What the CIE Color Diagram Tells You About Your TV

Why can one television show more saturated color than another? Why do reviews place triangles inside a horseshoe-shaped diagram? And what does the Color Space setting in your TV actually change?

The main answer is simple.

The horseshoe-shaped CIE diagram is a map of chromaticity for a defined standard observer. A television's red, green, and blue primaries form a smaller triangle inside that map. The triangle shows which chromaticities the display can make by mixing those primaries under the stated conditions.

The signal also has a defined color system. It may use BT.709, BT.2020, or another set of rules. The TV must interpret the signal according to those rules, then reproduce or map the requested colors within the display's capabilities.

That is the useful idea behind the geometry.

The diagram is not a literal map of every color experience a person can have. The triangle is not the display's complete picture-quality limit. A color space is not just three dots. Those distinctions matter, especially when we connect the diagram to HDR and television settings.

Three numbers can describe a color match

People with typical trichromatic color vision can make a wide range of color matches by adjusting three independent primary lights.

That does not mean the eye sends three simple RGB numbers directly to the brain. The retina contains overlapping cone responses, and the brain interprets those signals according to adaptation, surroundings, brightness, and other context.

Colorimetry deals with a narrower question:

When do two light stimuli produce the same color match under defined viewing conditions for a defined observer?

The CIE 1931 2° standard observer is a mathematical model based on average color-matching behavior. It is intended to represent normal color vision for small photopic fields within its stated scope. It does not mean that every individual observer sees or matches every color identically. ^[1][2]

That standardization is still powerful. Once a visual match can be described with three values, engineers can define coordinates, compare lights, specify display primaries, and build color systems that different devices can attempt to reproduce.

What Wright and Guild measured

The CIE 1931 system grew from earlier color-matching experiments, especially the independent work of W. D. Wright and J. Guild.

In these experiments, an observer viewed a divided field. One side contained a test light, often a narrow spectral stimulus. The other side contained a mixture of three adjustable primary lights.

The observer adjusted the primary amounts until the two halves matched in color.

Wright published his re-determination of trichromatic coefficients in 1929. Guild published his spectral colorimetric work in 1931. Their observer groups and exact procedures were limited compared with what we might expect from a modern population study, but the experiments supplied important numerical foundations for standardized color matching. ^[4][5]

The results included an important complication.

With the chosen physical primaries, some test colors could not be matched using only positive amounts of all three primaries on the mixture side. The experimenter could instead add one primary to the test side, then match the combination with the remaining primaries.

The calculation recorded that moved primary as a negative amount.

Nothing in the eye was producing negative light or a negative cone response. The negative value came from the selected primary set and the bookkeeping used to describe the visual match.

That distinction becomes important when the data are transformed into the CIE XYZ system.

From RGB matches to CIE XYZ

The CIE used a linear transformation to define three theoretical quantities called X, Y, and Z.

These are not three physical lamps inside a television. They are mathematical coordinates chosen to make standardized color calculations more convenient. Under the CIE system, the color-matching functions can be represented without the negative regions found in the earlier physical-primary functions.

The Y quantity was also chosen so that it corresponds to photopic luminance within the defined colorimetric system. Luminance is a physical quantity weighted by the standard visual sensitivity function. It should not be treated as a complete measurement of perceived brightness in every context. ^[1][6]

A color stimulus can therefore be represented by three tristimulus values:

• X
• Y
• Z

Those three values include both chromatic information and overall scale.

To create a two-dimensional chromaticity diagram, the values are normalized:

• x = X / (X + Y + Z)
• y = Y / (X + Y + Z)
• z = Z / (X + Y + Z)

Because x + y + z = 1, two coordinates are enough to identify the chromaticity. The familiar diagram plots x horizontally and y vertically.

This normalization removes the overall tristimulus scale. A dim stimulus and a brighter stimulus with the same relative X, Y, and Z proportions can occupy the same x, y point.

That is why the diagram shows chromaticity, not luminance.

What the horseshoe shows

The outer curved boundary is called the spectral locus.

Each point along most of that curve corresponds to the chromaticity of a monochromatic stimulus at a specified wavelength for the CIE 1931 standard observer. The standardized dataset runs through the visible spectrum from the short-wavelength end to the long-wavelength end. ^[3]

The two ends of the spectral locus do not meet naturally. A straight boundary called the line of purples connects them.

The line of purples represents mixtures of light from the long- and short-wavelength ends of the spectrum. These purple and magenta chromaticities do not correspond to one monochromatic wavelength.

