Colour shift coatings are thin-film interference (dichroic) coatings applied to glass that produce an angle-dependent change in reflected or transmitted colour. In industrial use, we treat them as bespoke optical thin-film stacks built for spectral filtering, engineered visual accents or sensing and display applications, each with explicit spectral and mechanical tolerances. If you are planning to specify one, your next step is an RFQ that states spectral targets and durability requirements up front.
TL;DR:
- Specify the actual incidence angle range first, because increasing angle shifts the spectral band toward shorter wavelengths and can invalidate normal incidence targets.
- Include center wavelength, passband or stopband edges, FWHM, and reflectance or transmittance tolerances, then require spectrophotometry, ellipsometry, and interferogram reports for acceptance.
- In laboratory tests, coatings made by reactive sputtering with buried layers reached 16 to 20 GPa hardness and nearly 8 nanometers FWHM.
- Catalogue coatings rarely satisfy aerospace or medical requirements; specify substrate and environmental limits, then verify a prototype against the complete specification before production.
Table of Contents
- How colour shift (dichroic) thin-film coatings work
- Design and specification checklist for your RFQ
- Manufacturing methods and materials behind the colour shift
- Durability, testing and realistic performance thresholds
- Application examples and integration notes
- Why we recommend bespoke specification over catalogue parts
- How we can help you get from concept to RFQ
- FAQ
- Sources
How colour shift (dichroic) thin-film coatings work
The colour shift comes from thin-film interference: light reflects at each boundary between layers of different refractive index, and those reflections combine constructively or destructively depending on wavelength. The result is a stack that reflects one band of the spectrum strongly while transmitting another, which is why a dichroic coating can look one colour in reflection and a different colour in transmission.
Two variables drive the design:
- Optical thickness of each layer (refractive index multiplied by physical thickness) sets which wavelengths interfere constructively, so the stack designer tunes layer thickness in quarter-wave optical thickness (QWOT) units to place the reflected or transmitted band where it is needed.
- Angle of incidence shifts the spectrum because the effective optical path length through each layer depends on the incident medium and the angle, broadly following the n·sinθ relationship; light hitting the glass at a steeper angle sees a shorter effective path, and the colour band shifts towards shorter wavelengths.
It is worth being precise with language here. In laboratory and industrial contexts, “dichroic” refers specifically to interference filters built from these layer stacks, while “iridescent” is often used loosely in marketing to describe any colour-shifting surface, including decorative effects with no engineered spectral control, as technical references on dichroic glass make clear. For an RFQ, use “dichroic” and state the mechanism, not the marketing term.
Design and specification checklist for your RFQ
A coating supplier can only quote accurately against numbers, not adjectives. Before you send a request, pin down the following:
- Spectral targets: centre wavelength, passband or stopband edges, full width at half maximum (FWHM), and absolute transmittance or reflectance tolerances at each key wavelength.
- Angular acceptance: the incidence angle range the part will actually see in service, since the n·sinθ relationship means a coating specified only at normal incidence can shift noticeably off-axis, a point a detailed RFQ-ready dichroic coating specification guide sets out in detail.
- Mechanical and environmental requirements: scratch resistance class, adhesion standard, operating temperature and humidity range, and thermal cycling limits.
- Substrate and process constraints: glass type, any prior tempering or lamination, and whether the coating must survive downstream cutting or edge-finishing.
- Acceptance tests: ask for spectrophotometry traces, ellipsometric fits, and interferogram reports rather than a single pass or fail statement.
Pro Tip: State your angular acceptance range before your spectral tolerance; getting the angle wrong invalidates every other number on the sheet.
Off-the-shelf dichroic coatings rarely meet the combined spectral and mechanical demands of aerospace or medical hardware, which is why most serious specifications end up bespoke rather than catalogue parts, as the same dichroic coating specification resource notes.
Manufacturing methods and materials behind the colour shift
Colour-shift stacks are almost always deposited by physical vapour deposition (PVD), and the variant matters for repeatability:
- Electron-beam evaporation gives good optical control and is common for lower-volume, high-precision runs.
- Sputtering, including reactive sputtering where a reactive gas forms the compound layer during deposition, tends to produce denser films with better adhesion and more consistent layer-to-layer uniformity across a batch.
- Both approaches depend on tight process control because small thickness errors shift the colour band, which is why reputable production runs are backed by numerical design and optical monitoring during deposition, using monochromatic, broadband or turning-point monitoring to catch drift before it becomes scrap.
Material choice sets the refractive-index contrast that makes the interference effect possible. Dielectric materials such as titanium dioxide (TiO2) and silicon dioxide (SiO2) are common high- and low-index pairings, while indium tin oxide (ITO) is used where some electrical conductivity is also needed alongside the optical effect. Larger index contrast between adjacent layers produces stronger reflectance peaks for a given number of layers.
More advanced constructions bury the active wavelength-selective layers beneath a thick, hard overcoat, a finishing choice explored in detail in the comparison of aqueous vs UV coating for packaging. This “buried layer” approach, along with modular multilayer stacks built through needle optimisation and gradual evolution design methods, lets engineers achieve narrowband colour response without sacrificing surface durability.
