What is dominant wavelength?

Dominant wavelength is the single spectral (pure rainbow) wavelength that a color most resembles in hue. On the chromaticity diagram you draw a straight line from the white point through the color's point and continue it until it meets the horseshoe-shaped spectral locus; the wavelength at that meeting point is the dominant wavelength. It captures the color's hue independently of how pure or pale it is.

What is excitation purity?

Excitation purity measures how far a color sits from white toward a pure spectral color, on a scale from 0 to 100 percent. It is the ratio of two distances on the chromaticity diagram: from the white point to the color, divided by from the white point all the way to the spectral locus along the same line. White is 0 percent purity; a pure spectral color is 100 percent.

Why do purple colors have a complementary wavelength instead of a dominant one?

Purples and magentas sit below the spectral locus, near the straight line of purples that connects the red and violet ends of the spectrum. A line from white through a purple exits the diagram along the line of purples, not the spectral curve, so there is no real dominant wavelength. Instead the line is extended backward through white to the locus, and that wavelength - reported with a suffix like c or a minus sign - is the complementary wavelength.

Dominant Wavelength and Purity: Naming a Color by Its Spectrum

"Teal" and "sky blue" and "#4ac0a0" all name a color, but none of them tells you its physics. There's an older, more precise way to name a color with two numbers: the single spectral wavelength it most resembles - its dominant wavelength - and how far it sits from white toward that pure color - its purity. Both are read straight off the chromaticity diagram with a ruler. This is the interactive guide to naming color by its spectrum.

Colorimetry · 92 4 Live Demos ~28 min read Chromaticity

λd

Dominant wavelength

Excitation purity 0–100%

a ruler

Read off the diagram

λc

Complementary, for purples

Two numbers instead of a name

Every color you can see plots as a point on the CIE chromaticity diagram - the familiar horseshoe. The curved boundary is the spectral locus: the pure, single wavelengths of the rainbow, from about 380 nm violet at one tip to 700 nm red at the other. Near the middle sits white. Every real color lives somewhere between the two: a mix of a pure hue and white.

That geometry gives a beautifully physical way to describe any color. Draw a straight line from the white point, through your color's point, and out to the horseshoe. Where it lands is the dominant wavelength - the spectral hue your color is "made of." And how far along that line your color sits - white at one end, the pure spectral color at the other - is its excitation purity. Hue and saturation, but defined by wavelength and ratio rather than by a name.

The core idea: a color = a spectral hue + a purity. The dominant wavelength is which rainbow color it points to from white; the purity is how far it has traveled from white toward that color. Both come from one straight line on the chromaticity diagram.

Reading it off the diagram

Here's the construction, live. Pick a color and the demo plots it on the chromaticity diagram, draws the line from white through it to the spectral locus, and reads off the dominant wavelength and purity. Move the color around and watch the wavelength slide along the rainbow edge and the purity grow as you head outward from white.

Interactive 01 · The construction

White, through the color, to the locus

The horseshoe is the spectral locus, ticked with wavelengths; the dot at the center is the white point. Choose a color: the line runs from white through the color to where it meets the locus - that meeting wavelength is λd. Purity is how far the color sits along that line (white = 0%, the locus = 100%).

Color

Sliding from white to pure

Fix the dominant wavelength and change only the purity, and you trace a straight line from white out to the spectral locus. At 0% you're at white; at 100% you're at the pure spectral color; in between you get every tint and pastel of that one hue. Same wavelength throughout - only the distance from white changes.

Interactive 02 · The purity axis

One hue, from pale to pure

Choose a dominant wavelength and slide the purity from 0 to 100%. The swatch travels from white to the fully saturated spectral color, and the marker moves along the line on the mini-diagram. Purity is the saturation axis of this naming scheme.

Dominant wavelength 520 nm

Purity 70%

The purple problem

Purples break the rule. They live in the region below the horseshoe, along the line of purples that joins the red and violet ends. A line from white through a purple exits along that straight edge, never touching the curved spectral locus - so a purple has no dominant wavelength. Instead we extend the line backward through white to the locus and report that as the complementary wavelength, flagged with a c.

