Dim-to-warm, tunable white and RGBW: how they work and when to specify them

At a glance

From US DOE and DiiA documents read on 6 November 2026. Ranges are the DOE's. The dim-to-warm end points are those of the products the DOE tested, not a promise for any fitting.

Four ways to light a warm room

Four options cover nearly every request for warm or changing light. They differ in what is inside the fitting, how many controls it needs and what colours it can make. The drawing shows the difference in one picture, and the table at the top gives the numbers we found.

• Fixed warm white has one white LED colour. The US DOE/RTI dim-to-warm report says that when ordinary LEDs are dimmed minimal change occurs in what they emit, so the colour temperature stays relatively stable. A fixed 2700 K fitting stays near 2700 K at every level.
• Dim-to-warm, also called warm dim, imitates incandescent or halogen dimming: it is designed for 2700 to 3000 K at full output and falls as the output is reduced, down to as low as 1800 K, the colour of candlelight (US DOE).
• Tunable white, also called Kelvin changing, lets the user or a scene choose a white from warm to cool, usually independently of brightness (US DOE).
• RGBW and full colour products have three or more LED colours, individually varied, to make white, a tint of white or a saturated colour (US DOE).

The UK Regulations draw the same lines. SI 2021/1095 defines a colour-tuneable light source as one that can emit a large variety of colours outside the white range as well as white, and says the term does not include tuneable-white sources that stay within the white range or dim-to-warm sources that shift their white light to a lower colour temperature as they dim.

Dim-to-warm: how it works and what it costs in complexity

Dim-to-warm is the closest an LED comes to a filament lamp, and the DOE says its light colour and dimming quality are prized in restaurants, hotel lobbies and guestrooms, ballrooms, theatres and homes. Because LEDs do not change colour when dimmed, it needs extra circuitry, and that is where the complexity sits.

• How it works. The DOE/RTI report describes products with two or more LED colours and a driver with preset current levels and logic circuits. At low power and deep dimming only the lower colour temperature LEDs run. At moderate dimming the lower and at least one higher colour temperature LED emit light. The products tested shifted from about 2700 K to between 1800 and 2200 K, depending on the model.
• One control. Colour is tied to the dimming, so there is one control signal and one controller for each group of fittings that dim together. The DOE says some systems can do this on a phase-cut dimmer, but with less dimming resolution or smoothness than 0-10V, DALI or DMX, which need separate wiring for the signal. Wireless receivers are another route. See dimming protocols.
• The lowest setting. Some products use white LEDs plus narrow-band amber ones, and at the very low end only the amber runs. The DOE says that can make skin tones and finishes look eerie, and suggests checking the colour rendering index near the lowest setting, with at least 80 as a rule of thumb, and judging it on a mock-up. See CRI, R9 and TM-30.
• What can fail. The DOE/RTI stress tests found failure modes particular to dim-to-warm products: chromaticity shifts of 0.007 or more in u′v′ from improper current control and from ageing red phosphors, and shifts above 0.018 in four module units whose logic circuits failed. The two candelabra-style lamps tested did worst under the harshest conditions, which the report puts down to less space in the lamp. The sample was small (seven products, 63 units in all), so treat this as a reason to ask for data, not a verdict.
• It also touches flicker. The same report found the flicker and stroboscopic readings of its lamps differed at each power level, on one dimmer. See flicker and low-end dimming.
• When it is the wrong tool. Sometimes light should dim without changing colour, as in a meeting room. The DOE says tunable white is the better choice then.

For the curve itself, the end points and how to write them, see colour temperature by space, which this sheet does not repeat.

Tunable white, RGBW and full colour

Tunable white

The DOE describes two common builds. The first has two sets of phosphor-coated LEDs, a warm white around 2700 K and a cool white of 5000 to 6500 K. Raising one and lowering the other makes whites on the straight line between them (linear white tuning). The black-body line is curved, so two whites cannot follow it, and the wider the range the further the mixed light strays from it (Duv): the white can look pinkish or purplish mid-range. Products with three or more LED colours can track the curve (non-linear white tuning) and keep Duv small.

