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CIE 1931 and 1976 Chromaticity Charts — Color, Transformations and NVIS
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3. Why CIE introduced 1976 u′v′

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A-P08 · Development edition / Draft

Chapter 3 of 9

Why CIE introduced 1976 u′v′

Explain why CIE introduced u′v′ · Compare spacing in the two diagrams · Distinguish chromaticity from CIELUV

Narration transcript

A useful map with uneven perceptual spacing

The nineteen thirty-one chart is excellent for many calculations, but distances on it are not a uniform ruler for perception. Equal numerical steps can produce very different visible changes in different regions. Color-discrimination experiments, including MacAdam’s work, made this limitation clear. The ellipses often reproduced on xy charts depend on experimental conditions and are frequently enlarged for visibility. We will use schematic local markers, not pretend they are measured discrimination data.

1960 connects 1931 and 1976

The history includes an intermediate step. CIE adopted the nineteen sixty uniform chromaticity scale diagram before the nineteen seventy-six version. The later system keeps u and scales the vertical coordinate by one and a half. That small-looking change matters: v is not v-prime. A document reporting u and v without primes may be using nineteen sixty. Check the definition before comparing numbers, especially when discussing correlated color temperature or distance from the Planckian locus.

1976 improves spacing; it does not perfect it

CIE nineteen seventy-six improves chromaticity spacing, but uniform remains an approximation. The distance between two u-prime, v-prime points is useful when a requirement specifies that metric. It is not automatically a full color difference, a fixed number of perceptual steps, or a luminance difference. The spectral locus changes shape because we changed the map. Its physical interpretation and the identity of each underlying stimulus remain the same.

u′v′ is not the full CIELUV space

The full CIELUV space adds relative lightness and a reference white. Its u-star and v-star coordinates depend on L-star and on the difference from the reference white’s u-prime and v-prime. That is why a chromaticity distance alone is not a full CIELUV color difference. The asterisks and primes are meaningful. Keep them in specifications, captions and exported results so the next engineer knows exactly which quantities you used.

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Chapter Reading

A useful map with uneven perceptual spacing

MacAdam’s 1942 experiments investigated visual color-matching variability in the xy diagram. The results are often summarized as differently sized and oriented ellipses. Those ellipses describe the experiment and its observer/conditions, not a universal visibility threshold for every display or person.

This motivates two cautions. First, an xy radius should not be transferred numerically to u′v′. Second, an engineering acceptance boundary is not automatically a just-noticeable-difference boundary. Use the metric specified by the requirement. A graphical scale with equal horizontal and vertical units is essential when visually comparing circles or Euclidean distances.

Check your understanding

Does a radius of 0.01 have the same meaning everywhere in xy?

Show worked answer

It has the same numerical meaning, but not a uniform perceptual meaning.

1960 connects 1931 and 1976

The 1960 UCS diagram was part of the effort to improve spacing relative to xy. The 1976 diagram uses u′=u and v′=1.5v. Both are projective representations derived from XYZ. A given pair such as (0.20,0.31) has a different meaning depending on whether the second coordinate is v or v′.

CCT and Duv conventions commonly use the 1960 (u,v) diagram. A vertical displacement drawn on a 1976 plot should not be casually labeled Duv. The course uses primes consistently. Before copying a specification, identify the system, observer, white reference where applicable, and distance definition.

Check your understanding

If v in CIE 1960 is 0.32000, what is v′?

Show worked answer

0.48000. The u coordinate is unchanged.

1976 improves spacing; it does not perfect it

For two points, Δu′v′ = √[(u′₂−u′₁)²+(v′₂−v′₁)²]. This coordinate distance is often convenient for radial chromaticity limits. It does not account for lightness, the adapting white or every nonuniformity of vision.

The 1976 chart has two axes; CIELUV is a three-dimensional space. Do not label a bare u′v′ plot “L*u*v*” as though it included L*. Chart conversion also cannot repair an incorrect observer choice or a poorly measured spectrum. More decimal places preserve numerical precision, but do not establish calibration or accuracy.

Check your understanding

Does Δu′v′=0.01 always mean the same perceived difference?

Show worked answer

No. Uniformity is approximate and appearance depends on other conditions.

u′v′ is not the full CIELUV space

CIELUV uses L*=116f(Y/Yn)−16, where f(t)=t^(1/3) above (6/29)^3 and f(t)=(841/108)t+4/29 below that threshold. Its other coordinates are u*=13L*(u′−u′n) and v*=13L*(v′−v′n). Yn and the subscript-n chromaticity describe the reference white.

At equal L* and the same white, ΔEuv=13L*Δu′v′ for a purely chromaticity difference. If L* differs, the full three-dimensional difference must be considered. This relationship does not turn ΔEuv into a perfect perceptual metric. CIELAB was also adopted in 1976, but its a*,b* coordinates are not the axes of this course’s u′v′ chart.

Check your understanding

Can you calculate complete CIELUV coordinates from u′v′ alone?

Show worked answer

No. You also need luminance relative to a specified white and its chromaticity.

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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