Beyond RGB. Building an RGBAL architectural light with WLED.

This project tests whether WLED, a platform normally associated with decorative LED strips, can control a high-quality indoor architectural luminaire. Producing five PWM outputs is straightforward. The real challenge is combining them accurately and repeatably.

WLED provides networking, presets, effects, a mature user interface and a practical route to an ESP32-based prototype. It already supports analogue RGB, RGBW and five-channel RGBCCT outputs, as well as addressable RGBW and selected addressable RGBCCT chipsets.¹ The electrical control needed for a five-output prototype is therefore already in place.

The missing part is the colour model. Stock WLED stores RGB, one white value and a separate CCT blend. It has no native model for mixing red, green, blue, amber and lime as five independent primaries. Stock WLED can drive a basic RGBAL prototype, but it cannot calculate how five measured spectra should be combined to produce a specific white, pastel or saturated colour.

The sdgelectronics/WLED_RGBAL fork adds that capability through four- and five-primary PWM buses, a spectrometer-derived lookup table, 16-bit internal channel data and a calibration API. The software and simulation work are complete. Closed-loop measurements on the finished hardware are the next major stage.¹³

Each extra letter usually represents another independently driven emitter colour. LED naming is not consistent across strips, fixtures and marketplaces, so the part number, pinout and channel diagram are more reliable than the acronym in a product listing.

So why combine amber and lime?

RGBAL gives the mixer more ways to reach the same target colour. Amber can produce warm oranges directly instead of relying on an inefficient red-and-green mix. Lime supplies efficient mid-spectrum energy for whites and pastels, while amber adds warmth to skin and other materials. Those extra choices can be used to improve brightness, colour rendition, electrical load or thermal headroom.

RGBAL does not automatically enlarge the outer colour gamut. Amber and lime often sit inside the triangle formed by the RGB primaries on a chromaticity diagram. Their main value is better spectral quality and efficiency within that gamut.

Multi-primary lighting matters because two sources can have nearly identical chromaticity and CCT while producing very different spectral power distributions. Materials reflect those spectra differently, so timber, red paint or skin may look natural under one source and flat under the other.

CRI is still a familiar shorthand, but it compresses a complicated subject into one average. TM-30 adds fidelity (R_f), gamut (R_g) and hue-specific information. The US Department of Energy’s review found that TM-30-based criteria captured preference better than the older single-number approaches in the studies it analysed.¹⁰

WLED’s documented light capabilities are RGB, white and CCT. Its JSON colour array contains three or four bytes for RGB or RGBW, while each segment stores CCT separately.³ This suits RGBCCT because the fourth colour value sets the white level and CCT divides it between the warm-white and cool-white emitters.

The official compatibility page dates analogue single-colour, CCT, RGB, RGBW and RGBCCT support to WLED 0.13.1. It also lists addressable WS2805 and FW1906 RGBCCT support from 0.15.0-b2.¹ Those are genuine five-channel lighting modes, but they still use the RGB-plus-white-and-temperature model rather than a general five-primary colour vector.

The current PWM bus code makes this explicit. The five-channel type is labelled PWM RGBCCT. It writes RGB directly, calculates the warm-white and cold-white values, then sends them to channels four and five.⁴

What happens if WW and CW become amber and lime?

The mapping works electrically. The nominal warm-white output drives amber and the cold-white output drives lime. WLED's white slider controls their combined level, while the CCT slider shifts the balance from amber to lime.

For a prototype, this is useful, but it is still an approximation. WLED continues to treat the two channels as a white-temperature pair. Auto-white can add them whenever it extracts white from RGB. Effects also tend to generate RGB values, so they may not use the extra pair in the way a five-primary mixer would. The CCT additive-blending setting changes how strongly both channels can be driven near the centre. WLED notes that high additive blending can increase heat and reduce LED life.²

At a fixed temperature and current, additive light is conveniently linear. Measure the CIE XYZ contribution of each emitter and place those measurements into a matrix. A requested target can then be expressed as a weighted mix of the five channels:

Three tristimulus targets and five controls mean that several valid mixtures can produce the same chromaticity and brightness. That freedom is why the extra channels are useful. The solver can favour a mixture that uses less power, stays within channel limits, reduces spectral discontinuities, balances junction temperatures or more closely matches a reference illuminant.

A practical solver uses non-negative channel values and respects current limits. It also needs measured dimming curves because PWM duty, optical output and the human impression of brightness are three different things.

Chromaticity matching is not spectral matching

The XYZ matrix can place the beam at a specified white point, but CRI and TM-30 depend on the complete spectral power distribution. Two mathematically matching white points can render objects differently. A calibrated mixer therefore needs a second objective based on measured spectra. The alternative is to use carefully measured recipes rather than claim universal colourimetric accuracy.

