Human centric lighting (HCL) is often sold as a wellness feature and specified as a colour temperature range. Neither framing survives contact with a project specification, because the quantities that determine whether an installation delivers a circadian effect are melanopic equivalent daylight illuminance (melanopic EDI), the timing of the schedule, and the accuracy with which the control system holds the CCT setpoint. This white paper sets out how to specify tunable white office lighting in measurable terms, how DALI-2 and Matter control architectures differ, and how QUEENDOM’s Z-17 Smart Panel Ceiling, Z-18 Smart Tunable White Panel, Z-19 Standard Recessed Panel and Z-20 Ultra-Slim Panel map onto an office retrofit.
1. From colour temperature to circadian stimulus
Correlated colour temperature describes the chromaticity of white light. It does not describe the spectral power distribution, and two sources at 4000 K can differ substantially in their short-wavelength content and therefore in their biological effect.
The recommended metric for that effect is melanopic equivalent daylight illuminance (melanopic EDI), defined in CIE S 026. It converts the spectral irradiance at the eye into the equivalent illuminance of CIE standard daylight D65 that would produce the same melanopic response. Melanopic EDI is measured in lux on the vertical plane at the eye, not on the horizontal working plane, and this distinction is the most common specification error in HCL projects.
Three quantities must therefore be controlled, not one:
| Cantidad | Unit | Where measured | Why it matters |
|---|---|---|---|
| Illuminance | lx, horizontal | Working plane | Visual task performance, EN 12464-1 |
| CCT | K | At the luminaire, or at the eye | Colour appearance, comfort |
| Melanopic EDI | lx, melanopic | Vertical plane at the eye | Circadian entrainment, alertness |
The relationship between the three is spectral, not arithmetic. Raising CCT at constant photopic illuminance raises melanopic EDI, but the ratio depends on the LED bin and the phosphor system. A luminaire specification that quotes only a CCT range leaves the melanopic EDI undefined and therefore leaves the HCL claim unverifiable.
2. Circadian schedules and dose targets
HCL schedules are built from three daily targets: a high melanopic dose in the morning to advance the circadian phase, a moderate level through the working day to sustain alertness, and a low melanopic dose in the late afternoon to avoid delaying sleep onset.
| Time of day | Photopic illuminance (lx) | CCT (K) | Melanopic EDI (lx) | Design intent |
|---|---|---|---|---|
| 07:00–09:00 | 500 | 5000–6500 | 400–500 | Phase advance, high alerting stimulus |
| 09:00–12:00 | 500 | 4500–5000 | 300–400 | Sustained alertness |
| 12:00–14:00 | 500 | 4000 | 250–320 | Neutral, lunch trough mitigation |
| 14:00–16:00 | 500 | 3500–4000 | 200–280 | Afternoon maintenance |
| 16:00–18:00 | 400 | 3000 | 130–200 | Wind-down, reduce phase delay risk |
| 18:00–20:00 | 300 | 2700 | 80–130 | Evening, protect melatonin onset |
| Night, if used | 100 | 2200–2700 | 20–50 | Minimal circadian disruption |
Two constraints limit how far a schedule can be pushed. First, minimum illuminance must still satisfy EN 12464-1 for the task at all times, so dimming toward evening is bounded below. Second, the CCT range is bounded by the luminaire’s physical capability and by colour consistency across the range: a tunable fixture that spans 2700 to 6500 K must hold SDCM ≤ 3 at every setpoint, otherwise side-by-side fixtures visibly disagree.
3. Melanopic EDI and CCT: the conversion problem
Melanopic EDI cannot be calculated from CCT and illuminance alone, but for a given luminaire the relationship is stable and can be characterised once. The ratio of melanopic EDI to photopic illuminance is the melanopic daylight efficacy ratio (melanopic DER), and it rises monotonically with CCT.
