Two films can both be described as "high clarity gloss" and look identical on a showroom sample board, then look different on a black bonnet in direct sun. The difference is measurable, and the measurements have names that appear on specification sheets without explanation: haze, gloss units, yellowing index, and the orange peel that is not on any sheet at all.
This article sets out what each of these numbers measures, what instrument produces it, what range is acceptable on automotive exterior film, and how to run a comparison between two films that produces an answer rather than an impression.
The reference is the SUMEI film range from Hangzhou Xingtuxintong Automotive Technology Co., Ltd., where the surfaces are specified numerically — for example the matte finish quoted at 22.3 gloss units — rather than described by name. The chemistry behind the clarity is covered in the specification guide.
Haze: the number that predicts dullness
Haze is the percentage of light that is scattered rather than transmitted straight through the film. On a transparent film, high haze means a milky, veiled appearance; low haze means the film is optically invisible against dark paint.
| Haze level | Appearance on black paint | Typical cause |
|---|---|---|
| Under 1% | Effectively invisible; paint reads as paint | Clean TPU extrusion, smooth adhesive, flat liner |
| 1–2% | Slight veil visible in raking light | Minor surface irregularity, or a coarser adhesive coat |
| 2–4% | Noticeable haze; the panel looks "filmed" | Liner texture transferred into the adhesive, or top coat unevenness |
| Above 4% | Clearly cloudy | Material problem, or contamination between layers |
Haze is measured with a hazemeter, which reports the ratio of scattered to total transmitted light through a specimen. It is worth knowing that haze measured on a flat specimen and haze seen on a vehicle are not the same thing: a film with low haze can still look hazy if the adhesive has been contaminated during installation, or if the film was stretched beyond its elastic limit during application and the polymer did not recover.
That second cause is the one buyers rarely connect to a complaint. A film installed with excessive heat and stretch over a complex curve can check back with visible distortion that reads as haze, even though the film's own haze figure is under 1%.
Gloss units: what GU measures and what 22.3 means
Gloss is measured by directing a light beam at a defined angle onto the surface and measuring the reflected intensity against a black glass standard. The angle matters enormously, and the automotive industry quotes at 20 degrees for high gloss and at 60 degrees for mid-range surfaces.
| Specimen | Gloss (60° GU) | Gloss (20° GU) | What it is |
|---|---|---|---|
| Black glass standard | 100 | 100 | The reference |
| High gloss automotive clear coat | 90+ | 80+ | Factory finish |
| Gloss protection film | 90+ | 80+ | Should match or slightly exceed the paint beneath |
| Matte protection film | Around 20–30 | Under 10 | Satin finish; the SUMEI matte is quoted at 22.3 GU |
| Gloss black film | 90+ | 80+ | Coloured film with a gloss clear coat |
The 22.3 GU figure on a matte film is a specification with a purpose: it tells a buyer that the surface is deliberately satin rather than accidentally dull, and it gives a way to check consistency between batches. A matte film quoted without a gloss number cannot be verified, and "matte" spans a wide visual range from near-gloss to a heavy flat finish.
For a distributor, the number matters commercially because a customer who ordered matte and received a 15 GU film has a legitimate complaint, and a customer who received 22 GU has not — but without the specification there is nothing to point at.
Yellowing index: the durability number
Yellowing is measured as a change in the yellowness index (YI), typically from a spectrophotometer reading before and after accelerated ageing. It is the single most commercially important optical number for exterior film, because a yellow film on a white car is an immediate warranty claim.
The reason it is quoted less often than haze or gloss is that it requires an ageing test to produce. A QUV accelerated weathering test — the standard method for film, typically run to 1000 hours — exposes the specimen to cycles of UV and condensation, and the YI shift afterwards is the durability evidence.
Two things to ask about a YI claim:
- What was the test duration and cycle? A YI figure without the test condition means nothing. QUV 1000 h is the common benchmark for film.
