Compare all 18 standard illuminants side-by-side: CCT, CIE xy, CRI Ra, and white-point swatch.
CIE 15:2018 — Colorimetry
CIE 15:2018 (4th edition) is the fundamental reference for colorimetric computation. Defines the CIE 1931 2° standard observer, illuminant SPDs (A, D50, D55, D65, D75, E), XYZ tristimulus values, CIELAB, and recommended illuminant-observer computations. Tables provide SPD data at 1 nm and 5 nm intervals from 300–830 nm.
CIE 13.3:1995 — Colour Rendering Index
Defines the CIE Colour Rendering Index (CRI). Ra is the general index (mean of R1–R8). R1–R14 are special indices for 14 test-colour samples. Reference illuminant is a Planckian radiator for CCT < 5000 K and a CIE daylight phase for CCT ≥ 5000 K. Adaptation uses CIE 1960 UCS Von Kries, and colour difference is in CIE 1964 U*V*W* space.
IES TM-30-18 — Colour Rendition
The Illuminating Engineering Society TM-30-18 method evaluates colour rendition using 99 colour evaluation samples (CES). Rf (fidelity index, 0–100) measures average colour shift from a reference. Rg (gamut index, ~60–140) measures change in gamut area. Together they address CRI’s known limitations — particularly with saturated colours and narrow-band sources.
ISO 10526:2007 — CIE Standard Illuminants
ISO 10526 specifies the spectral distributions of CIE standard illuminants A, D65, and supplementary illuminants D50, D55, D75. It establishes the normative SPD tables and computation rules for the daylight series using S0, S1, S2 basis vectors and the CIE daylight chromaticity formula.
ICC.1:2022 — Profile Connection Space
The ICC specification defines D50 as the PCS illuminant. All ICC profiles must express colorimetric data relative to D50. Bradford chromatic adaptation is mandated for converting from device white-points (often D65) to D50. The illuminant CIE xy values for D50 (0.3457, 0.3585) are normative.
CIE Standard Illuminant A
CIE Illuminant A represents typical domestic tungsten-filament lighting. It is
defined as a Planckian radiator at 2856 K. The relative SPD is computed
as:S_A(λ) = 100 · (560/λ)^5 · (exp(c2/(560·T)) - 1) / (exp(c2/(λ·T)) - 1)
where
c2 = 1.4388 × 10^7 nm·K. This has a warm yellowish spectrum peaking in
the infrared.
CIE D-Series Daylight Illuminants
D-series illuminants (D50, D55, D65, D75) are defined by the CIE daylight formula
using three basis vectors S0(λ), S1(λ), S2(λ) measured from
real daylight data by Judd, MacAdam & Wyszecki (1964). The SPD
is:S(λ) = S0(λ) + M1·S1(λ) + M2·S2(λ)
where
M1 and M2 are computed from the daylight chromaticity xD, yD derived from CCT.
- D50 (5003 K): ICC PCS, printing industry standard.
- D55 (5503 K): Mid-morning/afternoon daylight.
- D65 (6504 K): Average noon daylight, sRGB reference white.
- D75 (7504 K): North sky daylight, overcast conditions.
CIE Fluorescent Illuminants (F1–F12)
CIE defines 12 fluorescent illuminant types in three groups:
- F1–F6: Standard (halophosphate) fluorescent lamps. Broad emission with mercury lines at 405, 436, 546, 578 nm.
- F7–F9: Broadband (de luxe) fluorescent. Higher CRI, simulate daylight.
- F10–F12: Narrow-band three-phosphor lamps. Efficient but low CRI for saturated colours.
Planck’s Law — Blackbody Spectral Radiance:
h = 6.62607e-34 J·s (Planck constant)
c = 2.99792e8 m/s (speed of light)
k = 1.38065e-23 J/K (Boltzmann constant)
Second radiation constant: c2 = h*c/k = 1.4388e-2 m·K
Wien displacement: lambda_peak = 2.898e6 / T (nm)
Illuminant A uses this at T = 2856 K, normalised
relative to 560 nm for the CIE standard SPD.
