation (780–2500nm) blocked by the film. IR makes up roughly 53% of total solar energy. IR rejection is often advertised prominently because the numbers are high — but it only measures one portion of solar energy. See our full article on IR vs. TSER.
How Specs Are Measured
All specs on manufacturer data sheets are tested on 3mm clear float glass (the most common residential window glass) per NFRC (National Fenestration Rating Council) and ASHRAE protocols. This is the industry standard baseline.
Important: Your actual results will vary based on your glass type and thickness. Tinted glass, low-e glass, laminated glass, and tempered glass all interact differently with window film. A professional assessment of your specific windows will give you a more accurate performance estimate.
Manufacturer-Reported vs. NFRC-Rated
There is an important distinction between manufacturer-reported specs and NFRC-rated specs.
| Type | What It Means | Who Verifies It |
|---|---|---|
| Manufacturer-Reported | Tested by the manufacturer using NFRC/ASHRAE methodology. Accurate but self-reported. | Manufacturer's own lab |
| NFRC-Rated | Independently tested and certified by the National Fenestration Rating Council. Third-party verified. | Independent NFRC-accredited lab |
Most window film specs — including ASWF's — are manufacturer-reported. This does not mean they are inaccurate. ASWF uses NFRC/ASHRAE methodology and their numbers are consistent with independent testing. But it is worth knowing the distinction when comparing brands.
Every spec on this website is labeled as manufacturer-reported unless otherwise noted.
Reading an ASWF Spec Sheet — Example
Here is how to read the spec data for ASWF Twilight™ 35:
| Metric | Value | What It Means |
|---|---|---|
| VLT | 34% | 34% of visible light passes through. Room will be noticeably darker. |
| TSER | 55% | 55% of total solar heat is blocked. Significant cooling benefit. |
| UV Rejection | 99% | Blocks virtually all UV. Protects furniture and flooring from fading. |
| Glare Reduction | 63% | Reduces visible light glare by 63% vs. unfilmed glass. |
Questions About Specs for Your Windows?
Nicholas can assess your specific glass type and recommend the film that will give you the best real-world performance.
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