What happens to safety sign colours when the lights go out?

At a glance

What fails first

Red loses brightness fastest as vision shifts to the rods

Escape guidance

ISO 16069:2017 (guidance systems); ISO 17398:2004 (sign performance)

Ships

IMO Resolution A.752(18), 1993; ISO 15370:2021

Status checked

1 October 2026

Why red goes dark first

The eye's daytime sensitivity, the CIE's photopic function V(λ), peaks near 555 nm; its night-time sensitivity, the scotopic function V′(λ), peaks near 507 nm and falls almost to nothing in the red. In between, in the mesopic range of dusk, street lighting and a dim corridor, both systems contribute. Read against each other, the two curves show the problem for safety colour: relative to the rest of the spectrum, light around 630 nm carries about one eightieth of the weight in rod vision that it carries in cone vision, while blue-green gains. So the red of a prohibition sign or a fire-point marker sinks towards black as light fades, while the green of an escape sign holds up comparatively well. The mechanism itself, the Purkinje shift, is explained in the lighting pages; what matters here is that ISO 3864's colour coding was designed, as ISO 3864-4 says of its own requirements, primarily for daytime colour and normally lit places.

Red and black look alike in darkness. That is one reason the safety-sign system pairs every colour with a shape and a contrast colour: when hue is gone, the white border of a red disc or the black band of a yellow triangle is what is left to read.

What the rules ask for when power fails

Regulators deal with low light through illumination first and luminous materials second. Britain's Health and Safety Executive says that safety signs should keep their intrinsic features during a power failure, by emergency lighting or phosphorescent material, unless the failure removes the hazard, and that where natural light is poor, adequate lighting is needed and photoluminescent signs may also play a part. In the United States, OSHA's exit-route rule requires each exit sign to be lit to at least 54 lux by a reliable source, or to be self-luminous or electroluminescent at a stated minimum luminance, and to be distinctive in colour — without naming the colour. Neither rule makes a glowing sign a substitute for lighting; both treat light as the primary means and luminous material as the fallback.

How a photoluminescent sign glows, and why green

A photoluminescent sign is printed on, or made of, a long-afterglow phosphor — today usually strontium aluminate doped with europium and dysprosium, whose stored energy is released as light for hours after charging; the mechanism is on the phosphorescence page. Its glow comes from europium ions and peaks in the green near 520 nm, which happens to sit close to the peak of rod vision, so the afterglow is seen well by dark-adapted eyes. Two consequences follow for sign colour. First, the colour of a sign in the dark is the phosphor's emission, not its printed safety colour: ISO 3864-4 has a separate, normative annex classifying the emission colour of phosphorescent material, apart from the daylight colour requirements. Second, a sign glows only if it has been charged: the IMO's guidelines call for enough ambient light to charge the material, and ISO 16069 has its own illumination requirements for phosphorescent components.

Below the smoke: low-location marking

Smoke collects at ceiling height and swallows overhead emergency lights first. The International Maritime Organization responded for passenger ships: since the early 1990s, escape routes on ships carrying more than 36 passengers have been marked by lighting or photoluminescent strips no more than 0.3 m above the deck, leading to the exit door handle, with escape signs in the lowest 300 mm of the bulkhead and contrasting in colour with it. Its 1993 guidelines set photoluminescent strips at least 75 mm wide that still give 15 mcd/m² ten minutes after the lights go out and stay above 2 mcd/m² for an hour. ISO 15370, now in its 2021 edition, carries these requirements for ships. On land, ISO 16069:2017 sets out the same idea for buildings as a safety way guidance system — continuous guidance lines, marked stairs, door frames and decision points — and says plainly that such a system does not replace emergency escape lighting but can do better than it where smoke is present.

What the standards leave out

The ISO documents here divide the work. ISO 3864-4 fixes the colours of sign materials, including phosphorescent ones, but excludes retroreflective materials and combinations of retroreflective with fluorescent or phosphorescent material. ISO 17398 classifies signs by service environment, material, photometric properties and means of illumination, with tests for how long they keep performing; it is what a specification means by a sign's photoluminescent class. ISO 16069 governs layout and luminance of the whole guidance system on site. None of them is a risk assessment, and the luminance classes in ISO 16069 and ISO 17398 are sold standards whose figures are not reproduced here. National standards and trade-association classifications build on them; check which one a building regulation or fire authority actually cites.