| CCT (K) | Melanopic DER (typical, 80 CRI SPD) | Photopic E (lx) for EDI 250 lx | Photopic E (lx) for EDI 400 lx |
|---|---|---|---|
| 2700 | 0.42 | 595 | 952 |
| 3000 | 0.48 | 521 | 833 |
| 3500 | 0.57 | 439 | 702 |
| 4000 | 0.65 | 385 | 615 |
| 5000 | 0.82 | 305 | 488 |
| 5700 | 0.92 | 272 | 435 |
| 6500 | 1.02 | 245 | 392 |
The practical implication is significant. To reach a morning melanopic EDI of 400 lx at 4000 K requires 615 lx of photopic illuminance, which exceeds the 500 lx office requirement and therefore forces the fixtures to run above the base design level. At 5000 K the same melanopic dose needs only 488 lx, which is achievable within the standard 500 lx design. Higher CCT is therefore the cheaper route to a melanopic target, at the cost of a colour appearance that some occupancies find cold.
| CCT (K) | Melanopic DER | Photopic E for EDI 150 lx | Photopic E for EDI 250 lx |
|---|---|---|---|
| 2700 | 0.42 | 357 | 595 |
| 3000 | 0.48 | 313 | 521 |
| 3500 | 0.57 | 263 | 439 |
| 4000 | 0.65 | 231 | 385 |
| 5000 | 0.82 | 183 | 305 |
| 5700 | 0.92 | 163 | 272 |
| 6500 | 1.02 | 147 | 245 |
3.1 Melanopic EDI versus CCT at fixed photopic illuminance
The curve below shows melanopic EDI as a function of CCT at three fixed photopic illuminance levels, using the melanopic DER values above.
4. Dimming, control architecture and curve fidelity
A tunable white installation has two control variables — intensity and colour — and both must be transmitted with enough resolution to avoid visible stepping.
Dimming curve shape. Logarithmic dimming matches human brightness perception: equal percentage changes in perceived brightness require geometrically increasing luminous flux. Linear dimming produces a system that feels unresponsive at the top of its range and abrupt near the bottom. DALI-2 drivers are configured with a dim curve selection, and the choice must be declared because the same 0–100 percent command produces very different absolute flux on the two curves.
Dimming depth and minimum level. The minimum dim level sets how far the installation can descend for evening and out-of-hours scenes. A minimum of 1 percent is the practical floor for office work; lower values are achieved at the cost of colour stability.
Colour resolution. DALI-2 colour control uses device type 8 commands with colour temperature in mireds (DT8 Tc) or with RGBWAF channels. Mired-based control is the better choice for tunable white because the steps are perceptually even in colour appearance and because the driver handles the mixing internally.
5. Control architecture: DALI-2 and Matter
DALI-2 and Matter solve different problems and are complementary rather than competing.
DALI-2 (IEC 62386) is a wired digital bus for lighting. It provides guaranteed dimming resolution, addressing per luminaire or per group, sensor and push-button input devices, and standardised colour control including device type 8 for tunable white. It is deterministic, needs no network, and is the appropriate backbone for a building’s lighting infrastructure.
Matter is an application-layer interoperability standard for smart buildings. It allows a lighting system to be controlled by a building management platform or a consumer-grade controller over IP, with defined device types for colour temperature and dimming. It is the appropriate layer for occupant control, scene recall and integration with wider building services.
| Aspect | DALI-2 | Matter |
|---|---|---|
| Physical layer | Two-wire bus, 16 V, polarity free | IP over Ethernet or Wi-Fi, Thread border router option |
| Addressing | 64 short addresses per bus, 16 groups, 16 scenes | Per-device node identity |
| Colour control | Device type 8, mireds or RGBWAF | Colour temperature cluster in kelvin |
| Dimming resolution | 254 steps logarithmic, standardised | Platform-dependent, typically 1 percent steps |
| Sensors | Standardised occupancy and light sensor devices | Bridged through a Matter bridge |
| Informes de fiabilidad. | Deterministic, no network dependency | Depends on IP network availability |
| Commissioning | PC tool and DALI-2 commissioning process | Matter commissioning with QR code |
| Typical role | Lighting backbone | Occupant interface and BMS integration |
| Certification status | DALI-2-ready driver and control gear | Matter-ready with bridge firmware |
The recommended architecture for an HCL office is a DALI-2 bus carrying the luminaires and sensors, with a Matter-ready gateway bridging the bus to the building network. This keeps the light output deterministic and independent of network health, while exposing the schedule and scene control to the occupant app and the BMS.
