Technology
Solar spectrum in the car
11 minEstimated reading time — roughly how many minutes it takes to read at a normal screen pace.
- How solar spectrum splits into UV / VIS / IR and what each band does inside the car.
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Window film is not just a “dark window.” It is a spectral filter — it transmits visible light while blocking UV and IR. The difference between “works” and “does not work” is which part of the solar spectrum each film type blocks, and how effectively. Because each generation solves a different problem, the choice is non-trivial.
Solar spectrum in the car
Solar radiation hitting a vehicle divides roughly as follows:
| Band | Wavelength | Energy share | Effect |
|---|---|---|---|
| UV | 280–400 nm | ~5 % | Ages plastics, fades upholstery, skin risk |
| VIS (visible) | 400–780 nm | ~43 % | Light, glare, perceived “darkness” |
| IR (infrared) | 780–2500 nm | ~52 % | Heat — main driver of cabin overheating |
From an energy standpoint, more than half the problem is IR. A dark dyed window blocks VIS but passes IR — the cabin still overheats. That is why old-school film “does not cool,” even when it looks dark.
Three generations of window film
Interactive spectrum
Three generations of window film
Switch between film types and see how much UV / VIS / IR passes through, view the layers, and see whether the film blocks radio signals.
Nano-ceramic — selective IR rejection
10–15 letCeramic oxide nanoparticles (typically ATO / ITO / specialized SiO₂ composites) in a polymer matrix. Selectively block IR and UV while passing VIS and radio bands. UV >99%, IR 94–98% @ 1025 nm, no signal block.
UV block
99 %
IR rejection
95 %
VIS block (darkness)
40 %
1. Dyed film
Oldest and cheapest. Polyester matrix with dissolved organic dyes. Function:
- VIS: absorbs visible light → “dark window.”
- UV: partial block (~60–80 %, depends on dye load).
- IR: almost no block — dyes lack IR-active bands.
Weaknesses:
- Dyes degrade under UV — after 2–5 years the film turns purple-brown.
- Does not lower cabin temperature.
- Suitable for privacy and appearance only, not thermal protection.
Service life: 2–5 years.
2. Metalized film
Second generation. Thin vapor-deposited metal layer (aluminium, silver, nickel) in a polymer matrix. Acts as a Faraday cage in visible and radio spectrum.
- VIS: reflects → characteristic “mirror” look.
- UV: blocks nearly 99 %.
- IR: reflects 50–70 % → meaningful cabin cooling.
Critical problem: blocks radio signals:
- Dashboard GPS (radio + navigation) loses lock.
- GSM / LTE / 5G inside the car is heavily attenuated.
- ADAS sensors in the glass (camera, lidar) may fail.
On modern cars after ~2015 (windscreen camera, lane keep, adaptive cruise), metalized film is often technically unusable. Some installers cut out zones around sensors — but that breaks visual continuity.
Service life: 7–10 years.
3. Nano-ceramic film
Current standard. Polymer matrix with nano-scale ceramic oxides — typically ATO (antimony tin oxide), ITO (indium tin oxide), or specialized SiO₂ composites. Particle size ~10–50 nm, i.e. below VIS wavelength — particles do not absorb visibly; they transmit.
| Band | Block | Mechanism |
|---|---|---|
| UV | >99 % | UV absorption in oxides |
| VIS | 20–60 % (depends on VLT choice) | Intentional tint |
| IR @ 1025 nm | 94–98 % | Selective absorption by ceramic nanoparticles |
| Radio (GPS/GSM) | 0 % | Ceramics are radio-transparent |
Selectivity is key: particles have an IR absorption band where the sun radiates most heat (~700–1400 nm), while VIS and radio pass. Nano-ceramic therefore cools like metalized, without signal block.
Service life: 10–15 years.
TSER, VLT, and contact angle
Three core metrics for comparing window films:
- VLT (Visible Light Transmittance) — how much visible light passes. 90 % VLT ≈ nearly clear; 5 % VLT = limousine dark. Critical for Czech legislation (see Window tinting in the Czech Republic — what is legal).
- TSER (Total Solar Energy Rejected) — total % of solar energy blocked across UV + VIS + IR. Commercial nano-ceramic 58–90 %; premium 90+ %.
- Water contact angle — surface energy of the scratch coat. New film 90–105°; after 5 years often ~80°. Lower angle = more dirt adhesion, harder washing.
TSER is the best single number for thermal performance. Cheap dyed films sit at 30–40 % TSER; premium nano-ceramic exceeds 70 %.
94–98 %
Nano-ceramic IR rejection @ 1025 nm
Where the sun radiates most heat. This is the difference between a cool cabin and a beer keg in the parking lot.
Measurable cabin effects
| Quantity | No film | With nano-ceramic 35 % VLT |
|---|---|---|
| Cabin temp at noon (35 °C ambient) | 65–70 °C | 50–55 °C |
| Driver UV exposure | 100 % | <1 % (SPF equivalent >1000) |
| Upholstery fade (5 years) | Visible | Negligible |
| A/C load | 100 % | 60–75 % |
A 10–15 °C cabin drop is the difference between “unusable after 30 minutes in sun” and “can sit on the lot all day.” That is why OEMs in hot export markets spec nano-ceramic from the factory.
ADAS compatibility
Modern cars use near-infrared camera systems in the windscreen (lane keep, high beam assist). Film in that zone must maintain NIR transmittance at 850–940 nm — which metalized film typically cannot.
Nano-ceramic for ADAS-equipped vehicles should be OEM-certified or tested on the specific camera stack. Otherwise lane-keep may fault.
On pre-~2014 cars without ADAS, this is usually irrelevant.
Takeaways
- IR is 52 % of solar energy. If the film does not block IR, it only darkens — it does not cool.
- Dyed = cheapest, fades in 2–5 years, no IR. Metalized = good IR, blocks signal. Nano-ceramic = both without signal loss.
- TSER drives thermal protection; VLT drives legal compliance (Czech rules).
- ADAS cars need certified film with NIR transmittance.
Further reading
Topic: Film & glass
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