CIE Daylight Series SPD:
S0, S1, S2 = CIE mean daylight basis vectors (380-780 nm, 5 nm)
Daylight chromaticity from CCT:
4000 <= T <= 7000:
xD = -4.607e9/T^3 + 2.9678e6/T^2 + 0.09911e3/T + 0.244063
7000 < T <= 25000:
xD = -2.0064e9/T^3 + 1.9018e6/T^2 + 0.24748e3/T + 0.23704
yD = -3.0*xD^2 + 2.87*xD - 0.275
M1 = (-1.3515 - 1.7703*xD + 5.9114*yD) / (0.0241 + 0.2562*xD - 0.7341*yD)
M2 = (0.03 - 31.4424*xD + 30.0717*yD) / (0.0241 + 0.2562*xD - 0.7341*yD)
Tristimulus Values from SPD:
Y = k * sum[ S(lambda) * y_bar(lambda) * d_lambda ]
Z = k * sum[ S(lambda) * z_bar(lambda) * d_lambda ]
k = 100 / sum[ S(lambda) * y_bar(lambda) * d_lambda ]
x_bar, y_bar, z_bar = CIE 1931 2-deg colour matching functions
Summation over 380-780 nm at 5 nm intervals (81 wavelengths)
Chromaticity coordinates:
x = X / (X + Y + Z), y = Y / (X + Y + Z)
CIE Colour Rendering Index (CIE 13.3):
CCT < 5000 K: Planckian radiator at same CCT
CCT >= 5000 K: CIE daylight at same CCT
Step 2: For each TCS (i = 1..14):
a) Compute XYZ under test: integral of TCS_i * S_test * CMF
b) Compute XYZ under ref: integral of TCS_i * S_ref * CMF
c) Chromatically adapt test XYZ to ref white (Von Kries in UCS)
d) Convert both to CIE 1964 U*V*W*
e) dE_i = Euclidean distance in U*V*W*
Step 3: R_i = 100 - 4.6 * dE_i
Ra = (1/8) * sum(R_1..R_8) (general CRI)
IES TM-30-18 Colour Rendition:
Rf = 10 * ln(exp((100 - 6.73 * avg_dE) / 10) + 1)
where avg_dE = mean CIEDE2000 across 99 CES
(simplified: 14 TCS used here as approximation)
Rg (Gamut Index):
Rg = 100 * (test_gamut_area / ref_gamut_area)
Interpretation:
Rf = 100: perfect fidelity
Rf < 85: noticeable shifts
Rg = 100: same gamut area as reference
Rg > 100: larger gamut (more saturated)
Rg < 100: smaller gamut (desaturated)
Bradford Chromatic Adaptation:
| 0.8951000 0.2664000 -0.1614000 |
|-0.7502000 1.7135000 0.0367000 |
| 0.0389000 -0.0685000 1.0296000 |
Adaptation:
[L,M,S]_src = M * [X,Y,Z]_src_white
[L,M,S]_dst = M * [X,Y,Z]_dst_white
scale_i = dst_LMS_i / src_LMS_i
For any colour [X,Y,Z]:
[L,M,S] = M * [X,Y,Z]
[L',M',S'] = [L*scale_L, M*scale_M, S*scale_S]
[X',Y',Z'] = M_inv * [L',M',S']
Von Kries uses diagonal adaptation in
Hunt-Pointer-Estevez LMS space instead.
CCT Estimation (McCamy 1992):
CCT = 449*n^3 + 3525*n^2 + 6823.3*n + 5520.33
Valid for: 2000 K to 12500 K approx.
Duv (distance from Planckian locus):
u = 4x / (-2x + 12y + 3)
v = 6y / (-2x + 12y + 3)
Duv = sqrt((u - u_p)^2 + (v - v_p)^2)
where (u_p, v_p) is the nearest point on the
Planckian locus in CIE 1960 UCS.
Colour Difference Formulas:
dE = sqrt[(dL*)^2 + (da*)^2 + (db*)^2]
CIEDE2000 (dE00):
G = 0.5 * [1 - sqrt(C_bar^7 / (C_bar^7 + 25^7))]
a'i = ai * (1 + G)
C'i = sqrt(a'i^2 + bi^2)
h'i = atan2(bi, a'i)
SL = 1 + 0.015*(L_bar'-50)^2 / sqrt(20+(L_bar'-50)^2)
SC = 1 + 0.045 * C_bar'
SH = 1 + 0.015 * C_bar' * T
dE00 = sqrt[(dL'/SL)^2 + (dC'/SC)^2 + (dH'/SH)^2 + RT*(dC'/SC)*(dH'/SH)]
CIE Illuminants & Colorimetry
[2] ISO 10526:2007. CIE standard illuminants for colorimetry.
