Colour temperature by space

At a glance

Kelvin. There is no standard for these ranges: they are common practice, shown with where they come from. A lighting designer sets the figure for each room. SDCM is standard deviation of colour matching.

Read this first: a label, not a standard

Colour temperature is a label for how warm or cool a white light looks. The International Lighting Vocabulary defines correlated colour temperature (CCT) as the temperature of the black-body (Planckian) radiator whose colour is nearest to the light in question, in kelvin, and warns that the idea should not be used when the light’s colour is too far from the black-body line (CIE S 017:2020, e-ILV 17-23-068). Lower kelvin is warmer and more orange; higher is whiter and bluer.

No standard we could read says what colour temperature a restaurant, a bar or a guestroom should have. BS EN 12464-1 is paywalled and was not read. What exists is practice, and the table gives it: warm in the evening, cooler where the work is. It matches what we already publish in our hotel lighting design article (2200 to 2700 K where guests linger, 2700 K or below where people sleep) and on our hospitality lighting page. Treat every range here as a starting point for the lighting designer, not a requirement.

Two things the kelvin figure does not tell you. It does not tell you how faithfully colours are shown: that is colour rendering, which Approved Document M asks to be good for all surfaces (Volume 2, paragraph 4.34), and which is a separate number from CCT. And it does not guarantee that two fittings look the same: the same CCT can come from many different spectra, and the colour can look green or pink even when the CCT calculates correctly (US Department of Energy). Consistency is the second half of this sheet.

What 2200 K to 4000 K looks like

The drawing shows the colour of light at six common values and how a few materials would look under each. Read it as an illustration, not a measurement: it is computed from Planck’s law, your screen is not calibrated, and the eye adapts to the room it is in, so the shift looks less strong in person than a camera with a fixed white balance records. The shift is drawn at about half strength to allow for that.

What the swatches show, and what a lighting designer would check on real samples under the real lamp:

• 2200 K is amber. It is what a filament lamp does when dimmed low, and it suits a late bar or a spa. It deepens oak and brass and mutes blue-grey fabrics toward olive.
• 2700 and 3000 K are both “warm white”, and the difference between them is small on a screen and noticeable beside each other in a room. 3000 K is a little whiter, so plaster and pale fabrics look cleaner.
• 3500 to 4000 K is where the light stops reading as warm. Nickel and chrome look crisp here and brass looks paler; timber loses some of its glow. It is a back-of-house and task range more than a guest-area one.
• Metals follow the light. Warm light pulls nickel toward a warm tone and enriches brass; neutral light shows nickel as nickel. Choose the light and the finish together, and judge them on samples.

Warm-dim: what happens to colour when you dim

A filament lamp gets warmer as it is dimmed. A US Department of Energy poster for a typical incandescent lamp gives 2744 K at full power falling to 1600 K at 5 per cent. Ordinary LEDs do not do this: when an LED is dimmed, minimal change occurs to its colour, so the CCT stays near its setting (US DOE report on dim-to-warm LED products, RTI International, January 2020). So a room lit with ordinary LEDs keeps one colour from full brightness to the lowest level, where a filament lamp would have warmed.

Dim-to-warm (also called warm-dim) LED products add the missing behaviour. The DOE report describes them as a subclass of white-tunable products that mimic an incandescent lamp as they dim, with new electrical circuits to do it. The products it tested moved from about 2700 K at full output to between 1800 and 2200 K, depending on the model. A DOE page on colour-tunable products says dim-to-warm output is typically 2700 to 3000 K at full power, falling to about 1800 K at the low end.

• The curves differ. In the DOE tests only one of four lamps came close to the shape of the incandescent curve; others stayed near one CCT for much of the dimming range. Ask for the curve, not just the end points.
• Colour and brightness are one control. Dim-to-warm ties colour to the dimming signal, so each group of fittings that dims together needs one controller (DOE). That is a decision for the dimming protocols sheet.
• Check the lowest setting. DOE suggests checking colour rendering near the lowest setting, with at least 80 CRI as a rule of thumb, and judging in a mock-up, because deep amber can look unnatural.
• It is not tunable white. White-tunable products let a user or a scene choose any CCT across a range, usually from about 2700 K to 5000 or 6500 K, independent of brightness; dim-to-warm follows the dimming level.
• Durability. The DOE report found failure modes unique to dim-to-warm products, including chromaticity shift beyond 0.007 in u′v′ from improper current control and from ageing of red phosphors. It tested a small number of products, so treat it as a reason to ask for data, not a verdict.

Consistency across fittings

Consistency is where a 2700 K scheme goes wrong. A lobby can have a chandelier, pendants over a bar, wall lights and strip lighting in the joinery, each from a different maker with a different LED. All are sold as 2700 K. Seen together they may not match.

