A tunable white luminaire is judged by two spectra, not one. The first is the spectrum it produces at the correlated colour temperatures it advertises; the second is the spectrum it produces at the intermediate settings where it will actually spend most of its operating life. Simulating the first is straightforward and simulating the second is where most designs are won or lost, because the mixing behaviour of a two-channel or three-channel tunable luminaire is not linear in colour temperature and not intuitive. This white paper shows how to simulate a tunable white spectrum, how to convert the simulated spectrum into the melanopic equivalent daylight illuminance that human centric lighting schemes specify, and how to build a dimming and colour temperature map that holds the circadian target across the day.
The reader is a lighting designer, a building services engineer, or a product manager working on human centric lighting, healthcare or commercial office projects where a circadian or alertness outcome is part of the brief.
1. Why tunable white is a spectral problem, not a colour problem
A conventional white luminaire is specified by correlated colour temperature, colour rendering index and chromaticity tolerance, and those three numbers describe its spectrum adequately for most purposes. A tunable white luminaire is specified by the same numbers at the ends of its range, and this is insufficient, because the intermediate spectra depend on the mixing architecture.
The distinction matters because the melatonin suppression response of the human circadian system is driven by a specific photoreceptor — intrinsically photosensitive retinal ganglion cells containing melanopsin, with a peak sensitivity near 490 nm — and that response is not proportional to photopic illuminance, nor to colour temperature alone.
| Quantity | Symbol | Unit | What it measures |
|---|---|---|---|
| Correlated colour temperature | CCT | K | Chromaticity position |
| Colour rendering index | CRI Ra | — | Fidelity of 8 test colours |
| TM-30 fidelity | R_f | — | Fidelity over 99 test colours |
| TM-30 gamut | R_g | — | Saturation change |
| Melanopisches EDI | EDI | lx (melanopic) | Circadian-relevant light at the eye |
| Melanopischer DER | — | — | Melanopisches EDI divided by photopic illuminance |
| Circadian stimulus | CS | 0–1 | Modelled melatonin suppression |
The melanopic daylight efficacy ratio, M/DER, expresses how much circadian signal a spectrum delivers per unit of photopic illuminance. It is the key quantity that a tunable white luminaire changes when it changes colour temperature, and it is the quantity a human centric lighting schedule manipulates.
2. The human centric lighting target
Good practice in human centric lighting specifies a minimum melanopic EDI at the eye during the active part of the day and a maximum in the hours before sleep.
| Time band | Target melanopic EDI at eye | Photopic illuminance (typical) | Implied M/DER | Concrete guidance |
|---|---|---|---|---|
| Morning activation (07:00–09:00) | ≥ 250 lx EDI | 300–500 lx | ≥ 0.6 | High CCT, full output |
| Core working day (09:00–16:00) | ≥ 250 lx EDI | 500–750 lx | 0.4–0.6 | High CCT, full output |
| Late afternoon (16:00–18:00) | 100–200 lx EDI | 400–600 lx | 0.25–0.40 | Mid CCT, moderate dimming |
| Evening wind-down (after 19:00) | ≤ 50 lx EDI | 150–300 lx | ≤ 0.20 | Low CCT, dimmed |
| Night, sleep-critical areas | ≤ 10 lx EDI | 30–70 lx | ≤ 0.15 | Low CCT, low level, shielded |
The morning target is the demanding one, because it must be met while the space is still being lit to a normal office illuminance rather than to a special high level. Meeting a 250 lx melanopic EDI target at 500 lx photopic illuminance implies an M/DER of 0.5, which requires a cool spectrum; meeting it with a 2700 K spectrum would require roughly twice the photopic illuminance and would therefore breach the energy code.
3. Simulating the spectrum
A tunable white luminaire of the two-channel type mixes a warm white channel with a cool white channel, und die resulting spectrum is a linear combination in power, weighted by the relative drive of each channel. A three-channel or four-channel type mixes additional channels to control the position off the blackbody locus or to improve colour rendering at intermediate settings.
The three curves are not scaled copies of one another: as the setting moves from 2700 K to 6500 K, the blue content near 450 nm grows relative to the red content above 600 nm, und die peak wavelength of the combination shifts toward the short end. Because the melanopic response peaks at 490 nm and has appreciable sensitivity at 450 nm, the circadian-weighted power grows faster than the photopic-weighted power as the colour temperature rises. The M/DER therefore rises with CCT, which is the physical basis of the entire human centric lighting approach.
