Tunable White and Human Centric Lighting: CCT Control Over DMX
5000 K in the morning, 4000 K in the afternoon, 2700 K in the evening. That sentence has worked its way into office, hospital, school and hotel specifications over the past few years. The idea behind it is simple: the colour of light affects alertness and sleep. Executing it is less simple — and this is where marketing language and engineering part ways.
In short
- Changing colour temperature affects alertness and sleep through the ipRGCs.
- A serious specification asks for melanopic EDI (CIE S 026), not kelvin.
- Tunable white mixes two LED groups — do not confuse it with dim to warm.
- On DMX, processed mode (dimmer + CCT) suits architectural work; 16-bit dimming is close to mandatory.
- DALI DT8 on the building side, DMX on the show side; gateways let both coexist.
- Without binning (3-step MacAdam) in the specification, the whites will not match.
Why we change colour temperature at all
The eye contains a group of cells that have nothing to do with seeing: light-sensitive ganglion cells called ipRGCs. They are particularly sensitive to the blue region and carry a “day or night” signal to the brain. High colour temperature and high illuminance in the morning support alertness; low colour temperature in the evening suppresses melatonin less.
Lux is not the metric — melanopic EDI is
“Human centric lighting” has become a marketing label. A serious specification does not ask for lux and kelvin; it asks for melanopic EDI (m-EDI), a quantity defined in CIE S 026 that measures the effect of light on the ipRGCs. Visible illuminance can stay identical while the melanopic effect doubles. Being able to make that distinction when you quote is what separates a serious supplier from the rest.
How tunable white actually works
At heart it is a mix of two LED groups: a warm white (say 2700 K) and a cool white (say 6500 K). Change the ratio and you get every tone in between. But three different things are sold under similar names:
| Type | How it works | Where it belongs |
|---|---|---|
| Tunable white | Two channels: warm and cool separately | Office, school, hospital, hotel — our subject here |
| Dim to warm | One channel; warms as it dims | Restaurant, lobby, decorative — the incandescent feel |
| Multi-channel / full spectrum | RGBW, RGBWW, sometimes a separate amber | Better colour and CCT accuracy; stage and museum |
The two-channel mix has a weakness worth knowing: at intermediate tones the light can drift off the Planckian locus (Duv). The display says 4000 K but the wall looks slightly green or slightly pink. Good luminaires correct for this internally; cheap ones do not.
Channel layout on the DMX side
The same luminaire can be patched two different ways, and that choice shapes the whole project.
| Mode | Channels | Advantage | Drawback |
|---|---|---|---|
| Raw | 1: Warm white · 2: Cool white | Full control, flexible | The console has to do the kelvin maths; brightness drifts |
| Processed | 1: Dimmer · 2: CCT | Kelvin directly, brightness holds | You depend on the fixture’s algorithm |
| 16 bit | Dimmer coarse + fine | No stepping at low level | Twice the channels |
Architectural work almost always favours processed mode: the operator types 3000 K, the fixture works out the mix and the brightness does not move. Raw mode earns its place on stage, where effects matter.
16 bit is not a luxury here
Tunable white usually runs on slow transitions — going from 4000 K to 2700 K over an hour, for instance. With 8-bit dimming the steps become visible at low levels and the transition “jumps” along. On this kind of project 16-bit dimming is close to mandatory.
DMX or DALI for this job?
The honest answer: DALI on the building side, DMX on the show side. DALI’s DT8 device type supports colour temperature directly; one address carries both level and Tc, and the luminaire reports back. D4i adds energy and fault data from inside the fixture.
| DMX512 | DALI (DT8) | |
|---|---|---|
| Speed | 40+ refreshes per second | Slow, scene based |
| Feedback | None (partial with RDM) | Yes — faults, hours, energy |
| Addressing | Channel based | Device based, 64 addresses per line |
| Sending CCT | As a channel (manufacturer defined) | Standard command (Tc) |
| Where it fits | Stage, facade, effects | Office, hospital, school, hotel |
Both can live in one building: DALI in corridors and offices, DMX in the atrium and on the facade, with DALI-to-DMX gateways between them.
Commissioning and scheduling
- Astronomical clock. A schedule tied to sunrise and sunset follows the seasons on its own, unlike fixed times.
- Keep transitions slow. Fifteen to thirty minute fades go unnoticed; a two-minute fade is distracting.
- Define user priority. When someone picks a scene in a meeting room, the automatic programme must not cut in; it should return after a timeout.
- Keep the scene count low. Four well-tuned scenes beat twelve half-finished ones.
- Hand over with measurements. At the end of commissioning, measure illuminance and colour temperature at a few points and give it in writing.
Common mistakes
- Not specifying binning. LEDs from different batches give different whites at the same kelvin. Ask for 3-step MacAdam (SDCM) or tighter.
- Colour shift on dimming. Cheap drivers do not hold colour at 10%; test this before you buy.
- CRI falling at low kelvin. Colour rendering at 2700 K may not match 4000 K; ask for figures at both ends.
- Mixing manufacturers. Two brands’ “3000 K” will not look the same in one room.
- Talking only about kelvin. Light level and the direction light arrives from can matter more than colour temperature.
Related articles
- What Is DALI? IEC 62386 and Digital Lighting Control Basics
- DALI or DMX512? Use Cases and How to Choose
- DMX512 in Architectural Lighting: Facades and Outdoor Installations
Project and product support
Stuck somewhere in your installation, or unsure which product fits? Drop us a line for project consultancy and product support.