[3] Judd, D.B., MacAdam, D.L., Wyszecki, G. (1964). Spectral distribution of typical daylight as a function of correlated color temperature. JOSA, 54(8), 1031-1040.
[4] CIE (1986). Colorimetry, 2nd Ed. CIE 15.2:1986.
Colour Rendering
[6] IES (2018). IES method for evaluating light source color rendition. IES TM-30-18.
[7] David, A., Fini, P.T., Houser, K.W., et al. (2015). Development of the IES method for evaluating the color rendition of light sources. Optics Express, 23(12), 15888.
Chromatic Adaptation
[9] Fairchild, M.D. (2013). Color Appearance Models, 3rd Ed. Wiley-Blackwell.
[10] ICC (2022). ICC.1:2022 Image technology colour management.
Colour Temperature & Metamerism
[12] Robertson, A.R. (1968). Computation of correlated color temperature and distribution temperature. JOSA, 58(11), 1528-1535.
[13] Wyszecki, G. & Stiles, W.S. (2000). Color Science: Concepts and Methods, Quantitative Data and Formulae, 2nd Ed. Wiley.
About this tool
This tool implements CIE standard illuminant SPDs, CRI (Ra + R1–R14), TM-30 approximation (Rf/Rg), metamerism index, Bradford & Von Kries chromatic adaptation, and surface reflectance analysis — entirely client-side with zero network dependency. Not a substitute for calibrated measurement or official CIE/IES software.
CRI has known limitations with modern narrow-band LED and 3-phosphor fluorescent sources. The 14 TCS reflectances are relatively unsaturated, meaning CRI can rate a source highly even if it renders saturated colours poorly. TM-30 addresses this with 99 colour evaluation samples and separate fidelity/gamut metrics.
1. Only 14 TCS (mostly desaturated Munsell samples)
2. Uses CIE 1964 U*V*W* (non-uniform colour difference)
3. No gamut shift information (expansion vs contraction)
4. No hue-specific rendering data
5. Narrow-band sources can score Ra > 80 while
distorting saturated reds and deep blues significantly
TM-30-18 improvements:
- 99 CES spanning full hue/saturation range
- Uses CAM02-UCS and CIEDE2000
- Separate Rf (fidelity) and Rg (gamut) indices
- 16-bin hue-angle gamut vectors for directionality
Two objects can appear identical under one illuminant but different under another (illuminant metamerism). The metamerism index quantifies this risk. High values between illuminant pairs indicate that colour samples at risk — critical for textile, automotive, and packaging industries where products are viewed under varying store/office lighting.
For each of 14 TCS (or 99 CES in full TM-30):
XYZ_A = integral(TCS * SPD_A * CMF)
XYZ_B = integral(TCS * SPD_B * CMF)
Lab_A, Lab_B from respective XYZ
dE_i = CIEDE2000(Lab_A, Lab_B)
MI = mean(dE_1..dE_N)
Interpretation:
MI < 0.5: very low metamerism risk
MI 0.5-2: moderate risk — visible to trained observers
MI 2-4: high risk — clearly visible shifts
MI > 4: severe — different appearance guaranteed
Step-by-step method for evaluating a lighting source against requirements:
- Measure or select the test illuminant SPD.
- Compute CCT and Duv — confirm Duv < 0.006 for the source to be “on the Planckian locus.”
- Compute CRI Ra — general acceptability threshold is Ra ≥ 80 for most applications, Ra ≥ 90 for critical colour work.
- Check R9 (saturated red) separately — many LED sources score Ra > 80 but have R9 < 50.
- Compute TM-30 Rf and Rg for a more complete picture. Rf ≥ 85 and Rg 95–105 is considered excellent.
- If comparing two illuminants, compute the metamerism index — MI < 1 ensures minimal colour shift risk.
- Use the surface reflectance analysis to visually inspect how critical samples (skin, textiles, prints) appear.
- For ICC workflows, confirm the Bradford adaptation from test white to D50 produces acceptable ΔE₀₀.