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CIE 1931 and 1976 Chromaticity Charts — Color, Transformations and NVIS
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1. Why color needed coordinates

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A-P08 · Development edition / Draft

Chapter 1 of 9

Why color needed coordinates

Connect the history of 1931 and 1976 · Read coordinates and color-matching functions · Separate chromaticity from brightness

Narration transcript

One light, two maps

Welcome to CIE Chromaticity Charts from FPD.DEV, The Future of Display and Systems Development. We will read the nineteen thirty-one and nineteen seventy-six diagrams, transform real examples, and use the FPD.DEV calculator. Watch the same light appear on both charts. Its coordinates change, but the light itself does not. We will finish with an OLED and NVIS example, where color acceptance and spectral compatibility must be checked separately.

From visual matching to a shared standard

Before a common system, color descriptions depended heavily on the apparatus and the observer. Experimental color matching by W. David Wright and John Guild helped establish the CIE nineteen thirty-one system. Observers matched a test light using combinations of primaries. The resulting standard observer represents average matching behavior under specified conditions. It is a mathematical model for measurement, not a claim that every person sees every color identically.

Spectrum → XYZ → chromaticity

A spectrum becomes three numbers by weighting it with three color-matching functions and integrating over wavelength. X, Y and Z are tristimulus values. They are not the red, green and blue channels of a display, and they are not direct cone responses. With the appropriate photometric scaling, Y represents luminance. Divide each tristimulus value by their sum to remove overall intensity and obtain chromaticity.

Observer, coordinates and units are different choices

Keep three choices separate: the observer, the coordinate system, and the luminance scale. Converting x and y to u-prime and v-prime preserves the observer used to calculate the original values. It does not create a ten-degree measurement. Likewise, changing relative Y to a physical luminance unit requires a real measurement or a known calibration. Five decimal places in a calculator are display precision, not evidence of measurement accuracy.

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Chapter Reading

One light, two maps

A chromaticity diagram is a two-dimensional map derived from three tristimulus values. It removes the overall scale of those values, so it is useful for specifying color balance independently of luminance. This course uses the CIE 1931 2° standard observer throughout unless explicitly stated. The year 1976 identifies a coordinate system; it does not automatically select a different observer.

You will learn to identify the spectral locus and purple boundary; distinguish physical limits, device limits and specification limits; convert points, lines, rectangles and circles; and use chromaticity evidence appropriately in an engineering decision. Allow approximately 60–90 minutes including reading, calculator exercises and the existing narrated case study.

Check your understanding

Does switching charts change the spectrum?

Show worked answer

No. It changes the coordinates used to describe the same stimulus.

From visual matching to a shared standard

The 1931 system brought experimental color matching into a standardized numerical framework. Wright and Guild used different experimental primary sets; those measurements were transformed and reconciled in developing the system. Do not confuse the experiment’s actual colored lamps with the later mathematical XYZ primaries.

The familiar 2° label describes the standard observer’s small-field basis. ISO/CIE 11664-1 describes its use for roughly 1°–4° fields at photopic adaptation; the 1964 observer addresses larger fields. Individual observers, age, field size and adaptation can change visual matches. A coordinate pair without an observer specification is therefore incomplete documentation. Historical reading: Fairman, Brill and Hemmendinger (1997), and Broadbent (2004).

Check your understanding

Is the standard observer a specific test instrument or person?

Show worked answer

Neither. It is a standardized set of color-matching functions.

Spectrum → XYZ → chromaticity

For an emitting display, S(λ) can be spectral radiance; the common constant and integration units must match the intended absolute or relative result. A sampled calculation uses weighted sums with the wavelength interval included. For a reflecting object, the incident illumination and spectral reflectance jointly determine the light reaching the observer.

x = X/(X+Y+Z), y = Y/(X+Y+Z), and z = 1−x−y. Only two of x, y and z are independent. Recovering XYZ from xy requires a scale: with specified Y and y>0, X=xY/y and Z=(1−x−y)Y/y. Multiple spectra can produce identical XYZ for one observer: this is the basis of metamerism.

Check your understanding

Why do we retain Y alongside x and y?

Show worked answer

Normalization removes the overall magnitude. Y supplies the luminance or relative scale.

Observer, coordinates and units are different choices

A complete record might state: CIE 1931 2° observer; CIE 1976 u′v′; display white measured at normal incidence; luminance in cd/m²; instrument identity; drive level; temperature; sample identity; and uncertainty. Coordinates alone cannot supply these conditions.

The 1931 and 1964 observers require different weighting of the spectrum. There is no unique conversion from one observer’s xy pair to the other observer’s pair without spectral information or additional assumptions. An xy↔u′v′ coordinate transform, by contrast, is exact for the same underlying XYZ when the denominators are defined.

Check your understanding

Does CIE 1976 mean the CIE 1964 10° observer?

Show worked answer

No. Observer selection and coordinate representation are separate.

Open FPD.DEV CIE calculator Glossary

Sources and calculation assumptions

CIE 1931 2° observer. Coordinate conversions describe the same stimulus; chart colors are illustrative sRGB approximations. CIE dataset and adapted charts: CC BY-SA 4.0, modified by FPD.DEV. Chromaticity alone does not establish NVIS spectral compatibility.

  1. CIE data — CIE (2019), 1931 2° color-matching functions; 360–830 nm, 1 nm. CC BY-SA 4.0.
  2. CIE 1976 definitions — CIE e-ILV 17-23-073: UCS transformation and 1960 relationship.
  3. Purple boundary — CIE e-ILV 17-23-058.
  4. Spectral locus — CIE e-ILV 17-23-056.
  5. Dominant and complementary wavelength — CIE e-ILV 17-23-062 and 17-23-063.
  6. CIELUV — CIE e-ILV 17-23-074; ISO/CIE 11664-5:2024.
  7. Standard observers — ISO/CIE 11664-1:2019 overview.
  8. History — Fairman, Brill & Hemmendinger (1997), Color Research & Application 22, 11–23.
  9. History reconstruction — Broadbent (2004), Color Research & Application 29, 267–272.
  10. Color discrimination — MacAdam (1942), JOSA 32, 247–274.
  11. sRGB — ICC Three Component Color Encoding Registry.
  12. NVIS standard — MIL-STD-3009 base 2001; Notice 2 validation 2024; Figure 2 and Tables II–V.
  13. Standard reader — FPD.DEV retained MIL-STD-3009 reader.
  14. Calculator — FPD.DEV RGB / CIE Calculator A-P14; reused local delivery copy.
  15. Narrated case study — Engineering Shorts 04 A-P19, published 1 November 2026; illustrative spectral model.
  16. CIE tristimulus calculation — ISO/CIE 11664-3; spectral distributions are inputs to XYZ calculation
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