6. Common mistakes and how to avoid them
| Mistake | Consequence | Correction |
|---|---|---|
| Specifying CCT range only | Melanopic claim unverifiable, HCL benefit not delivered | Specify melanopic EDI at the eye per CIE S 026 |
| Measuring melanopic EDI on the working plane | Overstated dose by 30–60 % | Measure on the vertical plane at eye height |
| Linear dimming configured on a DALI-2 driver | Poor perceptual response, complaints | Select the logarithmic dim curve |
| Mixed tunable and fixed fixtures on one scene | Visible colour mismatch | Group by capability, or use Z-18 throughout |
| CCT out of SDCM range at intermediate setpoints | Side-by-side fixtures disagree | Verify SDCM ≤ 3 across the full CCT range |
| Network-dependent lighting control with no fallback | Lights fail when the network fails | Keep DALI-2 as the deterministic backbone |
| Schedule not adjusted for season or latitude | Circadian benefit lost in winter | Use a latitude-aware schedule with daylight sensing |
7. Verification and test methods
- Photometric and colour verification per IES LM-79 — measure flux, efficacy, CCT and CRI at the top, middle and bottom of the tunable range at thermal equilibrium.
- Colour fidelity — report IES TM-30 Rf and Rg at each setpoint, and confirm SDCM ≤ 3 across the range rather than only at the endpoints.
- Melanopic EDI measurement — measure spectral irradiance on the vertical plane at 1.2 m seated eye height, at three positions per workstation, and compute melanopic EDI per CIE S 026.
- Control resolution test — sweep the DALI-2 command level from minimum to maximum in the configured curve and record measured luminous flux at 10 points, verifying monotonic response and absence of visible steps.
- Schedule verification — log the actual CCT and dim level delivered over a full working week and compare against the design schedule, including response to occupancy and daylight override.
- Interoperability check — commission the Matter bridge and verify that colour temperature and dimming commands received from the gateway reproduce the same output as the equivalent DALI-2 commands.
8. Conclusion and selection guidance
For open-plan offices where the full circadian schedule is required, Z-18 Smart Tunable White Panel is the anchor product: it spans 2700 to 6500 K with DALI-2 device type 8 colour control, Matter-ready gateway support and SDCM ≤ 3 across the range. For cellular offices and back-of-house areas that only need a fixed setpoint, Z-19 Standard Recessed Panel at 30 mm depth provides a conventional recessed installation at lower cost, and Z-20 Ultra-Slim Panel at 10 mm depth covers refurbishments where void depth is restricted and the fixture must sit almost flush in a shallow ceiling. For residential-grade smart ceilings and small suites, Z-17 Smart Panel Ceiling offers the same control ecosystem in a consumer-friendly format. Build the specification on melanopic EDI rather than CCT alone, keep the DALI-2 bus as the deterministic backbone, and verify colour consistency at intermediate setpoints where the two-channel mixing is least constrained.
9. Referenced standards
- IEC 62386 — Digital addressable lighting interface (DALI), including Part 209 for colour control
- Matter 1.3 — Application layer specification for smart home and smart building interoperability
- CIE S 026 — System for metrology of optical radiation for ipRGC-influenced responses to light
- EN 12464-1 — Light and lighting of work places, Part 1: Indoor work places
- EN 17037 — Daylight in buildings
- IES LM-79 — Approved Method: Electrical and Photometric Measurements of Solid-State Lighting Products
- IES TM-30 — Method for evaluating light source colour rendition
- EU Ecodesign Regulation 2019/2020 — Ecodesign requirements for light sources and separate control gear
10. Contact us
QUEENDOM supplies the Z-17 Smart Panel Ceiling, Z-18 Smart Tunable White Panel, Z-19 Standard Recessed Panel and Z-20 Ultra-Slim Panel from stock, with DALI-2-ready drivers, Matter-ready gateway support and photometric files covering the full tunable range. Melanopic EDI calculations, TM-30 colour fidelity reports and HCL schedule proposals are available on request. Contact our architectural lighting group for sample quantities and a workstation-level schedule study.
Related products and applications
The tunable-white panels and control gear referenced in this paper are listed below.
- Smart panel light (Z-17)
- Tunable white panel (Z-18)
- DALI-2 panel (Z-19)
- Human-centric panel (Z-20)
- Application overview: Lighting application solutions
- More technical papers: Lighting knowledge resources