• MacAdam steps (SDCM). A MacAdam ellipse is an experimentally derived measure of how far apart two colours must be before a person can tell them apart; steps of it, also called standard deviation of colour matching (SDCM), are used to specify colour tolerance. One step is roughly 0.001 in u′v′ chromaticity. MacAdam’s just-noticeable difference for one observer was about 3 steps, but the PNNL review stresses that it depends on the viewer, the field of view, the surface, the proximity and the time.
• Bins are wide. The ANSI definitions of white light in the US allow tolerances that, according to the same review, can span about 14 steps from one edge of a bin to the other. That is well beyond the roughly 3 steps MacAdam found just noticeable for one observer, so two lamps from one bin can be visibly different side by side. Specify a tighter tolerance and ask the maker to declare it.
• Same CCT is not the same colour. The distance of a light’s colour from the black-body line, called Duv, can differ between products at the same CCT. DOE notes that matching another source’s colour is hard because many spectra give the same CCT, and recommends mock-ups and visual evaluation because numbers alone may not show the difference.
• Colour drifts with age. The DOE review found LED colour shift is a real issue for some products, using a shift of 0.007 in u′v′ as its failure criterion. Buy lamps and modules for a room from one batch, keep spares from the same bin, and record the bin on the schedule so a replacement does not stand out.
• Mock up the families together at the dimmed evening level, because that is when the differences show.

Putting it in the specification

Put the number and the way it is proved in the specification, and give the partners what they need to hold a bin. Ask each maker for:

• the nominal CCT and the declared tolerance, as SDCM steps or a named bin, for every fitting family in the room;
• the colour rendering (Ra and R9) and, if available, the distance from the black-body line (Duv);
• for dim-to-warm, the measured curve on a sample: CCT at several dimming levels, not only the end points;
• the bin and batch of the lamps or modules supplied, and the spares policy, so a replacement comes from the same bin.

The wording below fixes four things: the nominal CCT and tolerance, the dimming curve, the rule for mixing fittings in one space, and the approval mock-up.

Nominal correlated colour temperature [2700] K at full output for every light source visible in [space], within [3] SDCM (MacAdam steps) of nominal, and within Duv [ ] of the black-body line. Colour rendering not less than Ra [ ] and R9 [ ]. Maker to declare the colour bin and supply the test report for each fitting family.

Dim-to-warm [required / not required]. Colour temperature to follow this curve, measured on a sample before production: [2700] K at 100 per cent, [2400] K at 50, [2000] K at 10, [1800] K at the lowest level, each within [ ] K. The same curve for every fitting in [space], on the [DALI / phase-cut] control in the dimming schedule.

Pendants, wall lights, joinery lighting and any strip lighting visible together in [space] to come from [one bin / one batch] or to be matched on a mock-up at [100 / 50 / 10] per cent. Spares of [ ] per cent to be supplied from the same bin, labelled with the bin and batch, and recorded on the schedule. Replacement lamps and modules to be ordered by that record.

One of each fitting family to be lit together in a mock-up at the evening scene and at the lowest dimming level, with the real finishes and fabrics nearby. Colour temperature and chromaticity to be measured with a calibrated [colorimeter / spectrometer] and recorded. Approved in writing by [name] before production.

Standards and sources

• US ENERGY STAR, colour temperature chart (May 2018): descriptors for 2200 to 6500 K, read 5 Nov 2026. US consumer-level guidance.
• CIE S 017:2020 International Lighting Vocabulary (e-ILV): entry 17-23-068, correlated colour temperature, read 5 Nov 2026.
• US DOE: Dim-to-Warm LED Lighting stress testing (RTI, January 2020); Understanding LED Color-Tunable Products; PNNL (Royer), Chromaticity Adrift (2015): dim-to-warm, Duv, MacAdam steps, ANSI bins, colour shift, read 5 Nov 2026.
• BS EN 12464-1, ANSI C78.377 and IEC 62386-209: named, paywalled, not read: confirm against the current editions.

Questions we are asked

What colour temperature should a restaurant use?

There is no standard. Common practice, and our own published guidance, is warm in the evening: about 2200 to 2700 K where guests linger, with a little more white (up to about 3000 K) for daytime service. Dim-to-warm LEDs let the room move from one end of that range to the other as the evening goes on. The lighting designer sets the figure, ideally on a mock-up with the real finishes.

What is the difference between 2700K and 3000K?

Both are what the US ENERGY STAR programme calls soft or warm white, traditional incandescent light. 3000 K is a little whiter and less orange, so plaster and pale fabrics look cleaner; 2700 K is warmer and richer on timber and brass. The difference is small on a screen and noticeable side by side, which is the reason never to mix them in one space.

What is warm-dim or dim-to-warm?

LED lighting whose colour gets warmer as it is dimmed, as a filament lamp does. US DOE tests found products falling from about 2700 K at full output to between 1800 and 2200 K. The curves vary by product, colour and brightness share one control, and colour rendering should be checked at the lowest setting.

How do I keep colour consistent across different fittings?

Specify the tolerance as well as the kelvin: a number of MacAdam steps (SDCM) or a named bin, declared by each maker. Take lamps and modules for one space from one batch, keep spares from the same bin, and mock up all the families together at the dimmed evening level. A 7-step ANSI bin can span about 14 steps from edge to edge, so the kelvin figure alone does not guarantee a match.

What is SDCM?

Standard deviation of colour matching, counted in MacAdam steps. One step is roughly 0.001 in u′v′ chromaticity. For one observer a difference of about 3 steps was just noticeable, but visibility depends on the viewer, the surface and the setting. Fewer steps means a closer match between fittings.