Hunt (1994) Appearance Correlates (FLL=1 all surrounds)
| Colour | J | C94 | h° | M94 | s94 | Q94 |
|---|---|---|---|---|---|---|
| Primary | — | — | — | — | — | — |
| Comparison | — | — | — | — | — | — |
J=lightness · C₉₄=chroma · h°=hue angle · M₉₄=colourfulness · s₉₄=saturation · Q₉₄=brightness
Hue Composition H (Quadrature)
| Colour | H (quadrature) | Hue description | h° |
|---|---|---|---|
| Primary | — | — | — |
| Comparison | — | — | — |
H: Red(0)→Yellow(100)→Green(200)→Blue(300)→Red(400) · Fraction = % of adjacent unique hues.
Opponent Chromaticity Plane (a, b)
a = Ra − (12/11)Ga + (1/11)Ba · b = (1/9)(Ra + Ga − 2Ba) · filled = A · ring = B · unique-hue rays R/Y/G/B
Hunt FL Luminance Adaptation Factor
FL = 0.2·k4·(5LA) + 0.1·(1−k4)2·(5LA)⅓ · k = 1/(5LA+1) · Vertical line = current LA
Response Compression — Hunt 0.73 vs CIECAM02 0.42
Hunt: 40x0.73/(x0.73+2)+1 (gold) · CIECAM02: 400x0.42/(x0.42+27.13)+0.1 (blue)
ΔE Multi-Metric Comparison
CIE 1931 Chromaticity Diagram
x,y chromaticity · spectral locus · D65 white point · filled = A · ring = B
CIECAM97s Cross-Reference (FLL=0 for avg — same Bradford+HPE)
| Colour | J | C₉₇s | h° | M₉₇s | s₉₇s | Q₉₇s |
|---|---|---|---|---|---|---|
| Primary | — | — | — | — | — | — |
| Comparison | — | — | — | — | — | — |
Average surround difference: Hunt z≈1.45 (FLL=1) vs CIECAM97s z=1.0 (FLL=0) → Hunt gives steeper J gradient for dark colours.
CIECAM02 Cross-Reference (CAT02 + sigmoid 0.42)
| Colour | J | C₀₂ | h° | M₀₂ | s₀₂ | Q₀₂ |
|---|---|---|---|---|---|---|
| Primary | — | — | — | — | — | — |
| Comparison | — | — | — | — | — | — |
CIECAM02 uses CAT02 matrix + Michaelis-Menten sigmoid (exponent 0.42, baseline +0.1) and z=1.48+√n (no FLL flag).
CIELAB D65 Reference
| Colour | L* | a* | b* | C* | h° |
|---|---|---|---|---|---|
| Primary | — | — | — | — | — |
| Comparison | — | — | — | — | — |
Compare ΔEHunt, CIEDE2000, and ΔE₇₆ across average, dim, and dark surrounds for the current colour pair. Also shows J and M94 per surround to visualise how lightness and colourfulness shift.
| Surround | ΔEHunt | CIEDE2000 | ΔE₇₆ | JA | JB | M94A | M94B |
|---|
R.W.G. Hunt and the Evolution of the Hunt CAM (1958–1995)
Robert William Gainer Hunt (1923–2012) was one of the most influential colour scientists of the 20th century, working at Kodak Research Laboratories in Harrow, UK. His systematic investigation of colour appearance began in the early 1950s and produced a series of increasingly refined models over four decades.
| Year | Model | Key advance |
|---|---|---|
| 1958 | Hunt (1958) | First systematic hue-composition quadrature H; brightness modelling |
| 1982 | Hunt (1982) | Luminance-level adaptation (FL), non-linear response compression |
| 1987 | Hunt (1987) | Revised Bradford adaptation, surround factors c/F/Nc/FLL |
| 1991 | Hunt (1991) | Proximal field Yp, induction effects; full appearance attribute set |
| 1994 | Hunt (1994) | Consolidated model — this implementation; foundation for CIECAM97s |
| 1995 | Hunt (1995) | Final book edition; minor calibration updates |
Legacy: CIE TC1-34 (chair: Mark Fairchild) used Hunt (1994) as the primary source when developing CIECAM97s. The Bradford matrix, Hunt response function, hue composition H, and appearance correlate definitions were adopted wholesale. The main simplification in CIECAM97s was setting FLL=0 for average surround.
