LED CCT Spectral Half-Width
Typical spectral half-width of white LEDs from 2700 K to 7600 K — a component engineering white paper on how colour temperature and drive current set the width of the emitted spectrum
Queendom LEDs · Component Engineering Group
1. Why Spectral Width Is a Design Parameter
Colour temperature is the number printed on the label. Spectral half-width is the number that decides whether the fixture still matches that label after the driver has warmed up, after the dimmer has dropped to 10 %, or after the next production lot arrives. In a luminaire, in a display backlight, in a horticultural array and in a signalling head, the tolerance that matters is not the specified CCT on its own but the spread of the emitted spectrum around it.
A phosphor-converted white LED is a blue die with a phosphor layer in front of it. Both parts move with drive current and junction temperature: the blue peak red-shifts and the phosphor conversion efficiency falls, so the mixture drifts away from the point it was binned to. A part binned at 4000 K can sit 200 K or more from that point at the top of its current range, which is the same order of magnitude as a complete ANSI bin.
This paper collects the typical half-width values our component engineering group works with, covering 2700 K to 7600 K and 20 mA to 1400 mA, and explains how to use them when a colour-consistency specification has to survive volume production. The companion browser tool, the LED Spectrum Simulator, shows the same effect on a live spectral power distribution.
2. What CCT and Spectral Half-Width Actually Describe
2.1 Correlated colour temperature
Correlated colour temperature is the temperature of the black-body radiator whose chromaticity is closest to the source, evaluated on the CIE 1931 chromaticity diagram. It is a one-dimensional summary of a two-dimensional colour point, which is why a CCT alone never describes a white LED completely: two emitters can both be quoted at 4000 K and still look different, because their distance from the Planckian locus differs.
- Low CCT (2700–3500 K) — warm, yellow-shifted, comparable to incandescent sources; the usual choice for residential and hospitality lighting.
- Mid CCT (4000–5000 K) — neutral white, the default for office, retail and task lighting.
- High CCT (5700–7600 K) — cool, blue-shifted, used outdoors, in industrial halls and in display backlights.
2.2 Spectral half-width
The spectral half-width is the width of the emission band measured at half of its peak intensity: the full width at half maximum. For a white emitter there are two conventions. The first reports it on the wavelength axis in nanometres. The second converts it to the equivalent spread on the Planckian locus and reports it in kelvin. In a phosphor-converted white LED the emitted spectrum is a superposition of the blue pump peak and the broad phosphor band, so the half-width is set mainly by the phosphor blend and by the ratio of blue to converted light — not by the die alone.
2.3 Two different widths, and why the distinction matters
A narrow half-width in nanometres means a purer colour and a wider achievable colour gamut. A narrow equivalent spread in kelvin means a closer match to the nominal colour point. The two are related but not interchangeable, and a specification that quotes one quantity while the incoming inspection measures the other will not agree at the receiving dock. All tables in this paper are on the kelvin axis: they give the equivalent colour-temperature spread at half maximum.
2.4 Drive current and junction temperature
Raising forward current raises junction temperature. The blue peak red-shifts and the phosphor conversion efficiency falls. Both effects widen the effective spread, and the second one shifts the colour point. High-current operation therefore needs both a thermal path and a constant-current driver if the delivered colour is to stay inside the bin the part was bought in. Warm-white and cool-white emitters do not respond at the same rate, which is why the current axis matters more for some colour temperatures than for others.
3. How the Data Was Collected
3.1 Scope
The work reported here measures the spectral half-width of white LEDs from 2700 K to 7600 K at drive currents from 20 mA to 1400 mA, and expresses the result as an equivalent colour-temperature range in kelvin. The purpose is to establish the relationship between nominal colour temperature, drive current and delivered spread, so that a lighting design, a binning strategy or a purchasing tolerance can be set from data rather than from a single datasheet point.
3.2 Equipment and conditions
| Item | Specification |
|---|---|
| Samples | White LEDs covering the 2700, 3000, 3500, 4000, 5000, 5700, 6500 and 7600 K nominal bins |
| Drive | Constant-current source, 20–1500 mA, accuracy ±1 % |
| Spectral measurement | High-resolution spectroradiometer, resolution ≤ 1 nm, with integrating sphere |
| Temperature | Stabilised platform held at 25 °C ± 2 °C |
| Sampling | 10 acquisitions per sample and current step, averaged, outliers removed |
3.3 Method
- Fix the sample on the stabilised platform so that the thermal path is identical for every current step.
