Specifying Coloured Light That Still Looks Right in Ten Years
Colour is one of the few things in a building that can quietly stop being what you drew. The scheme opens, everyone is happy, and three or four years later somebody rings to say the blue has gone grey and the magenta has gone pink. By then the lighting contractor has moved on, the supplier has changed hands, and the person holding the drawings is you.
This is worth understanding properly, because it is not a quality problem. Most coloured light fades by design — it is a consequence of how the colour is made.
The short version
Coloured filters generally work by absorbing the wavelengths you do not want. That absorbed energy becomes heat, sitting in a dyed plastic film directly in front of a light source. The dye degrades. The colour drifts.
Dichroic glass works by reflecting the unwanted wavelengths instead. There is no dye to bleach, and the colour is a property of a coating on glass rather than a pigment suspended in polymer.
Why colour in light has a service life
A conventional colour filter is a polymer sheet carrying dye. White light hits it, the dye absorbs everything outside the intended band, and what passes through is your colour. The absorbed energy does not disappear — it is converted to heat, in the filter, continuously, for as long as the fixture is on.
So the filter is doing two jobs at once: making your colour, and cooking itself. Saturated colours suffer worst, because they absorb the most. Deep blues and reds are the usual first casualties.
For a theatre that is fine — filters are a consumable and get changed between productions. For a building it is a different proposition entirely. Nobody is going to re-gel a facade every eighteen months, and if the fixtures are at height or over water or on a heritage parapet, the access cost dwarfs the cost of the filter itself.
What dichroic glass does instead
Dichroic filters are optical glass carrying a vacuum-deposited dielectric coating — alternating layers of titanium dioxide and silicon dioxide. The coating is tuned so that the wavelengths you want pass through and the rest are reflected back out rather than absorbed.
Two consequences follow, and both matter on a building:
- Far less heat is trapped in the filter, because the energy is reflected rather than converted.
- There is no organic dye to break down, so the colour is not on a clock in the way a dyed polymer is.
It also transmits more light than a dyed filter of the same colour, which is worth knowing when you are working backwards from an illuminance target and a fixture count.
The figures worth writing into a specification
| Property | Published figure |
|---|---|
| Coating life | In excess of 10 years |
| Continuous operating temperature | 200°C (over 24 hours) |
| Short-term temperature range | −50°C to 450°C (under 1 hour) |
| Useable aperture | Greater than 95% |
| Coating | Dielectric TiO₂ and SiO₂, vacuum deposited; passes military adhesion and abrasion testing |
| Standard thickness | 1.75 mm ±0.2 mm; 1.1 mm and 3.3 mm available |
| Custom cut range | 20–343 mm round, 10–242 mm square, tolerance ±0.5 mm |
| Palette | 60+ standard colours |
The temperature figures are the ones that usually settle an argument with a contractor. A filter rated for 200°C continuous is not a consumable sitting in a gate hoping for the best — it is a component you can put in a luminaire schedule.
The detail that catches people out: batch consistency
This one rarely comes up at design stage and almost always comes up later.
Buildings get extended. Fixtures get damaged. A stage two happens three years after stage one. At that point the question is not “can I get that colour again” but “will the replacement actually match what is already there, under the same light, on the same surface, seen side by side.”
Rosco manufactures the standard Permacolor range in a single vacuum deposition facility and guarantees batch-to-batch consistency — the filter you order in ten years is the filter you have now. That guarantee is what makes it specifiable for a phased project.
The exception to know about: alongside the standard range there are “Library Colors”, which do not carry the same batch-consistency guarantee. If your scheme might be extended, patched or partially replaced later, specify from the standard range and say so in the spec.
Where the colour comes from matters to how it reads
A dichroic coating is an interference filter, and interference depends on the angle at which light passes through it. In practice that means the perceived colour can shift slightly across a very wide beam, or when the filter is viewed well off-axis.
For most architectural applications — a narrow or medium beam grazing a surface, a projector throwing onto a facade — this is a non-issue. It becomes something to check with wide-angle optics, or where the audience will see the fixture aperture itself from a shallow angle. Worth raising early rather than discovering at commissioning.
Reframing the cost conversation
Dichroic glass costs more per filter than a dyed filter. That comparison is close to meaningless on a building, because it compares a capital item against a consumable.
The comparison a client understands is total cost over the life of the scheme:
- Count the fixtures that need colour.
- Estimate the replacement interval for a dyed filter at that colour saturation and that source temperature.
- Multiply by the cost of access, not the cost of the filter — the EWP, the road closure, the out-of-hours labour, the permit.
- Compare that recurring figure against buying the colour once.
On anything at height, over water, in a public realm setting or inside an operating building, the access line usually decides it on its own.
What it looks like built
The Vienna work above is worth a second look for a different reason. Victoria Coeln's Chromotopia installations use etched dichroic filters as the artwork itself — colour and image in one piece of glass, projected onto buildings you are not allowed to touch. It is a useful reminder that on heritage fabric, light is often the only intervention that is genuinely reversible.
Practical notes for an Australian or New Zealand project
Lead time is the thing to plan around. We hold around twenty of the standard colours in Artarmon. The rest of the 60+ range, and anything custom cut, is an indent order. If colour is on the critical path, talk to us at design development rather than at procurement.
Custom sizes are normal, not exotic. Round and square cuts across the ranges above are routine, so the filter can suit the luminaire rather than the other way around.
Get a physical sample in front of the client early. Colour on a screen is not colour on a sandstone facade at night. We can get samples out for a mock-up while the scheme is still movable.
Test first: view the actual filter, in the actual fixture, on the actual surface, at night, before it goes in the spec. Substrate colour and texture change a coloured wash more than most people expect, and a mock-up is far cheaper than a variation.
A short specification checklist
- Name the specific colour number, not just “blue”.
- Specify from the standard range where the scheme may later be extended or patched, and state the batch-consistency requirement.
- Confirm the operating temperature at the filter position in the luminaire.
- Check the beam angle, and flag wide-angle optics for review.
- State the filter size and thickness, and whether it is a custom cut.
- Note access arrangements for the fixtures — it is the number that justifies the specification.
Working colour into a scheme?
Send us the fixture, the surface and the effect you are after, and we will tell you which filters do it, what is held locally, and what the lead time looks like. Samples for a mock-up are usually the fastest way to settle a colour conversation.
Sources
Technical properties — coating construction, temperature range, coating life, useable aperture, thickness, custom cut ranges and batch consistency — from Rosco's Permacolor product page and published datasheet. Always confirm current technical data for the specific colour and size before issuing a specification.
Image credits
All photography courtesy of Rosco, sourced from Rosco Spectrum, with photographers, artists and design practices credited where Rosco has identified them. Each caption links to the original article.
Creative Film & Theatre Solutions is the exclusive Rosco agent for Australia and New Zealand. This article is general guidance only and is not a substitute for current technical data or project-specific engineering advice. Rosco and Permacolor are trademarks of Rosco Laboratories Inc.