The Critical FLL=1 Difference — Hunt vs CIECAM97s Lightness
The most impactful practical difference between Hunt (1994) and CIECAM97s is the treatment of the FLL flag in the z exponent:
Hunt (1994): FLL = 1 for ALL surrounds
CIECAM97s: FLL = 0 for AVERAGE surround, = 1 for dim/dark
For Yb=20%, average surround:
Hunt z = 1 + 1·√0.2 = 1.447 → J = 100·(A/Aw)^(0.69×1.447) ≈ (A/Aw)^0.998
97s z = 1 + 0·√0.2 = 1.000 → J = 100·(A/Aw)^(0.69×1.0) = (A/Aw)^0.69
Hunt gives nearly linear J mapping; CIECAM97s compresses mid-tones. Neither is definitively "correct" — they represent different calibration choices.
Bradford Chromatic Adaptation Transform
Bradford CAT (Lam 1985, refined by Hunt & Pointer 1985) uses a spectrally sharpened cone space. The 3×3 sharpening matrix was derived from corresponding-colour experiments at the University of Bradford. It includes a mild blue-boost row to improve prediction of blues under illuminant change.
Pipeline: XYZ → M_Bradford → von Kries scaling (D) → M_Bradford⁻¹ → XYZ_adapted → M_HPE → LMS for appearance computation.
Proximal Field Yp and Chromatic Induction
Hunt (1991) introduced the proximal field — the region of the visual field immediately surrounding the stimulus, typically a few degrees of visual angle. This is distinct from the broader background (Yb) and the overall surround.
The proximal field contribution modifies the achromatic adaptation through an induction factor, modelling simultaneous contrast. When Yp = Yb, the standard Hunt (1994) formulation is recovered exactly.
Hunt (1994) vs CIECAM97s vs CIECAM02 — Key Differences
| Hunt (1994) | CIECAM97s | CIECAM02 | |
|---|---|---|---|
| CAT | Bradford | Bradford | CAT02 |
| Compression | Hunt 0.73 | Hunt 0.73 | Sigmoid 0.42 |
| FLL (avg) | 1 | 0 | N/A |
| z formula | 1 + FLL·√n | 1 + FLL·√n | 1.48 + √n |
| Baseline | +1 | +1 | +0.1 |
| Status | Historical | Historical | CIE 159:2004 |
Applications & Legacy
- CIECAM97s (1998): Entire model directly derives from Hunt (1994). CIE 131:1998.
- CIECAM02 (2002): Refined successor; replaced Bradford with CAT02 and Hunt power function with Michaelis-Menten sigmoid. CIE 159:2004.
- ICC v4: Early colour management systems built on Bradford adaptation from Hunt.
- Textile and automotive: Bradford CAT was standard in metamerism indices.
- Research: Understanding Hunt (1994) is essential for reading all pre-2002 CIE colour appearance literature.
Bradford Chromatic Adaptation Transform
[-0.7502, 1.7135, 0.0367],
[ 0.0389, -0.0685, 1.0296]]
Von Kries diagonal scaling:
Rc = (D·Yw/Rw + 1−D) · R
Gc = (D·Yw/Gw + 1−D) · G
Bc = (D·Yw/Bw + 1−D) · B
XYZadapted = MBradford−1 · [Rc, Gc, Bc]
Hunt Power-Function Response Compression
[L, M, S] = MHPE · XYZadapted
MHPE = [[ 0.38971, 0.68898, -0.07868],
[-0.22981, 1.18340, 0.04641],
[ 0.00000, 0.00000, 1.00000]]
Hunt compression:
x = FL · |L| / Lw
Ra = sign(L) · 40 · x0.73 / (x0.73 + 2) + 1
Asymptote: lim(x→∞) = 41 lim(x→0) = 1
The 0.73 exponent was determined by Hunt from psychophysical brightness-matching data.