Together, the spectral locus and line of purples enclose the chromaticities that can be formed from nonnegative mixtures of visible spectral power for the defined standard observer.

That wording is more precise than saying the diagram contains every color every person can see.

The diagram represents standardized color matches. Individual observers vary. Color appearance also changes with luminance, adaptation, surrounding colors, field size, and other conditions that the two-dimensional xy diagram does not encode.

White is a coordinate, not one universal sensation

White does not occupy one mandatory point in the middle of the diagram.

A color system adopts a white point. That point identifies the chromaticity treated as neutral under the system's reference conditions.

Modern television systems commonly use CIE standard illuminant D65. In BT.709, BT.2020, and BT.2100, the D65 chromaticity is:

• x = 0.3127
• y = 0.3290 ^[9][14][15]

Another defined white, equal-energy illuminant E, is at approximately:

• x = 0.3333
• y = 0.3333

The choice matters because chromaticity coordinates are interpreted relative to the adopted system and viewing assumptions.

A point near the center of the diagram is less saturated relative to a chosen white than a point near the boundary, but the xy diagram does not provide a perfectly uniform perceptual saturation scale. Moving the same geometric distance in two different regions does not guarantee an equally visible change.

What the diagram leaves out

The CIE 1931 xy diagram remains useful because it is familiar and because standards publish primary coordinates in that system.

It also has important limits.

It leaves out luminance

A very dim red and a much brighter red can share the same x, y chromaticity.

The diagram cannot tell you how bright a display becomes, how deep its black is, or how much color it can maintain at different luminance levels.

It represents a standard observer

The plotted boundary and coordinates come from defined average color-matching functions. Real people vary because of age, lens transmission, cone sensitivities, field size, adaptation, and other biological factors.

It is not perceptually uniform

Equal distances on the xy diagram do not represent equal perceived color differences.

CIE 1976 u'v' and CIELAB were developed to improve useful uniformity for specified measurement tasks. They are still approximations with defined conditions and limits, not perfect maps of every color appearance. ^[7][8]

A colored diagram cannot display all the colors it labels

A web image of the CIE diagram is shown on a monitor with its own limited gamut.

Colors on the rendered spectral boundary are therefore illustrations. The screen cannot literally show chromaticities outside its own primary triangle.

A technically responsible diagram should disclose this limitation. It can label coordinates accurately even when the visible fill is only an approximation.

Why a television makes a triangle

Most consumer displays create color through three main additive channels associated with red, green, and blue, although real subpixel layouts and additional white or auxiliary elements can vary.

For the simplest colorimetric model, assume three fixed primary chromaticities and nonnegative linear-light drive values.

Plot the red, green, and blue primary chromaticities on the xy diagram. Connecting the points forms a triangle.

By changing the relative amounts of the three primaries, the display can produce chromaticities on the triangle's edges and inside it. It cannot exactly produce a chromaticity outside that triangle using only those fixed primaries.

This is the display's two-dimensional chromaticity gamut under the stated measurement conditions.

The qualification matters.

A real display's primary spectra may change slightly with drive level, temperature, panel technology, viewing angle, or processing. Some displays include a white subpixel or use more than three physical emitters. The simple triangle remains useful, but it is a model of the measured primary chromaticities, not a complete description of every operating state.

The triangle is not the whole display

A larger triangle can include more saturated chromaticities, but triangle size alone does not tell you the complete color capability of a television.

Two displays can have similar primary coordinates while differing greatly in:

• peak luminance
• sustained luminance
• black level
• color luminance
• color volume
• near-black behavior
• white balance
• grayscale tracking
• transfer-function tracking
• uniformity
• viewing-angle behavior
• bit depth and gradient quality
• gamut-mapping behavior

The full color volume adds luminance to chromaticity.

A display might reach a saturated red coordinate at low luminance but lose saturation as brightness rises. Another display might preserve more chroma at higher luminance. Both can show a similar two-dimensional triangle even though their HDR color volumes differ.

This is why a review that reports only "percent P3 coverage" is incomplete unless it also states the measurement space and method.

Coverage based on xy area, u'v' area, sampled points, or three-dimensional color volume can produce different percentages.

What happens outside the triangle

Suppose the signal requests a color outside the display's measured gamut.

The display cannot reproduce that exact target with its available primaries. It must transform the request into a reproducible value.

Several approaches are possible.

Clipping

The display can push the requested value to a boundary.

This can preserve saturation up to the limit, but multiple source colors may collapse onto the same display color. Detail and separation can be lost.