• Two controls. One for intensity and one for colour. The DOE says the best products have algorithms that tune from one colour temperature to another without doubling the light level in the middle of the range, built into the driver or an interface. Wiring is more complex than for a fixed-colour fitting, and efficacy, though higher than full-colour systems, is lower than fixed white.
• Protocols. The DOE lists 0-10V, DMX, DALI, proprietary protocols and wireless. On DALI, Part 209 of IEC 62386 (also known as device type 8) defines colour control; the DALI Alliance describes the Tc colour type as controlling correlated colour temperature along the black-body line, from warm to cool, and says DALI-2 certification for it has been possible since February 2020. BSI Knowledge lists BS EN 62386-209:2011 as current.
• What it is used for. The DOE lists making a room feel cooler or warmer, matching finishes that change seasonally or after a remodel, suiting a new owner, simulating daylight or candlelight and matching light to artwork. It also lists support for the circadian system and says the exact mechanisms and effects are not yet fully understood. No health claim is made here.

RGBW and full colour

Full-colour products can move off the black-body line altogether, which the DOE says suits theatres, theme parks and restaurants. The DiiA guide to DALI describes RGBWAF, up to six channels (red, green, blue, white, amber and free colour), as cost-effective but not giving precise or consistent colour across different luminaires, so suited to decorative or ambient lighting in homes, restaurants or entertainment venues; the xy colour type gives repeatable colour for work where accuracy matters.

• Controls. A high-resolution protocol such as DMX, DALI or wireless, with power wired separately from the signals. Programming and reprogramming may not be intuitive, and the DOE suggests a consultant or a trained facility manager.
• The white is made, not given. The DOE says matching the chromaticity and rendering of another source completely may be difficult, because many mixes give the same colour temperature and none gives exactly the same rendering. Ask for the colour rendering of the whites you will actually use, not of the maximum output.
• Skewed colours. Mixed spectra can oversaturate or undersaturate the room. The DOE says excessive saturation can look cartoonlike and undersaturation drab. Efficacy is lower than fixed white because green LEDs are less efficient than white ones, which the DOE calls the green gap.

What the UK rules say about them

The Regulations and Approved Document L each treat these products slightly differently. We read the text, and what follows is what it says, not a ruling on any fitting.

• SI 2021/1095 (Great Britain). Tunable-white and dim-to-warm sources are defined in Schedule 1 and are not colour-tuneable light sources. A source that can be set to different characteristics is assessed at its reference control settings, which are the factory defaults a user meets at first installation (Schedule 3 paragraph 11). A genuine colour-tuneable source gets a 0.10 addition to the correction factor in the energy formula (Schedule 3 Table 3), and Schedule 4 paragraph 3(2)(n) exempts colour-tuneable sources that can reach at least blue, green and red with minimum excitation purities of 90, 65 and 95 per cent, measured at dominant wavelengths of 440 to 490, 520 to 570 and 610 to 670 nm, and are intended for high-quality coloured light. The exemption is for sources specifically designed and marketed for that use.
• Approved Document L Volume 1, 2026 edition (England, dwellings). Paragraph 5.57 asks for light sources of at least 105 light source lumens per circuit-watt, or the value in Table 5.7 for non-standard specialist light sources. Table 5.7 lists 65 for a high excitation purity light source and 80 for tuneable LED devices. The text does not define tuneable LED devices.
• Approved Document L Volume 2, 2026 edition. Paragraph 5.65 has a figure for high excitation purity light sources (above 65) and none for tuneable ones.

What this means for a specifier: ask the supplier which category each light source is declared as, what its reference control settings are, and whether the efficacy and colour rendering stated are those settings or the settings the room will use. See UK ecodesign rules for lighting products and Part L and decorative lighting.

Choosing between them

Choose from what the room has to do, not from the technology. Our arrangement, drawn from the DOE and DiiA remarks above, is in the table. It is a prompt for the design team and not a recommendation for any room.

Whichever you choose, it adds to the rest of the specification. Each extra LED colour is a channel for the driver, so check the outputs in LED drivers for decorative luminaires; ask for colour rendering at every colour point (CRI, R9 and TM-30), for flicker at low levels (flicker and low-end dimming), and for the scenes and settings in the handover pack (O&M information for decorative lighting). The drawing shows the colour temperature against brightness for each option.