4. Computing melanopic EDI from the simulated spectrum
The calculation follows CIE S 026 and proceeds in five steps for each spectral setting.
| Step | Operation | Input | Output |
|---|---|---|---|
| 1 | Obtain the relative SPD | Simulated spectrum, 380–780 nm in 5 nm steps | Relative power per wavelength |
| 2 | Convert to absolute spectral irradiance at the eye | Scale so the photopic illuminance equals the design value | W/m²/nm at the eye |
| 3 | Weight with the melanopic action function | Multiply by s_mel(λ) | Melanopic weighted power |
| 4 | Apply the standard constant | Multiply by 1 / 1.3262 W/lm equivalent | Melanopisches EDI in lx |
| 5 | Divide by photopic illuminance | EDI divided by E_v | M/DER |
The table below illustrates the result for a representative tunable white panel driving at constant power across the range.
| Setting (K) | Photopische Beleuchtungsstärke (lx) | Relative flux | M/DER | Melanopisches EDI (lx) | CRI Ra | R_f | R_g |
|---|---|---|---|---|---|---|---|
| 2700 | 400 | 0.86 | 0.31 | 124 | 83 | 82 | 98 |
| 3000 | 422 | 0.91 | 0.37 | 156 | 84 | 83 | 99 |
| 3500 | 442 | 0.95 | 0.45 | 199 | 85 | 84 | 100 |
| 4000 | 465 | 1.00 | 0.52 | 242 | 85 | 85 | 101 |
| 5000 | 490 | 1.05 | 0.66 | 323 | 84 | 84 | 102 |
| 5700 | 502 | 1.08 | 0.74 | 371 | 83 | 83 | 103 |
| 6500 | 515 | 1.11 | 0.82 | 422 | 82 | 82 | 104 |
Read this table carefully, because two effects work together. At constant drive power the luminous flux itself rises with colour temperature, because the cool channel has higher luminous efficacy of radiation. Simultaneously the M/DER rises, because the spectrum shifts toward the melanopic peak. The product of the two means that melanopic EDI at 6500 K is roughly three and a half times the value at 2700 K, on the same panel und die same power. This is the mechanism that makes a single luminaire capable of both a circadian-activating and a sleep-supporting condition.
| Observation | Value at 2700 K | Value at 6500 K | Ratio |
|---|---|---|---|
| Photopic illuminance | 400 lx | 515 lx | 1.29 |
| M/DER | 0.31 | 0.82 | 2.65 |
| Melanopisches EDI | 124 lx | 422 lx | 3.40 |
The ratio of ratios is the design headroom available from spectrum alone. It is worth noting for specification purposes that the same headroom could be obtained by dimming, but dimming reduces photopic illuminance proportionally, whereas a spectrum change increases the circadian signal per unit of visible light.
5. The dimming and colour temperature map
A human centric lighting installation is operated as a two-dimensional schedule: a photopic illuminance for the task, and a colour temperature for the circadian target. The simulation output is therefore best presented as a set of achievable combinations.
The contour map is the working document for a commissioning engineer, because it converts the abstract melanopic EDI target into a pair of control settings. To deliver 250 lx EDI, the installation may be run at full output and 4000 K, or at seventy percent and 5000 K, or at fifty percent and 6500 K. The choice between these operating points depends on the other requirements: full output at 4000 K keeps the colour rendition stable und die electrical load high; running at 5000 K with reduced output reduces energy consumption while maintaining the circadian target, at some cost in visual warmth.
| Operating point | Dim level | CCT | Photopic illuminance | Melanopisches EDI | Abtausch |
|---|---|---|---|---|---|
| A | 100 % | 4000 K | 465 lx | 242 lx | Neutral colour, highest energy |
| B | 80 % | 5000 K | 392 lx | 259 lx | Meets target, moderate saving |
| C | 60 % | 6500 K | 309 lx | 253 lx | Meets target, largest saving, cooler colour |
| D | 100 % | 6500 K | 515 lx | 422 lx | Morning activation boost |
| E | 40 % | 3000 K | 169 lx | 62 lx | Evening wind-down |
| F | 20 % | 2700 K | 80 lx | 25 lx | Night condition |
Operating point C is the interesting one for energy-conscious projects: the same circadian outcome as point A, achieved with forty percent less electrical energy, at the cost of a cooler appearance. Whether that trade is acceptable is a client decision, und die simulation is what makes the decision explicit rather than accidental.
