Review: Philips Hue network enabled multicolour lightbulbs

Colour temperature

Colour temperature is a stupid attempt to sum up a spectrum in one number. What you really need is a spectroscope. You'll then see that an incandescent bulb of any sort emits a nice smooth spectrum (which can indeed be derived from the filament's temperature). What comes out of a CFL is mostly green and all lumps and bumps, and what comes out of an LED is a rather smoother two-humped distribution with an unnaturally high amount of blue.

Our eyes are evolved to work best with a single-hump spectrum a.k.a. sunlight. A halogen bulb comes closest, but is not as hot as the sun so it's a bit deficient in blue and violet. An old-fashioned "yellow" filament bulb generates less green and virtually no blue and violet, but the spectrum is still smooth.

There's also mounting evidence that blue and violet light is involved in maintaining our bodies' circadian rhythms. Yellowish lighting for night-time use was probably a very sound non-choice, and the medical impact of switching to CFLs and LEDs may yet to be appreciated. If anyone is suffering from insomnia, I'd suggest it's worth a try to kick out all the CFL and LED bulbs from your house, and run halogen bulbs on a dimmer in the evening to suppress their blue output. Conversely on grey winter mornings, a full-on halogen bulb or even the bluer LED illumination may be a very good thing.