L*, a*, and b* describe a color's position in three-dimensional color space: L* is lightness (black to white), a* is the red-green axis, and b* is the yellow-blue axis. Every color a spectrophotometer reads gets expressed as a specific point using these three values.
ΔE is the distance between two of those points — your mixed sample and the target formula. A lower ΔE means a closer match; most shops treat anything under roughly 1.0 as visually imperceptible under normal lighting, though tolerances vary by job and customer expectations.
Illuminant matters because color can shift depending on the light source it's viewed under — a phenomenon called metamerism. That's why a spectro reading specifies an illuminant like D65 (standard daylight); a formula that matches under one light source can look slightly off under another.
In practice, you don't need to calculate any of this by hand — the spectro does the math and gives you a pass/fail read. But understanding what's behind the numbers helps when a match comes back close but not exact, and you're deciding whether to adjust the formula or accept the result.
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