The data

What goes wrong, and what to do

How to check it yourself

1. Switch off all lighting, including emergency lighting if it can be safely isolated, and stay in the dark for 10–20 minutes before judging what remains visible.
2. Look for whether red signs and markings have become indistinguishable from black, and whether the escape direction can be read from shapes and arrows alone.
3. Note what charges each photoluminescent sign during normal use, and whether that light is on long enough before an evacuation could be needed.
4. Check the sign's documentation for the standard and class it claims (for example ISO 17398 or ISO 16069) rather than relying on how bright it looks when new.

What the evidence says

Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.

• FactStrong evidence

  The CIE photopic luminous efficiency function V(λ) peaks at 555 nm and the scotopic function V′(λ) at 507 nm; at 630 nm the scotopic value is about 0.003 of its peak against about 0.27 for the photopic.

  Source: CIE spectral luminous efficiency for photopic vision, V(λ) (CIE 018:2019 dataset); CIE spectral luminous efficiency for scotopic vision, V′(λ) (CIE 018:2019 dataset)

• Colourwise analysisStrong evidence

  Relative to each function's peak, light at 630 nm has about one eightieth of the weight in scotopic vision that it has in photopic vision, while light at 450 nm has about twelve times as much.

  Based on: Ratio V′(λ)/V(λ) computed from the CIE 018:2019 data tables at the wavelengths shown; both functions are normalised to 1 at their own peaks.

  Caveat: A ratio of relative weights, not absolute sensitivity: rod vision is far more sensitive overall. Real safety colours are broad spectra, and mesopic vision lies between the two curves.

  Source: CIE spectral luminous efficiency for photopic vision, V(λ) (CIE 018:2019 dataset); CIE spectral luminous efficiency for scotopic vision, V′(λ) (CIE 018:2019 dataset)

• StandardStrong evidence· Great Britain

  HSE guidance on the Health and Safety (Safety Signs and Signals) Regulations 1996 (L64, third edition, 2015) says safety signs should keep their intrinsic features under power failure, from emergency lighting or phosphorescent material, unless the hazard is removed by the failure, and that photoluminescent signs may also have a role where natural light is poor.

  Source: Safety signs and signals: The Health and Safety (Safety Signs and Signals) Regulations 1996 — Guidance on Regulations (L64, third edition, 2015); The Health and Safety (Safety Signs and Signals) Regulations 1996 (SI 1996/341)

• StandardStrong evidence· United States

  OSHA's 29 CFR 1910.37(b)(6) requires each exit sign to be illuminated to at least five foot-candles (54 lux) by a reliable light source and to be distinctive in colour, and permits self-luminous or electroluminescent signs of at least 0.21 cd/m².

  Caveat: Federal general-industry rule; building and fire codes adopted by states and cities add their own exit-sign requirements, which were not checked.

  Source: 29 CFR 1910.37 — Maintenance, safeguards, and operational features for exit routes

• StandardStrong evidence

  IMO Resolution A.752(18) (1993) sets photoluminescent low-location strips on passenger ships at least 75 mm wide, giving at least 15 mcd/m² ten minutes after external light is removed and more than 2 mcd/m² for 60 minutes, placed no more than 0.3 m above the deck.

  Caveat: The 1993 guidelines; current ship requirements are in SOLAS chapter II-2, the Fire Safety Systems Code and ISO 15370:2021, which were not read and may differ.

  Source: Resolution A.752(18): Guidelines for the evaluation, testing and application of low-location lighting on passenger ships (adopted 4 November 1993)

• FactStrong evidence

  ISO 16069:2017, the second edition, covers the design of safety way guidance systems with electrically powered or phosphorescent components, excludes ships under IMO rules, and states that such a system is not intended to replace emergency escape lighting.

  Source: BS ISO 16069:2017 — Graphical symbols. Safety signs. Safety way guidance systems (SWGS): catalogue record and public preview

• FactStrong evidence

  ISO 3864-4:2011 relates its colour requirements primarily to daytime and normally lit environments, also covers phosphorescent material that operates unlit, with a normative annex classifying its emission colour, and does not specify retroreflective signs or retroreflective material combined with fluorescent or phosphorescent material.

