Lumen Docs
Spectral Distribution
Spectral Distribution — Citation Version
1. What Spectral Distribution Means
Spectral Power Distribution (SPD) describes how much radiant power a light source emits at each wavelength across the visible spectrum (≈380–780 nm).
This definition is consistent with radiometry and color‑science standards, where SPD represents power per unit area per unit wavelength.
- Smooth SPDs (e.g., incandescent, daylight) show continuous energy across wavelengths. Incandescent lamps exhibit a smooth curve peaking in the red region. (Wikipedia)
- Spiky SPDs (e.g., fluorescent, many LEDs) contain narrow peaks caused by phosphors or discharge lines. Fluorescent lamps show multiple narrow spikes from phosphor blends and mercury emission lines. (Wikipedia)
The shape of the SPD determines how colors appear because reflectance and sensor response depend on wavelength.
2. How SPD Affects Color Perception
A. Saturation and Vividness
Color saturation depends on whether the spectrum contains the wavelengths a material reflects. Missing wavelengths reduce saturation — a principle supported by SPD‑based color‑rendering analysis.
B. Color Accuracy
Two sources with identical CCT can have very different SPDs.
LEDs, in particular, can be engineered with structured spectra that score well on CRI while still producing poor real‑world color rendition.
This is why CCT alone does not guarantee color accuracy — it describes only the correlated tint, not spectral balance.
C. Contrast and Visual Comfort
Human visual response and adaptation depend on wavelength distribution. Smooth spectra align more naturally with the eye’s photopic response, while spiky spectra can create harsh transitions.
D. Material Appearance
Materials reflect specific wavelengths; if those wavelengths are absent in the SPD, the perceived color shifts. This principle underlies the use of full‑spectrum sources in inspection, museum, and color‑critical environments.
3. Comparing Common Spectral Profiles
Incandescent / Halogen
- SPD: Smooth, continuous, red‑weighted.
- Effect: Natural, warm color rendering.
Fluorescent
- SPD: Multiple narrow spikes from phosphors + mercury lines.
- Effect: Uneven color, sometimes greenish.
High‑Pressure Sodium (HPS)
- SPD: Dominant yellow‑orange peak, very weak blue.
- Effect: Poor color rendering, near‑monochromatic appearance.
Metal Halide
- SPD: Multiple peaks with broader distribution than fluorescent.
- Effect: Better color rendering but variable depending on chemistry.
(Supported by general SPD behavior of discharge lamps.) Wikipedia
LED (White)
- SPD: Blue pump + phosphor blend; can be smooth or highly structured depending on design.
- Effect: Highly variable color quality; LEDs can be engineered to manipulate CRI without improving true fidelity.
4. Why SPD Matters More Than CRI
Traditional CRI uses only eight low‑chroma samples and averages their shifts, which can mask spectral deficiencies.
Two sources can have the same CRI but radically different SPDs — especially LEDs, which can be tuned to score artificially high on CRI despite poor real‑world color performance.
TM‑30 improves this by analyzing 99 color samples and providing two key metrics:
- Rf (fidelity) — accuracy of color rendering
- Rg (gamut) — saturation shift
TM‑30 derives these metrics directly from the SPD.
5. Practical Takeaway
When evaluating light quality:
- CRI → quick, limited fidelity snapshot
- CCT → overall warm/cool tone
- SPD + TM‑30 → true insight into color richness, accuracy, and visual comfort (TM‑30 is now widely adopted due to superior accuracy and expanded information.) Department of Energy
For architectural, retail, museum, or inspection lighting, always review the SPD curve — it reveals the actual spectral makeup of the light source.