Spectral Power Distribution
A plot of how much energy a light source emits at each wavelength across the visible spectrum.
Measured in
W/nm
In depth
The distribution explains what a colour temperature figure hides. Daylight and an incandescent bulb both have smooth continuous spectra, so colours render predictably; fluorescent tubes and cheap LEDs have spikes and gaps, so some pigments receive almost no light to reflect and go dead on camera. No white balance can restore a wavelength that was never emitted.
Examples in practice
A cheap LED with a deep gap in the red region makes a crimson dress photograph as a dull brown.