Interactive 03 · Complementary wavelength

When the line points the wrong way

Pick a color. If the line from white hits the spectral curve, you get a normal dominant wavelength. If it's a purple, the line hits the line of purples instead - so the demo extends it backward through white to find the complementary wavelength (the spectral color opposite the purple). Try a magenta or violet.

Color

Dominant is not peak

A common trap: dominant wavelength is not the same as peak wavelength. Peak is just where a spectrum is tallest; dominant is where the perceived hue points, after the eye weighs the whole spectrum. For a broad or lopsided emitter - a real LED, say - the two can differ by many nanometers. That's why LEDs are specified by dominant wavelength: it's what the color actually looks like.

Interactive 04 · Dominant vs peak

Where the hue points, not where it's tallest

A model LED spectrum: a peak you can move and widen. The demo marks the peak wavelength (tallest point) and computes the dominant wavelength (the perceived hue via the color-matching functions). Widen or skew the spectrum and watch the two separate.

Peak wavelength 525 nm

Spectral width 30 nm

Where it's used

Dominant wavelength and purity are working tools, not just diagram trivia.

LED binning

LEDs are sorted and sold by dominant wavelength, because that - not peak - is the hue a buyer sees.

Signal colors

Traffic and aviation signal specs set legal color by dominant wavelength and purity bounds.

Display primaries

A panel's red, green, and blue are often quoted as dominant wavelengths, summarizing the gamut corners.

Quality control

Purity gives a single, intuitive saturation number for pass/fail on dyes, filters, and phosphors.

Teaching hue

It ties the abstract diagram back to the rainbow: every color points to a real spectral wavelength.

Complementary pairs

The complementary wavelength names the spectral color that would neutralize a purple back to white.

Best practices and pitfalls

State the white point

Dominant wavelength depends on which white you draw from (E, D65…). Always say which, or the number is ambiguous.

Don't confuse with peak

Peak is a spectrum's tallest point; dominant is the perceived hue. For broad sources they differ - quote dominant for color.

Purity isn't chroma or saturation

Excitation purity is a specific ratio in xy; it won't match HSL saturation or CIE chroma. Keep the definitions separate.

Purples: use complementary

A magenta has no dominant wavelength - report a complementary one (with a c suffix) instead of forcing a value.

It's hue + purity, not lightness

Chromaticity ignores brightness. Two colors can share λd and purity yet differ in luminance entirely.

Mind the diagram's non-uniformity

Equal purity steps aren't equal perceived saturation - CIE xy is perceptually uneven. Use it to specify, not to judge spacing.

"A name like 'teal' tells you what someone calls a color. A dominant wavelength tells you which thread of the rainbow it's woven from, and its purity tells you how much white got woven in with it. Two numbers, and the color has nowhere left to hide." Editorial summary · a hue and how much of it

The takeaway: dominant wavelength and excitation purity name a color by geometry on the chromaticity diagram - the spectral hue a line from white points to, and how far along that line the color sits. Purples use a complementary wavelength instead. Dominant is the perceived hue, not the spectral peak, which is why LEDs and signal colors are specified this way.

Test your understanding

Six questions on dominant wavelength, purity, the complementary case, and dominant-versus-peak. Instant feedback, no scores recorded - a wrong answer comes with a short explanation.

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Dominant wavelength lives on the chromaticity diagram and feeds the specification of real sources - here's where to go next.

Colorimetry · 10

Dominant Wavelength and Purity: Naming a Color by Its Spectrum

Two numbers instead of a name

Reading it off the diagram

White, through the color, to the locus

Sliding from white to pure

One hue, from pale to pure

The purple problem

When the line points the wrong way

Dominant is not peak

Where the hue points, not where it's tallest

Where it's used

Best practices and pitfalls

Test your understanding

Quick check

Continue your journey

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CIE XYZ Explained

The Standard Observer: 2° vs 10° and the Average Eye

Emission: Neon, LEDs, and Gas-Discharge Light

The Vocabulary of Color: Hue, Saturation, Value, Tint, Tone, and Shade

Metamerism Explained