Light
Spectral Power Distribution
SPD
Also known as SPD
The distribution of optical power a light source emits across wavelengths. The spectral fingerprint underneath color appearance metrics such as CCT.
In more detail
Spectral power distribution (SPD) describes how much optical power a source emits at each wavelength across the spectrum of interest — typically plotted as a curve from near-ultraviolet through visible light into near-infrared. From an SPD you can derive luminous quantities, chromaticity, correlated color temperature, color-rendition metrics, and melanopic quantities. The SPD is the underlying physical description; the single-number labels on a cut sheet are compressions of it.
That distinction matters because correlated color temperature (CCT) only describes the apparent color of white light — warm versus cool looking. Two sources can share a CCT and still differ substantially in SPD. Those spectral differences change how surfaces render and how much the spectrum stimulates melanopsin-weighted pathways, even when the white light "looks" similar.
SPD does not by itself prescribe a lighting design, circadian outcome, or product quality. It is the measurement that lets those claims be checked.
Why it matters
Most residential lighting is still specified by watts, lumens, CCT, and maybe CRI. Those numbers leave the spectrum mostly invisible. In a high-performance house, spectrum affects color rendering and melanopic equivalent daylight illuminance — quantities you cannot read from CCT and illuminance alone. Asking for an SPD, or for products that disclose one, is how lighting stops being specified only by how warm or cool the white looks.
The common misunderstanding
That matching CCT means matching light. CCT is an appearance label derived from the SPD; it is not a substitute for the SPD. Equal CCT can hide different spectra, different color rendering, and different melanopic potency.
Related terms
On Compound
Where this term does real work — and where the decisions get made.
Sources
Last reviewed August 2026 by Compound Editorial. Definitions are educational and not project-specific engineering or medical advice.