Nearest Match

HLC Colour Atlas XL — Spectra, λ* Metrics & Nearest Match

Interactive HLC Viewer

This page provides an interactive viewer for the HLC Colour Atlas XL. Colors are organized by:

• H (Hue)
• L (Lightness)
• C (Chroma)

For each color tile you can inspect ID, H/L/C, Lab, (if available) HEX, and the measured reflectance spectrum over 380–730 nm.

Spectral metrics: λ^*_V2, λ^*_EE and Δλ^*

In addition to the spectrum R(λ), the atlas includes three compact descriptors derived from it:

1) λ^*_EE (Equal-Energy reflectance centroid)

λ^*_EE is the centroid (“center of mass”) of reflected energy in the 380–730 nm range under equal-energy weighting (no illuminant weighting):

λ*EE = ( ∫ λ · R(λ) dλ ) / ( ∫ R(λ) dλ )

Intuition: where the spectrum reflects “on average” — more toward shorter wavelengths (bluish) or longer wavelengths (reddish).

2) λ^*_V2 (Integrated-reflectance descriptor)

λ^*_V2 is defined by the integral balance below. Its physical interpretation depends on the measurement conditions and assumptions. With the absorption proxy A(λ) = 1 − R(λ), λ^*_V2 is the unique wavelength that satisfies:

∫380→λ* A(λ) dλ  =  ∫λ*→730 R(λ) dλ

This equation simplifies exactly to λ*V2 = 380 nm + ∫380→730 R(λ) dλ, with dimensionless reflectance and wavelength in nm. It describes integrated reflectance; it does not locate a local transition from reflection to absorption. The proxy 1 − R is physical absorptance only when transmission is negligible and the reflectance measurement supports that energy balance.

Intuition: the accumulated absorption on the shortwave side equals the accumulated reflection on the longwave side.

3) Δλ^* (Difference of two descriptors)

Δλ* = λ*V2 − λ*EE

Intuition: Δλ^* quantifies how far the equilibrium point and the centroid differ. This combines integrated reflectance and wavelength centroid. It is not the normalized skewness γ₁ and is not a standalone metamerism measure. Its usefulness depends on the comparison task.

Note: The viewer/plugin does not compute these values live. Spectra and λ* metrics are already prepared in the dataset and are only loaded and displayed.

Nearest Match — comparison without spectral computation

The Nearest Match plugin finds the closest atlas entry by direct comparison and the reference engine below returns a matched reference card. Its matching route is displayed with the result.

Supported inputs:

• HEX / RGB requests: the reference engine below uses its documented D50/2° ΔE00 route. This is separate from the Browser Edition’s frozen RGB-only pixel binding.
• Lab requests: D50/2° ΔE00 comparison against this engine’s installed master. Exact HLC and FP requests look up the stated reference.
• Descriptor comparison: absolute differences in nm can describe stored reference attributes ( λ^*_V2, λ^*_EE or Δλ^*)

“No computation” means:

• no spectrum parsing, no λ* recomputation,
• reference lookup and comparisons against existing atlas fields; these do not reconstruct a measured spectrum from RGB.

Technical note on color appearance

On-screen color appearance depends on display calibration, browser, and color management. For production (e.g., print), measured values and standardized viewing/measurement conditions remain decisive.