Direct mode versus calibrated mode

Direct control remains useful for bench measurements and selecting individual emitters, but it does not provide fixture-to-fixture matching. Professional manufacturers measure and calibrate individual luminaires because LED wavelength and output vary by bin and temperature. Even nominally identical emitters differ slightly.¹¹

The model shows that amber and phosphor-converted lime sit inside the triangle formed by red, green and blue on a CIE chromaticity diagram. They do not add a new boundary colour, but they provide brighter and spectrally fuller ways to produce colours that RGB can already match.

The initial model uses the Cree XNPAPL package, with red around 625 nm, green around 528 nm, blue around 458 nm and a modelled PC-lime peak around 558 nm. Adding lime leaves the chromaticity hull area unchanged to four decimal places, but substantially changes the predicted brightness and colour-rendering performance.¹³

The emitter choices are deliberately different

The first hardware build uses RGBL rather than the final five-colour arrangement. The four-die Cree package places every source within one optical footprint, simplifying colour mixing on the first board. The later RGBAL version will use discrete emitters, which adds flexibility but makes mechanical alignment and beam mixing more difficult.

Why the working white point became 3000 K

A sweep of the Planckian locus from 2200 K to 6500 K found that 3000 K gave the best overall balance of CRI Ra, TM-30 Rf and saturated-red rendering R9. Moving to 2700 K improved Ra by only 0.1 point but reduced R9 by almost 12 points, which is a poor trade for skin, timber and other warm materials.

Two findings to demonstrate in the video

Royal blue can extend the gamut. Unlike amber and lime, replacing blue with a shorter-wavelength royal blue moves an outer vertex of the RGB triangle. The current model predicts about 4.8% more chromaticity area, together with better TM-30 Rf and efficacy. Standard blue retains a small Ra advantage, showing that Ra and TM-30 measure different aspects of colour quality.

An optimised warm-white recipe may exclude the green die. At and below 3000 K, the model's best white switches the narrow green emitter off. Broad lime peaks in a similar region and supplies the required luminance more efficiently. Green remains important for saturated greens and cyans, but it is not the preferred contributor to this white recipe.

The fork avoids teaching hundreds of effects about amber and lime. In the WLED 0.16-dev codebase used for this project, WLED applies its perceptual processing before the PWM bus sees the colour. The multi-primary bus then receives a linear-light RGB request, interpolates the calibrated table and produces four or five physical channel values.¹³

What the simulation says so far

For the modelled Cree RGBL fixture at 3000 K, the current 17³ profile produces CRI 89.1, TM-30 Rf 87.8 and a 3005 K white. Of 4,913 grid points, 327, or 6.7%, need gamut mapping. Across 2,898 in-gamut test colours passed through the quantised LUT, the mean ΔE 2000 is 0.02 and the maximum is 1.40.¹³

These low interpolation errors show that the lookup table adds little error to the underlying model. They do not validate the emitter model itself. An inaccurate spectral model can still produce inaccurate output even when the lookup table reproduces it precisely.

Controller

The design will use an ESP32 because it provides the processing headroom and LittleFS capacity needed for the calibration profile, together with a much more camera-friendly default PWM frequency than the ESP8266.

Output stage

WLED’s GPIOs are control signals, not LED power outputs. A constant-voltage strip needs a suitable switching device per channel, while high-power emitters need regulated constant-current drivers. The output stage will include defined off-state bias, gate or input resistors, hardware current limits, fusing and a safe power-up state. WLED’s analogue-strip guidance also calls for external MOSFET circuitry and protection against overload and over-temperature.¹

Thermal design

Thermal testing will use the worst credible channel mixture rather than a single emitter at full output. A centre blend can activate amber and lime together, while a custom RGBAL recipe may run several channels at high current. A temperature sensor near the LED substrate will control thermal derating, and measurements will be taken after the heatsink reaches equilibrium. Stable temperature is essential for stable colour.

Optics

Several visible source colours need enough mixing distance or a suitable reflector or homogenising optic. The four-die Cree package gives the first RGBL build a shared optical footprint, while the discrete RGBAL version will be less forgiving. A diffuser may hide the individual emitters when viewed head-on but still cast coloured shadows onto nearby objects. Beam uniformity will therefore be tested at the intended installation distance, across the edge of the field and at low dim levels.

WLED’s documentation lists a default analogue PWM frequency of 19,531 Hz on ESP32, compared with 880 Hz on ESP8266.¹ This is one reason for choosing the ESP32. The higher frequency should avoid many common camera artefacts, but it is not a guarantee. Shutter time, frame rate, rolling-shutter readout, PWM depth, driver behaviour and low-duty operation can still interact.