6. Colour quality across the tuning range
A tunable white luminaire that meets its colour specification at the two end points may fail it in the middle, and this is the most common technical defect in two-channel mixing architectures.
| Setting (K) | Ra | R_f | R_g | Distance from Planckian locus (SDCM) | Typical failure mode |
|---|---|---|---|---|---|
| 2700 | 83 | 82 | 98 | 2 | Slight red deficiency |
| 3000 | 84 | 83 | 99 | 2 | Within tolerance |
| 3500 | 85 | 84 | 100 | 2 | Within tolerance |
| 4000 | 85 | 85 | 101 | 2 | Within tolerance |
| 5000 | 84 | 84 | 102 | 3 | Green excess on the locus |
| 5700 | 83 | 83 | 103 | 3 | Green excess increasing |
| 6500 | 82 | 82 | 104 | 4 | Gamut widening, Ra falling |
The degradation pattern is characteristic of a simple two-channel mix in which the intermediate chromaticity is achieved by linear blending. Because the blended point follows a straight line in the chromaticity diagram rather than the curved Planckian locus, the intermediate settings sit progressively further from the locus as the two end points diverge, which manifests as a green or magenta tint. The visible consequence is a luminaire that looks correct at 2700 K and at 6500 K but slightly green at 5000 K.
Three architectural remedies exist, und dieir selection is a design decision rather than a purchasing one.
| Architecture | Channels | Intermediate colour quality | Cost | Beste Passform |
|---|---|---|---|---|
| Two-channel warm/cool mix | 2 | Degrades toward mid-range, SDCM 3–4 | Lowest | General commercial, non-critical |
| Three-channel with lime | 3 | Maintains SDCM ≤ 3, R_g controllable | Medium | Office, healthcare, retail |
| Four-channel RGBW | 4 | Fully controllable chromaticity and gamut | Highest | Colour-critical, studio, HCL premium |
| Two-channel with tuned blend curve | 2 | Improved by channel-specific drive scheduling | Low | Retrofit where colour point is near-locus |
The two-channel architecture with a tuned blend curve deserves a note because it improves the result at no hardware cost. The chromaticity of the intermediate settings depends not only on the nominal blend ratio but on the actual drive levels and junction temperatures of each channel at the moment of measurement; scheduling the two channels with a calibrated curve that accounts for the differential thermal droop can move the intermediate SDCM from 4 to 2 in a well-controlled luminaire.
7. Mapping to products
| Product | Steuerschnittstelle | Channel architecture | CCT range | CCT, tunable | Colour quality | Application fit |
|---|---|---|---|---|---|---|
| Z-18 Smart Tunable White Panel | DALI-2, Matter-ready | Multi-channel tunable white | 2700–6500 K | Yes | SDCM ≤ 3 across range | HCL office, healthcare, premium retrofit |
| Z-17 Smart-Panel-Decke | DALI-2 control | Tunable white | 3000–6000 K | Yes | SDCM ≤ 3 at end points | Commercial office, general retrofit |
| Z-13 Reinraum-LED-Panel | Fixed setting, dimmable | Single-CCT white | 4000 K nominal | No | SDCM ≤ 3 | Reference case where spectral stability is required |
The Z-18 panel is the platform for human centric lighting because it combines a wide tuning range with the control interface the schedule requires. DALI-2 gives individual addressing and scene recall, so a room can be driven at 6500 K and full output for the morning activation period und dien stepped down through the day without any change to the physical installation. Matter-ready connectivity extends the same luminaire to a building management or consumer ecosystem, which matters where the lighting schedule should follow an occupancy or a wellness application. The specification should describe these as DALI-2 and Matter-ready, consistent with the certification status at the time of tendering.
The Z-17 panel covers the range needed for a standard commercial office circadian scheme where the tuning range can be narrower because the day-end warm condition does not need to go as low. Where a project requires only daytime boost and a fixed warm evening setting, the narrower range is sufficient und die product is the more economical choice.
It is worth stating the negative case explicitly, because it is frequently proposed and it does not work. A fixed-CCT luminaire such as the Z-13 cleanroom panel cannot substitute for a tunable white product in a circadian scheme. A Z-13 panel at its nominal 4000 K setting delivers an M/DER of approximately 0.52, and its melanopic EDI therefore tracks photopic illuminance in a fixed ratio. Reducing the level to create an evening condition reduces the circadian signal only in proportion to the visible light, which means an evening EDI target in the region of 50 lx could only be reached by dropping the illuminance to about 100 lx — far below what the space requires for safe working. The spectral axis is what makes the evening condition achievable at a usable illuminance, and no amount of dimming a single-CCT luminaire replaces it. Where a project mixes tunable white panels with fixed panels in the same space, the fixed panels should be confined to areas that are unoccupied during the evening hours.