Hue Composition H (Hunt 1958)
Red: hi=20.14° ei=0.8 H=0
Yellow: hi=90.00° ei=0.7 H=100
Green: hi=164.25° ei=1.0 H=200
Blue: hi=237.53° ei=1.2 H=300
Quadrature formula:
H = Hi + 100·(h−hi)/ei / ((h−hi)/ei + (hi+1−h)/ei+1)
Reading: H=275 → 75% Blue, 25% Green = "75B 25G"
Hunt (1994) Appearance Correlates
A = (2Ra + Ga + 0.05Ba − 0.305) · Nbb
Nbb = 0.725 · (1/n)0.2 n = Yb/Yw
Lightness J:
z = 1 + FLL·√n (FLL=1 for Hunt, 0 for CIECAM97s avg)
J = 100 · (A/Aw)c·z
Brightness Q:
Q = (1.24/c) · (J/100 + 0.305)0.9 · (Aw+0.305)0.9 · FL0.9
Chroma C, Colourfulness M, Saturation s:
t = (50000/13)·Nc·Ncb·et·√(a²+b²) / (Ra+Ga+(21/20)Ba)
C = |t|0.9 · √(J/100) · (1.64−0.29n)0.73
M = C · FL0.25
s = 100 · √(M/Q)
Hunt FL Luminance Adaptation Factor
FL = 0.2·k4·(5·LA) + 0.1·(1−k4)2·(5·LA)1/3
Notable values:
FL(0.01) ≈ 0.001 FL(1) ≈ 0.09 FL(64) ≈ 0.56 FL(1000) ≈ 2.0 FL(10000) ≈ 4.3
At low luminance, the k4 term dominates (scotopic). At high luminance, the cube-root term dominates (photopic).
Colour Difference Formulas
ΔE = √((J₁−J₂)² + (aM1−aM2)² + (bM1−bM2)²)
where aM = M·cos(h) bM = M·sin(h)
ΔE₇₆ (CIELAB Euclidean):
ΔE₇₆ = √((L₁*−L₂*)² + (a₁*−a₂*)² + (b₁*−b₂*)²)
CIEDE2000:
Full parametric formula with L/C/H weighting, rotation term RT, and chroma-dependent hue shift correction.
Surround Parameters (Hunt, 97s, 02)
Average: c=0.69 F=1.0 Nc=1.0 FLL=1
Dim: c=0.59 F=0.9 Nc=0.9 FLL=1
Dark: c=0.525 F=0.8 Nc=0.8 FLL=1
CIECAM97s — FLL=0 for average:
Average: c=0.69 F=1.0 Nc=1.0 FLL=0
Dim: c=0.59 F=0.9 Nc=0.95 FLL=1
Dark: c=0.525 F=0.8 Nc=0.8 FLL=1
CIECAM02:
Average: c=0.69 F=1.0 Nc=1.0
Dim: c=0.59 F=0.9 Nc=0.9
Dark: c=0.525 F=0.8 Nc=0.8
z = 1.48 + √n (no FLL flag)
Primary Sources — R.W.G. Hunt
- Hunt, R.W.G. (1982). A model of colour vision for predicting colour appearance in various viewing conditions. Color Research & Application, 7, 95–112.
- Hunt, R.W.G. (1987). A revised colour appearance model for related and unrelated colours. Color Research & Application, 12, 73–83.
- Hunt, R.W.G. (1991). Revised colour-appearance model for related and unrelated colours. Color Research & Application, 16, 146–165.
- Hunt, R.W.G. (1994). An improved predictor of colourfulness in a model of colour vision. Color Research & Application, 19, 23–26.
- Hunt, R.W.G. (1995). The Reproduction of Colour, 5th ed. Kingston-upon-Thames: Fountain Press.
- Hunt, R.W.G. (2004). The Reproduction of Colour, 6th ed. Chichester: John Wiley.
CIE Publications
- CIE 131:1998. CIE 1997 Interim Colour Appearance Model (Simple Version), CIECAM97s.
- CIE 159:2004. A Colour Appearance Model for Colour Management Systems: CIECAM02.
- CIE 015:2018. Colorimetry, 4th ed.
Secondary Sources
- Fairchild, M.D. (2013). Color Appearance Models, 3rd ed. Chichester: John Wiley.
- Li, C. et al. (2002). A revision of CIECAM97s for practical applications. Color Research & Application, 27, 49–59.
- Luo, M.R. & Hunt, R.W.G. (1998). The structure of the CIE 1997 colour appearance model. Color Research & Application, 23, 138–146.
- Lam, K.M. (1985). Metamerism and Colour Constancy. PhD thesis, University of Bradford.
Generate 200 random sRGB colour pairs, compute Hunt (1994) ΔE and CIEDE2000 for each, and display a histogram of ΔEHunt distribution under the current viewing conditions. Results are shown in the table below (first 50 pairs).
| Colour A | Colour B | ΔEHunt | CIEDE2000 | JA | JB |
|---|