CCT compresses a spectrum into one number. The spectral power distribution, usually printed as an SPD curve on a good datasheet, keeps the whole story: every wavelength from 380nm to 780nm and the relative power at each. Two LEDs with identical CCT can have visibly different SPDs, and the difference shows up in rendering, glare perception and camera response. Reading the curve takes a few habits worth building.
What the Axes Mean
The horizontal axis is wavelength in nanometers; the vertical axis is relative spectral power, either per nanometer or normalized to a reference point such as the peak or a green wavelength of 555nm. Because the axis is relative, the shape matters more than the absolute height. When comparing two parts, normalize both to the same reference or the comparison is meaningless.
Width matters as much as position. A narrow spike concentrates energy at one wavelength; a broad distribution spreads it. Narrow sources make saturated colors pop but render mixed colors poorly, which is why laser-like emitters score badly on rendering metrics despite pleasing saturation.
The Anatomy of a White LED
A phosphor-converted white LED has a characteristic three-part shape. The blue pump peak sits around 450nm. A valley follows where the phosphor takes over absorption. The broad phosphor hump covers the green through red region, its position set by the phosphor chemistry. The depth of the blue-yellow valley and the height of the red tail are the two regions that decide rendering quality.
| Region | Typical location | What it drives |
|---|---|---|
| Blue pump | 445-455nm | Perceived brightness, circadian stimulus |
| Blue-yellow valley | 470-490nm | CCT accuracy, tint preference |
| Phosphor hump | 500-600nm | General rendering, efficacy trade-off |
| Red tail | 620-700nm | R9, skin and meat rendering |
A red tail that dies before 650nm is the classic signature of a low-CRI economy part. Meat, wood and skin all reflect strongly there, so the failure is visible in daily use, not only in a lab report.
What the Curve Predicts
CRI and TM-30 metrics are computed from the SPD by mathematical comparison against a reference illuminant, so the curve is the input that produces the number. Spikes and valleys map directly to poor rendering of the reflected wavelengths. The R9 red sample is the usual casualty: an SPD cut short in the red gives R9 values below zero while R1 through R8 look acceptable, because the general index averages across many patches and hides a single deep failure.
Flicker, efficiency and lifetime are not SPD properties. A curve says nothing about how the part behaves electrically, so it complements rather than replaces the electrical and thermal sections of a datasheet.
Measuring and Comparing
Spectroradiometers capture SPDs directly; integrating spheres add spatial averaging so the reading does not depend on where the detector happens to point. Report the drive current and junction temperature with every curve, because the phosphor hump shrinks as junction temperature rises, a shift of a few nanometers and several percent in height between 25C and 85C. Comparing curves measured under different conditions produces false differences.
For procurement, ask for the SPD at the actual drive condition, not the best marketing condition. A supplier who provides it has measured the part; one who only has a CCT bin label has not.
FAQ
Why does my 3000K LED look pinker than another 3000K LED? Their SPDs differ in the red tail and around the blue-yellow valley. Same CCT, different spectra, different visual result.
Does a higher peak mean a brighter LED? No. Perceived brightness integrates the curve weighted by eye sensitivity, which peaks at 555nm. A tall deep-blue spike contributes almost nothing to perceived brightness.
Can I judge quality from the curve alone? Rendering, yes. Reliability, no. The SPD says nothing about lumen maintenance, solder integrity or moisture sensitivity.
