V Australia (a new Australia-USA airline) planes have this feature. The lighting inside the plane varies depending on the time of 'day' they want you to think it is. They often ask to have the window shades closed to enhance the effect. The colours aren't completely natural - they range from a deep blue (daylight) to an orange (sunset/sunrise) to a purple-blue (nightime). It's all done gradually and very subtle.

Subjectively it helped me to adjust to the different timezone during the flight, but it might have just been the 'hey, neato' factor as well.

Or even RGB LED lighting that can plug into an Arduino, so you could make that yourself.

Or, I suppose, in the vein of a plant that twitters about its thirst, a bot that performs sentiment analysis on your tweets and adjusts the light accordingly.

We've done the RGB led --> Arduino --> Internet trick.

This link will let you change the light color in the steering house of a barge in the Copenhagen harbour owned and run by artists and hackers. http://liljedahl.dk/christian/illutron/picker/

And this one lets you see how much power is currently being used on the barge. http://illutron.appspot.com/PowerGauge.html

Were planning on doing more with this. Throw me an e-mail if anybody wants to play :-)

RGB LEDs tend to have very unnatural spectrum with three narrow spikes, while normal white leds (blue led + some phosphor) has very wide spectrum (albeit different from incadescents, but probably better than common fluorescent lights). Other problem that I see with most RGB LED designs is that different colors are emitted from different spots, which tends to give unnatural shadows.

Three years ago I built LED lamp out of some no-name high-power white LEDs (~1€/piece) and last year I got Phillips LivingColors lamp and subjectively I find the light given by bunch of no-name white LEDs more pleasant.

Really what you need to do is match the wavelenghts that our rods and cones in our eyes are most sensitive to. That should be the optimal solution in terms of energy usage.

Perhaps. But it might not be pleasant to the eye.

The 'red' cones alone can't tell the difference between orange and brick red. The idea is to stimulate all 3 types of cones and the rods in the same proportion as daylight does. RGB LEDs should have the cones covered; I wonder if the problem are the rods? Do we need 4-component LEDs?

I don't know if all the cells in your eye are perfectly tuned to three peeks. I expect biological systems to show some variance.

The mechanism is chemical/physical, though, and depends on the absorption spectrum of a certain pigment molecule for each type of receptor. I strongly suspect the variance in those is likely to be tiny.

The fact that you can see a full spectrum -- as in a rainbow, thin-film refraction on a puddle on a paved street, etc. -- should be an indication that the band-pass for neural firing is relatively wide. If each optical receptor had a narrow acceptance window (the way that individual atoms absorb), there would be gaps in the rainbow. Remember that ALL of the colours you see in a rainbow are pure, composed entirely of photons of a particular wavelength proper to each of those colours. It's not RGB; it's not CMYK.

We can, to an extent, simulate a full spectrum through additive and subtractive colour mixing of "primaries", but it is only a simulation. (And Edwin Land demonstrated that we don't actually need all three of R, G and B in order to create the illusion of a full-colour picture with only a comparatively tiny gamut loss. It is a fragile thing, though, and requires the integrative function and cognitive mapping of our brains.

Similarly, we can easily tell the difference between full-spectrum lighting and discontinuous spectrum lighting. Discontinuous spectrum lighting makes us uncomfortable. None of the colours looks quite right. Food is unappetizing, pictures and people less attractive. Did the light-makers just miss the right mix of frequencies by a hair? No -- any scheme that relies on a small number of discreet wwavelengths will cause a similar discomfort. We may not know why we feels the way we feel at the time, but we sho' 'nuff knows we'se feelin' it.

You make a good point - I forgot about indirect light and banded spectra interacting with surfaces.

They'll probably get there eventually, but considering that CFLs are only just now starting to become legit replacements for some incandescent bulbs---the light quality in a house that went all-CFL during the first big pushes a few years back is appalling---I wouldn't hold my breath.

Except with incandescent bulbs banned in the EU, there's now a strong economic incentive to build cheap, good and efficient replacements. Mind you, I'm not sure banning them outright is the way I'd have done it. (I'd have taxed the environmental impact per amount of light emitted and lifetime)