Durability, testing and realistic performance thresholds
Thin films scratch more easily than bulk glass because the active layers sit at or near the surface, directly exposed to abrasion and handling. The practical fix is mechanical, not optical: bury the interference layers under a hard overcoat several microns thick, trading a small amount of optical amplitude for a large gain in scratch resistance.
Reactive-sputtered wavelength-selective coatings using this buried-layer strategy have demonstrated nanoindentation hardness in the range of 16 to 20 gigapascals 16–20 GPa, alongside narrowband transmission with a FWHM close to 8 nanometres in laboratory prototypes, a useful benchmark when you are assessing a supplier’s durability claims.
For an industrial order, ask your supplier for a test report covering nanoindentation hardness, scratch-panel results against a defined load, adhesion testing (tape or cross-hatch methods), and environmental cycling across your operating temperature and humidity range. Medical and aerospace buyers in particular should expect these figures stated as numbers against a named standard, not as a general assurance of “durability”.
Application examples and integration notes
The right specification depends on what the coated surface has to do in service:
- Sensing and display accenting usually prioritises a sharp, repeatable cut-on or cut-off wavelength over broad chroma, since the coating is filtering a signal rather than creating a visual effect.
- Decorative or branded visual effects on dashboards or instrument housings prioritise high-chroma reflectance and a pleasing colour travel across viewing angle.
- Protective optical windows often need high transmission with only a slight tint, which demands tighter absolute transmittance tolerances than a purely decorative part, as covered in our optical window coating specification guide.
Integration matters as much as the coating itself. Edge finishing and bonding surfaces should be masked or processed before coating where possible, cleaning protocols need to avoid abrasive contact with the active layer, and optical fabrication guidance covers safe handling during assembly. Always verify a prototype batch against your full spectral and mechanical spec before committing to volume production.
Why we recommend bespoke specification over catalogue parts
Experience with bespoke coating projects across defence, aerospace and medical programmes suggests that the recurring failure point is not the coating chemistry, it is the specification itself. The most common RFQ mistakes we see are missing angular acceptance ranges, vague durability language instead of named test methods, and no agreed acceptance report format. Each of these causes rework that a clear spec sheet would have prevented.
A recommended approach is to treat the RFQ as the engineering deliverable it is, not paperwork to finish quickly.
— Alexandra
How we can help you get from concept to RFQ
Engineering teams can turn a colour-shift concept into a coating specification that a production line can hold to. Relevant services include:
- Drafting RFQ-ready spectral and mechanical specifications from your target performance.
- Prototyping coated glass samples for verification before volume commitment.
- Optical testing, including spectrophotometry and interferogram reporting.
- Batch-level traceability for regulated sectors such as medical devices and aerospace.
Before you contact us, have your substrate drawings, target spectral curves, and environmental operating limits ready. This lets us scope a coating design against your real constraints rather than generic assumptions. Visit the services page to see full coating and finishing capabilities, or submit drawings and target spectra directly for an RFQ.
FAQ
What is the difference between dichroic and iridescent coatings?
“Dichroic” is the correct technical term for interference filters and thin-film stacks engineered to show different colours in reflection and transmission. “Iridescent” is often used loosely in marketing for any colour-shifting surface, including decorative effects that are not built with the same spectral control, as technical sources on dichroic glass distinguish.
How does angle of incidence affect a colour-shift coating’s performance?
The reflected or transmitted colour band shifts towards shorter wavelengths as the viewing or incidence angle increases, following the n·sinθ relationship for the incident medium. Any RFQ should state the real angular acceptance range the part will see in service, not just a normal-incidence figure, as set out in our dichroic coating specification guide.
How hard or scratch resistant can a colour-shift coating be made?
Reactive-sputtered wavelength-selective coatings using a buried-layer strategy have reached nanoindentation hardness of around 16 to 20 GPa in laboratory tests, which is a reasonable benchmark to request from a supplier. Burying the active interference layers beneath a hard overcoat is the main strategy for improving scratch resistance without losing the colour effect.
What should I include in an RFQ for a dichroic coating?
State your spectral targets (centre wavelength, passband or stopband, FWHM, and tolerances), the angular acceptance range, and your mechanical and environmental requirements. Ask for acceptance tests such as spectrophotometry, ellipsometry and interferogram reports rather than a general durability claim, a structure our RFQ-ready specification page walks through.
Can off-the-shelf coatings meet demanding industrial specifications?
Catalogue dichroic coatings rarely meet the combined spectral and mechanical demands of sectors such as aerospace or medical devices, which is why bespoke fabrication is the common route for these applications. A specification built around your actual angular range, environment and test requirements gives a far more reliable result than selecting a standard part.
Sources
- Wavelength‑Selective Coatings on Glass with High Hardness and Damage Resistance
- Optical coatings: design, characterization, monitoring — SPIE
- Dichroic glass — Wikipedia