8. Common simulation and specification errors
| Error | Consequence | Correction |
|---|---|---|
| Specifying only the SPD at the range endpoints | Mid-range green tint discovered at commissioning | Simulate and specify the mid-range settings |
| Quoting melanopic EDI without the photopic illuminance | M/DER cannot be verified | Always report both values |
| Assuming M/DER is constant across the range | Circadian target missed at low CCT | Use the simulated M/DER per setting |
| Designing the activation period at low CCT | Target unreachable within the energy budget | Use the cool setting for the morning boost |
| Ignoring the view direction and eye position | EDI at the eye differs from EDI at the desk | Evaluate at the eye, vertically |
| Treating DALI-2 as sufficient for HCL alone | No schedule, no commissioning protocol | Define the schedule und die control scenes |
| Assuming flux parity across the range | LPD compliance fails at high CCT | Check power and flux per setting |
| Overlooking the maintenance factor interaction | Maintained illuminance falls below the target | Apply the maintained value to the EDI target |
The seventh row is a compliance trap worth stating explicitly. Because luminous flux varies with colour temperature at constant power, a specific power calculation performed at one setting does not describe the luminaire across its range. The calculation must be performed at the operating point with the highest power, which is typically the coolest setting.
9. Verification
Tunable white performance is verified spectrally, und die equipment differs from conventional photometric verification.
| Check | Method | Acceptance |
|---|---|---|
| SPD at each setting | Spectroradiometer, integrating sphere or goniometer | Matches the simulated SPD within 5 % in band power |
| Chromaticity at each setting | Spectroradiometer | SDCM ≤ 3 across the specified range |
| M/DER at each setting | Derived from the measured SPD | Within 5 % of simulated value |
| Photopic illuminance per setting | Illuminance meter at the working plane | Within 10 % of design |
| Melanopisches EDI at the eye | Spectroradiometer, vertical plane at the eye position | Meets the schedule target |
| Control scene recall | DALI-2 controller, scene by scene | Each scene within tolerance |
| Power per setting | Power analyser | Within 5 % of the specified value |
| Transition smoothness | Visual and measured during a ramp | No perceptible colour jump |
The last check is a quality-of-experience item that no standard requires and every occupant notices. A luminaire that produces the correct spectra at each setting but transitions between them with a visible tint sweep will be perceived as defective. The transition should be characterised across the full range in steps of 200 K.
10. Conclusion
Tunable white simulation is the discipline of computing a spectrum rather than a colour. The working sequence is: simulate the SPD at each setting across the range, derive the melanopic EDI and M/DER from each simulated spectrum, build the dimming and colour temperature map showing which combinations deliver the circadian target, and verify the intermediate settings for colour quality rather than only the end points.
For a human centric lighting installation, the Z-18 tunable white panel with its DALI-2 and Matter-ready control interface provides the range und die addressability the schedule requires, and it holds SDCM within 3 across the tuning range. Where the scheme needs only a daytime boost and a warm evening setting, the Z-17 panel covers the requirement at lower cost. In both cases the specification should state the simulated SPD, the melanopic EDI und die photopic illuminance together for each operating point, because a circadian claim that quotes only one of those three numbers cannot be verified.
11. Referenzierte Standards
- CIE S 026 — CIE system for metrology of optical radiation for ipRGC-influenced responses to light
- CIE 015 – Farbmetrik
- CIE 224 — Colour fidelity index for accurate scientific use
- IES LM-79 – Zugelassene Methode: elektrische und photometrische Messungen von Festkörperbeleuchtungsprodukten
- ANSI C78.377 – Amerikanischer nationaler Standard für elektrische Lampen: Spezifikationen für die Farbart von Festkörperbeleuchtungsprodukten
- ANSI/IES TM-30 – Methode zur Bewertung der Farbwiedergabe von Lichtquellen
- IEC 62386 — Digital addressable lighting interface (DALI-2)
- EN 12464-1 – Licht und Beleuchtung: Beleuchtung von Arbeitsplätzen, Innenbereich
12. Kontaktieren Sie uns
QUEENDOM supplies tunable white SPD simulations and melanopic EDI reports for the smart panel range, including the operating point tables und die control schedules that deliver a specified circadian target for the Z-18 DALI-2 and Matter-ready panel und die Z-17 panel. Provide the room dimensions, the required daytime and evening targets, the photopic illuminance requirement und die control protocol, und die engineering group will return the simulated spectra, the melanopic EDI per setting and a commissioning-ready operating point map.
Verwandte Produkte und Anwendungen
The tunable-white luminaires referenced in the SPD simulation are listed below.
- Abstimmbares weißes Panel (Z-18)
- Intelligente Panelleuchte (Z-17)
- Industrielle Hallenleuchte (Z-13)
- Anwendungsübersicht: Lösungen für Beleuchtungsanwendungen
- Weitere Fachbeiträge: Wissensressourcen zum Thema Beleuchtung