Is this because if we don't use pure enough red and blue wavelength components, then the resulting 'white' will be tinged green?
White LED light typically has a poor red-green color contrast due to the nature of how the light is generated. Blue light from the diode emitter causes a yellowish phosphor to glow. The emission spectrum of the phosphor is a hump-like distribution of wavelengths centered around yellow to greenish yellow. There are some red wavelengths, and some green wavelengths, but it is mostly yellow. Red objects are going to be rendered very orange-colored, and green objects are going to appear yellowish.
This is for low CRI LEDs, particularly the ones that only use a single type of phosphor in their formulation. 80 CRI LEDs are better, but still have this fundamental problem.
Tri-color fluorescent tubes have a different set of issues, however.
I know this is off-topic now, but do you know what the CRI would be if we used just 3 LEDs to create a 6000K white: deep red at 650nm, green at 555nm, and deep blue at 450nm? I assume because the spectrum is 'spiked', the CRI would be very low, though I can't help but feel the light would look fine.
A typical
RGB white LED is 67-81 CRI. However, the CRI index in this case is not a very good indicator of color accuracy, and there are problems rendering both yellow colors and skin tones accurately with this approach while achieving any level of red saturation. An RGBA white LED (basically a RGB that adds an amber emitter) is a little better, and can achieve 92 CRI.
An RGB setup by itself, because of the poor color rendering, would probably not be suitable for actual room lighting. Most RGB LED stage spotlights (for special effects) also combine some white emitters in there too.
"Spikiness" in the spectrum does not
necessarily mean low CRI, it really depends how the wavelengths are distributed. It just happens to be the case that most artificial sources of lighting have spiky spectrums, and when a spectral emission is smooth, that generally means it has more coverage across different wavelengths in the spectrum.
Also an RGB setup typically has about
half the efficiency of a comparable white LED. This is because current green wavelength LED emitters have less efficiency. This is somewhat ironic because green should be the
most efficient color, in terms of lumens, since the human eye is so much more sensitive to it. This is actually a major problem they are working on. Another approach they are beginning to use is phosphor-converted green. This is just a blue emitter with greenish phosphor, and actually has an extremely high lumen per watt efficiency. When it comes to trying to produce the color green from LED, it is actually much more efficient to produce a hump of various greenish wavelengths via phosphor than to produce a single specific wavelength peak directly. The downside to this approach is lack of color purity, but this does not matter in white setups for general illumination.
(for some further information, you might read these articles:
Osram increases efficiency in green LEDs, The
"Green Gap", and see this thread:
EQ-white )
It can also be noted here there is the Philips Luxeon
Z Lime emitter (actually more greenish-yellow color), which has a very high 190 l/w efficiency, and is useful for customizable-color changing LED fixtures, where better color rendering ability is desired, since phosphor-converted LEDs have broader spectral bandwidth coverage than bare LED emitters that only emit a single peak wavelength. These emitters were made use of in the Philips Hue LED bulb.
(more information here:
Lime-Green LEDs Encourage Color-Tunable Lighting )
Sorry for getting off-topic a little bit, but one really can't discuss the maximum theoretical efficiency of white LEDs without mentioning the subject of phosphor-converted green LEDs.
Do you have any comparison images so I can see the differences for myself?
Unfortunately, there is not really any way to post an accurate colored picture of exactly what the color rendering looks like.
This might give you a rough idea of the type of difference CRI can make though:
http://www.donsbulbs.com/bulbs/g623/glossary/cri.examples.jpg
Also see the two pictures at the bottom of this page:
http://lowel.tiffen.com/edu/color_temperature_and_rendering_demystified.html
http://2bora.com/wp-content/uploads/cri-lamps-new.jpg
http://www.fosilum.si/static/uploaded/htmlarea/cri-comparison.jpg
http://www.naturalux.com/CRI Examples.html
http://www.fusionlamps.net/assets/img/CRI-comparison-apples.jpg