- Is the number a shift or an absolute value? A post-test YI of 3 on a scale where 1 is clear is a different claim from a shift of 3 points from a base of 1.
The material reason aliphatic TPU is specified for exterior film is entirely about this number: aromatic TPU yellows under UV because of the chemistry of its polymer chain, and no additive fixes it permanently. That mechanism is explained in the specification guide.
Orange peel: the number that is not on the sheet
Orange peel is a surface waviness that makes a reflection look dimpled, like the skin of an orange. It is not measured on most film specifications, but it is the visual defect buyers notice first on a horizontal panel.
| Source of orange peel | Where it originates | Whether the film can fix it |
|---|---|---|
| Paint beneath | Factory or refinish clear coat application | No — film conforms to what is there, and a gloss film can make it more visible |
| Liner texture | Adhesive wets out against a textured liner | Partly — a smooth liner prevents it |
| Adhesive coating unevenness | Coating process | No, once laminated |
| Installation stretch | Film stretched unevenly during application | No, once set |
This is why an installer's answer to "why does this look dimpled" is often "look at the paint". A gloss film with a smooth adhesive on a flat factory panel shows the paint's own texture faithfully, and on a refinished panel the film will faithfully show the refinisher's work. Before blaming the film, examine the reflection on an uncovered adjacent panel of the same vehicle.
The practical test for a buyer comparing films: take two samples, apply both to the same small panel or mirror, and compare the reflection of a straight edge at a low angle in daylight. The one that distorts the line less is the flatter film.
Running a comparison that produces an answer
A film comparison that produces a decision needs four steps and about an hour.
- Get the numbers in writing — haze, gloss at a stated angle, YI with the test condition, and thickness. A supplier who cannot state them has not characterised the product.
- Specimen test on a dark panel — apply both films to a black panel, let them dwell 24 hours, and assess in daylight and under a raking light. This catches haze and orange peel differences that a sample board hides.
- Reflection test — hold a straight edge, a fluorescent tube, or a ruled line across the reflection at a low angle and photograph it. The photograph is the evidence, and it is comparable between films.
- Ask for the ageing report — the QUV result and the YI shift. This is the only evidence about year three.
Two samples and one hour of work will separate films that a specification sheet describes identically, and the photograph from step three is also the artefact to send a supplier when the film on arrival does not match the sample that was approved.
Where each number belongs in a purchase decision
The numbers are not equally important for every customer segment.
| Segment | Number that decides the sale | Number that decides the complaint rate |
|---|---|---|
| Premium full-body installation on dark paint | Haze and orange peel | YI after two summers |
| Matte finish request | Gloss units, stated and consistent | Batch-to-batch gloss variation |
| Fleet and commercial | Thickness and impact resistance | Edge lift, not optics |
| Colour change | Colour consistency across rolls | Clear coat gloss retention |
A distributor stocking one range for all four segments will optimise for the wrong number. The optical specification should be chosen to match the segment a series is sold into, which is why a range typically contains a premium series with a tight haze specification and an entry series where the number is simply acceptable.
That range structure is covered in planning a film range, and the incoming test that catches a batch drifting outside its stated numbers is covered in roll inspection and acceptance.
What to put on the specification sheet you send buyers
If you are the distributor rather than the factory, the sheet you send your own customers should carry four numbers, because they answer the questions customers actually ask: haze percentage, gloss units with the measurement angle, yellowness index with the test duration, and thickness in mil with its tolerance.
Two more are worth adding for a trade audience: the adhesive chemistry and the surface type, since a customer choosing between gloss and matte is choosing a number, not a word.
Xingtuxintong specifies the SUMEI range numerically per series — the matte at 22.3 GU, the infrared-rejecting surface at 93% rejection at 1400 nm, and QUV 1000 h ageing data across the F Series — with the full parameter sets on the X Series and F Series pages. The product knowledge collection covers the rest of the construction, and series-specific optical data is available through the contact page.