Compression

The display can compress a wider source gamut into the smaller destination gamut.

This can preserve relationships among colors, but it reduces saturation or changes spacing between colors.

More selective mapping

A system can protect some hues, luminance regions, or memory colors while changing others.

That may improve one kind of content while creating different errors elsewhere.

ITU-R Report BT.2407 describes multiple gamut-conversion approaches and explains why no single method is automatically best for every purpose. The best compromise depends on the source, destination, creative priority, and viewing application. ^[16]

For that reason, it is too simple to say that the display always chooses the "closest" available color.

Closest according to which color-difference formula? Should the method preserve hue, luminance, saturation, gradients, or detail? Different mappings answer those questions differently.

Why no fixed RGB triangle covers the entire horseshoe

The spectral locus is curved.

A triangle has three straight sides.

A triangle whose vertices remain inside the realizable chromaticity boundary cannot exactly equal the curved boundary. It will leave regions outside the triangle.

That does not make an RGB display defective. It means a fixed three-primary system is a practical subset of the standard-observer chromaticity region.

Other systems can use additional primaries or different spectral strategies, but adding primaries also adds engineering, calibration, conversion, and metamerism questions. More primaries do not automatically guarantee more accurate reproduction of existing three-primary video signals.

A color space is more than a triangle

People often use gamut and color space as though they mean the same thing.

They are related, but they are not identical.

A primary triangle identifies a chromaticity boundary for a three-primary system. A complete video or computer color specification can also define:

• primary chromaticities
• white point
• transfer function or transfer characteristics
• signal encoding
• code-value range
• matrix coefficients
• reference viewing or production conditions
• metadata and signaling rules

Two systems can share the same primary coordinates and white point while using different transfer functions or reference assumptions.

That is exactly what happens with BT.709 and sRGB.

BT.709: the familiar HDTV triangle

Recommendation ITU-R BT.709 defines parameter values for HDTV production and international programme exchange.

The edition reviewed for this article is BT.709-6, published in June 2015. The recommendation's history reaches back to 1990. ^[9]

Its primary chromaticities are:

BT.709 colorimetry is widely associated with high-definition SDR television.

That does not mean every SDR source of every resolution uses BT.709. Standard-definition systems have their own historical colorimetry and matrix conventions. A DVD should not automatically be described as BT.709 merely because a modern television displays it.

It also does not mean that every file labeled 1080p is encoded, tagged, or converted correctly.

The recommendation defines a reference system. Real distribution chains and devices still have to follow it.

BT.709 and sRGB share primaries, but they are not identical

sRGB uses the same red, green, and blue chromaticities and D65 white point as BT.709. ^[10][11]

That shared geometry is useful. It does not make the two systems completely interchangeable.

Their transfer behavior and reference assumptions differ. BT.709 specifies an opto-electronic transfer characteristic for television production, while practical SDR display behavior is also shaped by other television recommendations and viewing conditions. sRGB defines its own piecewise transfer relationship and computer-display assumptions.

A conversion workflow must therefore consider more than the triangle.

Saying "BT.709 and sRGB have the same primaries and white point" is accurate.

Saying "BT.709 and sRGB are the same color space" is too broad.

P3 is a family, not one universal consumer-video signal

P3 primaries form a wider chromaticity triangle than BT.709, especially toward red and green.

However, the label P3 can refer to more than one system.

DCI-P3

The cinema-oriented DCI-P3 registry entry uses these primary chromaticities:

Its registered cinema definition uses a DCI white point near x = 0.3140, y = 0.3510 and a gamma value of 2.6. ^[12]

P3-D65

P3-D65 uses the P3 primary chromaticities with D65 as the white point.

This is a useful way to describe P3 primaries in a D65-based environment, but a complete workflow still needs transfer and encoding information.

Display P3

Display P3 uses the P3 primaries, D65 white, and an sRGB-type transfer function. The ICC registry identifies Apple as the responsible organization for the registered Display P3 space. ^[13]

These distinctions matter.

A television review may report P3 coverage as a measurement of the panel's primary reach. That does not mean the incoming consumer HDR signal is encoded as DCI-P3.

Likewise, selecting a menu option labeled DCI-P3 is not automatically the correct choice for HDR television.

BT.2020: the wider television color system

Recommendation ITU-R BT.2020 defines parameter values for ultra-high-definition television production and international programme exchange.

The edition reviewed for this article is BT.2020-2, published in October 2015. The recommendation's history began in 2012. ^[14]

Its primary chromaticities are:

These primaries form a much wider triangle than BT.709.