Higher PWM frequency also reduces the available PWM resolution and increases switching loss. WLED’s current bus code derives bit depth from the PWM frequency and clock, so the highest possible frequency is not necessarily the best setting.⁴

Frequency is only one part of flicker performance. Modulation depth, duty cycle and waveform also matter, particularly on camera. The US Department of Energy recommends testing across the dimming range and measuring the actual output rather than assuming a driver is flicker-free.¹²

1. Stabilise the mechanics, currents and temperature. Finalise the heatsink, optic, drive current and permitted simultaneous-channel load. Record cold-start drift and wait for thermal equilibrium before building the profile.
2. Drive one emitter at a known duty. The fork's raw override bypasses the LUT, auto-white handling and WLED's perceptual brightness curve. That bypass matters: without it, a nominal 50% request can be closer to 12% duty and the calibration would learn the wrong transfer function.
3. Capture every spectrum at several levels. An Avantes AvaSpec 2048-2 will measure each die across a sweep of duty levels. Multiple levels are needed because driver droop, LED heating and supply sag can distort the flux-versus-duty curve.
4. Build the real channel model. Integrate each spectrum against the CIE colour-matching functions to obtain its XYZ contribution. Stack those results into the 3×N matrix and replace the provisional emitter model with the finished fixture's data.
5. Choose white point and headroom. Sweep the available luminance ceiling and choose the knee where more brightness starts costing a disproportionate amount of colour quality.
6. Solve the colour cube. The current toolchain solves 4,913 small convex problems for the 17³ grid. In-gamut points balance colour, rendering and efficiency. Out-of-gamut requests are reduced in chroma while hue and lightness are held as steady as possible.
7. Upload safely. The device checks the profile signature, version, geometry and CRC, then hot-loads it. A bad file is rejected and the previous calibration remains active.
8. Close the loop. Command known colours, re-measure the beam and calculate ΔE 2000 against intent. Repeat at low output, after thermal soak and across the expected ambient range.
9. Turn measurements into presets. Put the proven architectural and camera looks into WLED so the YouTube demo and final installation recall exactly the same states.

Why an 85% luminance ceiling can improve overall performance

At the absolute luminance ceiling, the three colour constraints can completely determine the channel mix, leaving the optimiser no freedom to improve the spectrum. Reducing the target luminance creates room to choose a better mixture.

In the model, reducing peak luminance by 15% improves CRI by 8.9 points, R9 by nearly 38 points and efficacy by about 5%. WLED's master brightness then scales all channels together, preserving that spectral mix at lower brightness levels.¹³

What if there is no spectrometer?

A tristimulus colour meter can provide useful xy/uv and luminance data for the basic mixing matrix. It cannot reveal spectral discontinuities or calculate reliable rendering metrics without spectral information. A camera or phone is useful for demonstrating the result, but it is not suitable as the only calibration instrument. For one comprehensive calibration session, borrowing or hiring a spectrometer is likely to provide more value than another iteration of unmeasured firmware.

What can still go wrong on the real fixture

• Ambiguous naming: RGBWW does not tell us whether there are four or five controllable channels.
• Common-anode versus common-cathode hardware: the driver topology and polarity setting must match the LEDs. Software cannot correct a hardware mismatch.
• Auto-white surprises: WLED can derive white from RGB. In the repurposed RGBAL mapping, that means it may add amber or lime unexpectedly. Use manual white during characterisation.
• CCT blend heat: additive centre blending can drive both secondary channels hard. Test that case against the electrical current limits and the thermal design.
• Power estimation: WLED’s brightness limiter is useful, but a custom high-power analogue engine still needs hardware limits and a supply sized from measured loads.
• Thermal colour drift: flux and wavelength move with temperature. A temperature-stable fixture is easier to calibrate and easier to match.
• Optical separation: a diffuser that appears uniform at ten centimetres can still cast separated colour shadows at the wall.
• Low-level dimming: the bottom few per cent expose PWM resolution, driver turn-on thresholds, channel mismatch and camera banding.
• Calibration by eye: our visual system adapts quickly. Side-by-side references and instruments prevent “looks about right” from becoming a permanent coefficient table.
• Network exposure: WLED is a convenient local controller. Do not expose a luminaire’s web interface directly to the public internet.

RGB supplies the edges. Amber gives warmth without a red-green compromise. Lime fills a useful, efficient part of the middle spectrum. Together they create a flexible platform for an architectural light that can do bold colour and far more nuanced pale light than a basic RGB engine.

WLED is a suitable foundation because it already provides Wi-Fi provisioning, a mature interface, presets, APIs and effects. Stock WLED's RGBCCT path supports initial hardware commissioning, while the custom fork adds fixture-specific conversion from ordinary colour requests to measured RGBL or RGBAL drive values.

The results so far make a strong engineering case, but they are still model results. Measurements from the first Cree XNPAPL RGBL board, including its lime spectrum, PWM waveforms, optical mixing and closed-loop ΔE, will determine whether the model is accurate.

The YouTube video will begin with the familiar WLED control system before explaining why five physical outputs are not the same as five-primary colour management. The key demonstrations will be the unchanged gamut triangle, the green channel switching off in warm white and the colour-quality improvement gained by reducing peak luminance by 15%.

The next stage is to turn these model results into measured hardware results and develop the experimental colour fixture into a repeatable architectural luminaire.