How much energy a light gives out at each wavelength — the light's full recipe, of which its colour temperature is only a summary.
A function of wavelength, usually plotted from about 380 to 780 nm. Two lights with the same correlated colour temperature can have very different distributions and so render surfaces differently.
Kelvin alone does not describe a light. A 3000 K LED and a 3000 K halogen lamp look similar as light but can make the same fabric look different.
Low-pressure sodium street lamps emit almost all their light near 589 nm, which is why colours vanish under them.
It is the reason colour rendering, metamerism and 'this paint looks wrong under my lights' exist at all.
A spectroradiometer splits the light with a grating and records the power in each narrow band across the visible range, in absolute units or relative to a reference wavelength. From that one curve everything else about the light is calculated: chromaticity, colour temperature, luminous output, CRI and the TM-30 measures. Sampling matters for lamps with sharp emission lines. Reading a fluorescent spectrum at single points 10 nm apart, instead of averaging each band, can give a badly wrong colour.
| Illuminant | Strongest band | Power below 500 nm | Power from 600 nm up |
|---|---|---|---|
| Daylight, CIE D65 | 460 nm | 35% | 34% |
| Incandescent tungsten, CIE A | 730 nm | 9% | 66% |
| Narrow-band tri-phosphor fluorescent, CIE FL11 | 610 nm | 27% | 34% |
| Phosphor-converted white LED, CIE LED-B3 | 450 nm | 21% | 37% |
Illuminant A sends 66% of its visible power out at 600 nm and beyond and only 9% below 500 nm, which is why reds glow and blues go flat under a filament lamp. The white LED peaks at 450 nm, the blue chip that drives its phosphor. The tri-phosphor tube puts 34% of its power at 600 nm and beyond, much the same as D65's 34%, but piles it into a narrow band around 610 nm where daylight spreads it evenly. All four rows are white light.
How this was calculated: Shares are sums over the CIE illuminant tables sampled every 10 nm from 380 to 730 nm, as a fraction of the total over that range; they are shares of radiant power, not of visual brightness.
A gallery, a clothes shop or a food counter chooses lamps by colour temperature and wattage.
A lamp with a gap in the deep red makes meat, wood and skin look dull whatever its kelvin rating, and one that emits ultraviolet adds to the fading of light-sensitive exhibits. The spectrum, or at least the fidelity figures calculated from it, should be on the specification.
A camera is white-balanced perfectly under a narrow-band LED.
Neutrals come out neutral and some colours are still wrong, because balancing fixes the light's colour and cannot restore wavelengths the lamp never emitted.