Fittings in [space] to be dim-to-warm. Colour temperature [2700] K at 100 per cent output, falling to [1900] K at the lowest level (not warmer than [1800] K, not cooler than [2200] K), measured on a sample at [100, 50, 25, 10 and the lowest] per cent and recorded; each point within [ ] K of the approved curve. Ra not less than [90] and R9 not less than [50] at 100 and 50 per cent, and Ra not less than [80] at the lowest level, measured. One control signal for each dimming group: [DALI-2 / 0-10V / trailing-edge phase-cut].

Range [2200 or 2700] K to [4000 or 5000] K at any brightness from [100] to [1] per cent. Distance from the black-body line (Duv) within [ ] at [2700], [3000] and [4000] K. Ra and R9 as above at each of those points. Control by [DALI-2 device type 8, colour type Tc / two 0-10V signals / DMX512]; number of independently controlled outputs for each fitting [ ], stated on the shop drawing. Scenes set out in the schedule as [name, kelvin, per cent].

RGBW to be used only for [effect lighting in space], on its own circuit and control, apart from the white lighting that carries [ ]. White points used: [2700] and [3000] K, with Ra and R9 measured at those points. Colour repeatable from fitting to fitting within [ ] of the approved setting [state the colour type, for example xy, or none]. Programming and any reprogramming by [party], with the settings sheet in the handover pack.

One of each fitting family to be run on the final controls through every scene and the whole dimming and tuning range, with colour temperature, Duv, Ra, R9 and flicker recorded at the stated levels, and approved in writing by [lighting designer]. Any later change of driver, module or control product to be tested again.

Standards and sources

• SI 2021/1095, Lighting Products Regulations 2021 (Sch 1, Sch 3, Sch 4): revised text, read 6 Nov 2026.
• Approved Document L, Vol 1 (2026) para 5.57 and Table 5.7; Vol 2 (2026) para 5.65: England; takes effect 24 Mar 2027.
• US DOE, Understanding LED Color-Tunable Products: undated page, read 6 Nov 2026.
• US DOE and RTI, Dim-to-Warm LED Lighting: Stress Testing Results for Select Products (January 2020): small sample.
• DALI Alliance colour page; DiiA Specifying with DALI (SE001a); BS EN 62386-209:2011: colour types Tc, RGBWAF, xy; BSI lists the 2011 edition as current; IEC 62386-209 not read.

Questions we are asked

What is the difference between dim-to-warm and tunable white?

Dim-to-warm ties colour to brightness: one control, and the light warms as it dims. Tunable white has two controls, one for brightness and one for colour, so you can have a warm white at full brightness or a cool one dimmed. Dim-to-warm is simpler to wire and use; tunable white is more flexible and more complex.

How warm does dim-to-warm go?

The US DOE says products are usually designed for 2700 to 3000 K at full output and fall to as low as 1800 K, the colour of candlelight. In the DOE/RTI tests the products fell from about 2700 K to between 1800 and 2200 K, depending on the model. Ask the partner what end colour temperature its module reaches in the glass, and for the measured curve.

Do I need DALI for tunable white?

No. The DOE lists 0-10V, DMX, DALI, proprietary and wireless protocols as common, each with advantages and disadvantages. DALI has a colour control part, IEC 62386-209 (device type 8), with a Tc colour type for tunable white. Which protocol suits depends on the project: see our dimming protocols sheet.

Can a chandelier be dim-to-warm?

The DOE/RTI report tested candelabra-style dim-to-warm lamps, so lamps for candle bases exist, and its two candelabra lamps did worst under the harshest test conditions, which it ascribes to the limited space in the lamp. Whether a given partner fitting can take dim-to-warm lamps or a module, and what end colour it reaches, is for the partner to say.

Is RGBW suitable for general lighting?

It suits coloured effects. The DiiA guide calls RGBWAF suitable where absolute colour accuracy matters less, such as decorative or ambient lighting in restaurants. The DOE notes that the white is mixed, that matching another source is difficult, and that mixed spectra can oversaturate or undersaturate the room. Ask for the colour rendering of the whites you will use.

Does tunable white help sleep or wellbeing?

No health claim is made here. The DOE lists support for the circadian system among the uses of tunable white and says the exact mechanisms and effects are not yet fully understood. We keep to what a specifier controls: colour appearance, range, scenes and how the controls work.