- Drive the sample from the constant-current source at 20, 60, 100, 350, 700, 1000 and 1400 mA.
- Acquire the spectral power distribution in the integrating sphere and record peak wavelength and half-width.
- Convert the half-width to an equivalent colour-temperature range to give the values tabulated below.
- Compare the trends across colour temperature and current, and plot them as the curves described in section 5.
3.4 Data basis and reliability
Three sources are combined. Typical parameters from Queendom datasheets and specification sheets establish the nominal points. Laboratory measurements on the spectroradiometer provide the current dependence. The definitions of colour-temperature drift and spectral width used here follow the CIE and ANSI documents, so that a value quoted in this paper can be compared directly with a value quoted against those standards. Every number is the mean of repeated acquisitions with outliers removed.
4. Typical Spectral Half-Width by Colour Temperature and Drive Current
The matrix below is the core result. Each cell gives the typical equivalent colour-temperature spread at half maximum for that nominal CCT at that drive current, in kelvin. Read across a row to see what a change of drive current does to a single bin; read down a column to see how the spread changes with colour temperature at a fixed current.
| CCT (K) | 20 mA | 60 mA | 100 mA | 350 mA | 700 mA | 1000 mA | 1400 mA |
|---|---|---|---|---|---|---|---|
| 2700 K | 100–160 | 110–170 | 120–180 | 130–190 | 140–200 | 150–210 | 160–220 |
| 3000 K | 120–180 | 130–190 | 140–200 | 150–210 | 160–220 | 170–230 | 180–240 |
| 3500 K | 140–220 | 150–230 | 160–240 | 170–260 | 180–280 | 190–300 | 200–320 |
| 4000 K | 160–260 | 170–280 | 180–300 | 200–320 | 220–340 | 240–360 | 260–380 |
| 5000 K | 200–320 | 220–340 | 240–360 | 260–380 | 280–420 | 300–440 | 320–460 |
| 5700 K | 230–360 | 250–380 | 270–400 | 290–420 | 310–460 | 330–480 | 350–500 |
| 6500 K | 260–420 | 280–440 | 300–460 | 320–500 | 340–540 | 360–560 | 380–580 |
| 7600 K | 300–480 | 320–500 | 340–520 | 360–560 | 380–600 | 400–620 | 420–640 |
Note: values are typical. The actual range depends on the supplier binning coordinate strategy, on the phosphor blend and on the measurement conditions, and should be confirmed against the datasheet or a type test for a specific part number.
5. How to Read the Matrix
The colour-temperature axis dominates. At 20 mA the quoted band grows from about 60 K at 2700 K to about 180 K at 7600 K. The nominal colour temperature is therefore the first decision: warm bins are intrinsically tighter, cool bins intrinsically wider.
The current axis shifts the band upward and widens it slightly. From 20 mA to 1400 mA both bounds rise, because the blue pump red-shifts and the phosphor contribution changes. The span grows by roughly 15 to 25 %, so the dominant variable remains the nominal CCT.
Low current is comfortable, high current is not. Between 20 mA and 100 mA the movement is small: the part behaves close to its bin. Between 700 mA and 1400 mA the movement is significant and a driver without current regulation will show visible drift.
The upper bound moves faster than the lower bound at high CCT. At 7600 K the lower bound rises about 120 K while the upper bound rises about 160 K from 20 mA to 1400 mA. A specification written only around the nominal point will not capture that.
Warm white is more sensitive per unit of current. Because its absolute band is narrow, an absolute drift of a few tens of kelvin consumes a larger fraction of the available tolerance. A design that is comfortable at 2700 K has less headroom than the same design at 5000 K, even though the cool bin is nominally wider.
6. Binning: What the Standards Ask For and What Suppliers Do
ANSI and NEMA define nominal colour temperatures with a tolerance around each one. The table below lists the nominal range for the five most common bins. The third column gives the band that suppliers in this product family typically hold in production, expressed in MacAdam steps, which is the unit used to compare visual difference rather than absolute kelvin offset.
| Nominal CCT (K) | ANSI/NEMA nominal bin range | Indicative supplier band (MacAdam steps) |
|---|---|---|
| 3000 K | ±100–150 K, approx. 3 SDCM | 3–4 SDCM |
| 4000 K | ±150–200 K, approx. 4 SDCM | 3–5 SDCM |
| 5000 K | ±200–250 K, approx. 5 SDCM | 4–5 SDCM |
| 5700 K | ±250–300 K, approx. 5–6 SDCM | 5 SDCM |
| 6500 K | ±300–350 K, approx. 6 SDCM | 5–6 SDCM |
One MacAdam step is the colour difference that a trained observer can just detect under controlled conditions. Three steps is the usual target for parts that will be mixed inside one luminaire; five to six steps is common for parts that will be installed far apart. A standard-compliant bin is a floor and not a target: a luminaire that mixes two production lots and has a tight field-uniformity requirement should be built from tighter bins, from a single lot, or from a fixture-level colour check.