  Source: BS ISO 3864-4:2011 — Graphical symbols. Safety colours and safety signs. Colorimetric and photometric properties of safety sign materials: catalogue record and public preview

• FactModerate evidence

  The long-lasting green emission of strontium aluminate phosphors used for glow-in-the-dark safety markings comes from europium in the Eu²⁺ state, with emission near 520 nm.

  Source: Combustion Synthesis of SrAl₂O₄:Eu²⁺,Dy³⁺ Phosphorescent Pigments for Glow-in-the-Dark Safety Markings (Nanomaterials, 2023)

• Colourwise interpretationModerate evidence

  In darkness a photoluminescent sign communicates by the shape and layout of its glowing areas, not by its printed safety colours; a red element that does not itself glow is read as dark.

  Based on: Follows from the scotopic sensitivity data, which give deep red almost no weight, and from ISO 3864-4 classifying a phosphorescent material's emission colour separately from its daylight colour. No source was found that states it for red sign elements directly.

  Source: CIE spectral luminous efficiency for scotopic vision, V′(λ) (CIE 018:2019 dataset); BS ISO 3864-4:2011 — Graphical symbols. Safety colours and safety signs. Colorimetric and photometric properties of safety sign materials: catalogue record and public preview

Related questions

Why do glow-in-the-dark exit signs glow green and not the colour they are printed in?

The glow is light emitted by the phosphor, which for the strontium aluminate used in most signs is green near 520 nm. The printed colours are only seen when light falls on the sign.

Can a photoluminescent sign replace emergency lighting?

Not in the sense the standards use. ISO 16069 says a guidance system is not intended to replace emergency escape lighting, and HSE treats photoluminescent signs as an addition where light is poor.

Why are emergency escape markings on ships placed so close to the floor?

Smoke fills a space from the ceiling down and hides overhead lights first. IMO guidelines therefore put lit or photoluminescent marking within 0.3 m of the deck along the whole escape route.

Related topics

Go further

Sources

• CIE spectral luminous efficiency for photopic vision, V(λ) (CIE 018:2019 dataset) — CIE (Official standard or government)
• CIE spectral luminous efficiency for scotopic vision, V′(λ) (CIE 018:2019 dataset) — CIE (Official standard or government)
• Safety signs and signals: The Health and Safety (Safety Signs and Signals) Regulations 1996 — Guidance on Regulations (L64, third edition, 2015) — Health and Safety Executive (Official standard or government)
• The Health and Safety (Safety Signs and Signals) Regulations 1996 (SI 1996/341) — legislation.gov.uk (The National Archives) (Official standard or government)
• 29 CFR 1910.37 — Maintenance, safeguards, and operational features for exit routes — US Occupational Safety and Health Administration (Official standard or government)
• Resolution A.752(18): Guidelines for the evaluation, testing and application of low-location lighting on passenger ships (adopted 4 November 1993) — International Maritime Organization (Official standard or government)
• BS ISO 16069:2017 — Graphical symbols. Safety signs. Safety way guidance systems (SWGS): catalogue record and public preview — BSI (British Standards Institution), BSI Knowledge (Standards body)
• BS ISO 3864-4:2011 — Graphical symbols. Safety colours and safety signs. Colorimetric and photometric properties of safety sign materials: catalogue record and public preview — BSI (British Standards Institution), BSI Knowledge (Standards body)
• Combustion Synthesis of SrAl₂O₄:Eu²⁺,Dy³⁺ Phosphorescent Pigments for Glow-in-the-Dark Safety Markings (Nanomaterials, 2023) — Lazău R. et al., Nanomaterials 13, 687 (Primary research)
• BS ISO 15370:2021 — Ships and marine technology. Low-location lighting (LLL) on passenger ships. Arrangement: catalogue record — BSI (British Standards Institution), BSI Knowledge (Standards body)
• BS ISO 17398:2004 — Safety colours and safety signs. Classification, performance and durability of safety signs: catalogue record and public preview — BSI (British Standards Institution), BSI Knowledge (Standards body)

Reviewed 1 October 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.