BT.2020 defines the coordinate system. It does not guarantee that a consumer display can physically reproduce its boundary primaries.

It also does not guarantee that a program uses colors near every boundary.

A title can be encoded in a BT.2020 system while its actual pixel colors occupy a smaller region. The available container, the mastering display's capability, the camera and production pipeline, and the creative grade all affect the colors that are used.

How HDR fits into the picture

Recommendation ITU-R BT.2100 defines image parameter values for HDR television using the perceptual quantizer, called PQ, or hybrid log-gamma, called HLG.

The edition reviewed for this article is BT.2100-3, approved in February 2025. It uses BT.2020 primary chromaticities and D65 colorimetry for HDR television. ^[15]

That leads to an important distinction:

• BT.2020 identifies the wide television colorimetry.
• BT.2100 adds HDR system parameters, including PQ or HLG.
• A mastering display may reproduce only part of the BT.2020 boundary.
• The actual program colors may occupy a smaller region still.

People sometimes say that HDR is "P3 in a BT.2020 container."

That can be a useful shorthand when a particular program's mastered colors remain within a P3-sized region, but it is not a complete standard definition. Consumer HDR television is signaled using the applicable television system. The actual content gamut must be determined from the content and workflow, not assumed from one slogan.

What the Color Space setting changes

Many televisions include a control called:

• Color Space
• Color Gamut
• Color Range
• Gamut
• Color Space Settings

The exact name and behavior depend on the manufacturer, model, input, picture mode, signal type, and firmware.

In general, this control affects how the television maps signal values to its physical primaries.

It does not move the panel's physical primary chromaticities beyond what the hardware can produce.

Auto

Auto is usually the safest starting point for normal viewing when the source and television exchange correct format signaling.

In a well-behaved signal path, the television can apply the appropriate interpretation for SDR and HDR without the viewer forcing one gamut for everything.

Auto is not a guarantee.

Incorrect metadata, unusual converters, older source devices, PC output modes, capture equipment, or manufacturer behavior can still cause a mismatch. When colors look clearly wrong, verify the source format and consult the model's manual before changing several controls at once.

Native

On many wide-gamut televisions, Native exposes more of the panel's physical gamut instead of constraining narrower-gamut content to its intended primary triangle.

When BT.709 content is expanded toward a wider panel gamut, colors can become more saturated than the source specification calls for.

Some viewers may prefer the stronger look. That is a viewing preference, not evidence that the source contained those expanded chromaticities.

Native does not have one universal technical meaning across all brands. A manufacturer's manual or direct measurement is needed to describe a particular model precisely.

Manual BT.709, P3, or BT.2020 choices

Manual choices can be useful for calibration, testing, troubleshooting, or a known source path.

They are risky as permanent global settings.

Forcing BT.2020 on a BT.709 source can expand or misinterpret color. Forcing BT.709 on a BT.2020 signal can compress or distort it. Forcing DCI-P3 simply because the program is HDR can apply the wrong white point, transfer assumptions, or mapping behavior.

The correct choice is the one that matches the signal and the television's documented handling.

What calibration can and cannot do

Calibration can reduce measurable errors in the way a display reproduces a target within its usable range.

It may improve:

• white balance
• grayscale tracking
• transfer-function tracking
• primary and secondary color accuracy
• color-management behavior
• consistency between inputs or modes

Calibration cannot create a physical primary that the panel does not have.

It cannot make an out-of-gamut chromaticity appear exactly if the display cannot produce it. It can only help the display map that request in a controlled way.

This is why gamut coverage and color accuracy are different measurements.

A display can have a wide native gamut and poor accuracy.

Another display can reproduce a smaller target very accurately within that target.

The useful question is not only, "How large is the triangle?"

It is also, "How closely does the display reproduce the requested colors, at what luminance, under which settings and measurement conditions?"

Where this leaves us

The horseshoe-shaped CIE diagram is a standardized map of chromaticity, not a literal picture of every color experience.

The triangle inside it comes from a display's measured additive primaries. It shows a two-dimensional chromaticity boundary, not the display's complete color volume or picture quality.

BT.709, P3, and BT.2020 use different primary coordinates, but their names also carry different white-point, transfer, encoding, and application assumptions.

For your television, the practical starting point is straightforward:

Use an accurate picture mode, leave Color Space or Color Gamut on Auto unless you have a documented reason to override it, and do not assume that Native or the widest available setting is more faithful.

The correct result is not the largest triangle the television can produce.

It is the closest practical reproduction of the color system the program actually uses, within the display's measured capabilities.