Because the equivalent spread widens with colour temperature, the same absolute tolerance means a different visual tolerance at 3000 K and at 6500 K. When a project fixes a colour-consistency requirement, the requirement should be written in MacAdam steps and applied at the CCT concerned, not copied from a soft-white example.
7. What the Numbers Mean in Each Application
7.1 Lighting design
Colour temperature and spectral half-width together decide both the look of a space and the consistency of a run. Warm white at 2700–3500 K gives the narrower band and suits interiors where the same fixture type is repeated many times. Cool white at 5000–6500 K gives a wider band and suits large-area and industrial installations where absolute uniformity is less visible. Where fixtures are dimmed, remember that the current axis in section 4 is a dimming axis: a deep dim throws the delivered point off the nominal colour, and the compensation has to be designed in.
7.2 Display and backlight
In a display, a narrower half-width in nanometres raises colour purity and widens the achievable gamut; a wider emitter is easier to blend but covers less. For high-end panels targeting Rec.2020 or DCI-P3, the emitter choice is a half-width decision before it is a brightness decision.
7.3 Horticulture
Photosynthesis responds to spectrum, not to colour temperature. The blue band near 450 nm and the red band near 660 nm carry most of the photosynthetically active radiation, so a narrow half-width puts more of the emitted power inside the absorption peak and less outside it. The matrix in section 4 is the white-light reference point; horticultural arrays normally mix narrow-band emitters instead.
7.4 Automotive and signalling
Automotive lighting and traffic signals are regulated on colour, and a drift that would be invisible in an office is a non-conformity in a lamp. Narrow-half-width emitters make the compliance margin easier to hold, and the current axis matters because these parts are often driven hard.
7.5 Medical, stage and museum lighting
Where spectrum influences a diagnosis, a colour effect or the ageing of an artefact, half-width control is part of the specification. Museum and conservation lighting additionally requires the ultraviolet and infrared content to be held down, which is a separate but related spectral decision.
8. Checks Before You Commit to a Bin
- Confirm the half-width convention: nanometres or equivalent kelvin. A value without its axis is not a specification.
- Ask for the value at the drive current the product will actually run, not at the test current printed in the header.
- Check the current-derating curve and the thermal path together; the current matrix here assumes the junction temperature it produces.
- Express field-uniformity requirements in MacAdam steps and state the colour temperature they apply to.
- If two lots will be mixed, buy tighter than the standard bin, or buy one lot and store the remainder.
The failure patterns we see most often are the mirror image of those checks: quoting a lifetime or consistency figure from a single test current, assuming that a standard-compliant bin is automatically tight enough for a mixed-lot installation, and treating a typical value from a datasheet as a guaranteed limit. A datasheet typical is a design input; only a type test is a guarantee.
9. Related Reading
- LED Spectrum Simulator
Compose a phosphor-converted white spectrum in the browser and watch CCT, Duv and Ra respond to drive current. - Ceramic Package Selection White Paper
Choosing a ceramic package by power class, thermal path and reliability target. - UV and IR Wavelength Selection White Paper
Wavelength, bandwidth and binning as yield-critical parameters. - Automotive AEC-Q102 Qualification White Paper
What AEC-Q102 proves, and what it does not. - LED Component Knowledge & Resources
The parent section: package fundamentals, binning, wavelength and lifetime terminology. - High-Power LED Test Reports
Measured data behind the claims in these papers.
Full edition: LED CCT Spectral Half-Width White Paper — complete edition with the full current matrix, the binning comparison and the application worksheets.
PDF: available on request from the component engineering group.
Queendom · Component Engineering Group.
This white paper is published as part of the Queendom LED technical library. For datasheets, test reports and application notes referenced above, see the LED Components Support and Resource Center sections. Engineering enquiries: sales@queendomlamp.com.