A lot of people think that the colour temperature relates to the brightness of light output – this is entirely false. The colour temperature relates solely to the colour of the light and nothing else. A light with higher kelvin can have less light output than one with lower Kelvin. The reason for this is that we use Correlated Colour Temperature (CCT) to describe colour of light, rather than the true kelvin scale that relates to light output based on heat.
Here is a simple list of light sources and their approximate CCTs in kelvins to give you a better idea:
- 1700 K: match flame
- 1850 K: candle flame
- 2700-3300 K: incandescent light globe
- 3500 K: 100W quartz halogen light globe
- 4100 K: moonlight
- 5000 K: horizon daylight
- 5500-6000 K: daylight
- 6500 K: overcast daylight
The short of it is, the lower the Kelvin, the light appears more yellow, the higher the Kelvin, the light appears more blue. At 6000 Kelvin (close to daylight), the light appears cool white with a slight blue halo.
Quartz halogen bulbs are generally rated at around 3500K, which is in the yellow end of the colour temperature spectrum. HID systems typically come in either 4200K or 6000K. 4200K is whiter than a QH globe, but still sits just inside the yellow spectrum. LED products are normally rated between 5000K and 6000K which is considered to be close to daylight.