While it's important to understand how much light output you'll get from a bulb, lumens won't tell you whether the bulb produces a warm glow or a bright white color. For that, you'll need to pay attention to the measurement for correlated color temperature, or CCT (also referred to as light or color temperature, or light appearance).
Measured on the kelvin temperature scale (K), warmer lights (as found with most incandescent bulbs) have a yellowish-white color that measure between 2700K and 3000K. For whiter light that's optimal for kitchens and workspaces, look for bulbs that measure between 3500K and 4100K. And for the bluest light (akin to natural daylight), looks for bulbs marked 5000K to 6500K.
Color temperatures over 5000K are called cool colors (blueish white), while lower color temperatures (2700–3000 K) are called warm colors (yellowish white through red).
Starting in 2011, the FTC will be requiring a new label on light bulbs that will not only display the watts of energy used by a bulb but also its brightness level in lumens plus its light color appearance on a spectrum of warm to cool. Additionally, it will include the Energy Star logo if it meets qualifications. And if it's a CFL bulb, it will include a warning that the bulb contains mercury.
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| Temperature | Source |
|---|---|
| 1,700 K | Match flame |
| 1,850 K | Candle flame, sunset/sunrise |
| 2,700–3,300 K | Incandescent lamps |
| 3,000 K | Soft (or Warm) White compact fluorescent lamps |
| 3,200 K | Studio lamps, photofloods, etc. |
| 3,350 K | Studio "CP" light |
| 4,100–4,150 K | Moonlight, xenon arc lamp |
| 5,000 K | Horizon daylight |
| 5,000 K | tubular fluorescent lamps or Cool White/Daylight compact fluorescent lamps (CFL) |
| 5,500–6,000 K | Vertical daylight, electronic flash |
| 6,500 K | Daylight, overcast |
| 5,500–10,500 K | LCD or CRT screen |
| 15,000–27,000 K | Clear blue poleward sky |
| These temperatures are merely characteristic; considerable variation may be present. | |
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