Updated Aug 2012
Fluorescent and CFL light is a composite, chemical light, produced b y a mix of (usually three) phosphors trying to emulate incandescent light, with varying degrees of success. Even in some of the best, is still an artificial replica of the real thing that just doesn’t feel the same and does not behave in the same way, and for technical reasons can never have the same radiance and glow. FL/CFL or LED light is no more the same thing as incandescent light than a gold-coloured alloy can be called real gold, or synthetic microfiber real silk. Both the original and the copy have their respective uses, but in many cases a substitute just won’t do.
• When dimmed, CFL light just turns cooler and more grey and dull, not warmer. The same happens when you dim an LED.
• CFLs give a duller, non-radiating light which makes colours look pale and dead due to lower colour rendering (CRI 81-83), spiky spectral distribution that lacks parts of the spectrum. Check this out for yourself with the back of a CD under a fluorescent light source, and see the spectrum broken up into 3 blocks of colour with all the wavelenghts inbetween more or less missing. These spectral power distribution charts show the differences between incandescent and fluorescent light spectra:
Lighting designer Gad Giladi, D.E.S.A., M.F.A. FPLDA, explains what happens when wavelenghts are missing:
Not only are the quantities of light of CFL ‘equivalents’ not equal to those of the planned replaced incandescents but also the quality of the light greatly differs. This is due to the fact that the spectrum of the incandescent is a continuous one, i.e. has energy in all wavelengths of the visible electromagnetic spectrum while the spectrum of the CFLs, like all discharge lamps is a discontinuous one, i.e., depending on the composition of the phosphor coating of the tubes will present a lack of or a deficiency in energy at certain wavelengths of the visible spectrum.
This characteristic is not immediately visible to the human eye until the emitted light falls on a surface or an object: the energy in each different wavelength corresponds to a colour perceived by the human visual system. If that colour does not exist in the light, its corresponding pigment in materials cannot be perceived by the eye; if the energy in a specific wavelength is deficient, the corresponding pigment in materials will be perceived as dead, washed-out and distorted. (…)
That means that where colour perception is important, i.e. everywhere the human being lives and spends time, the replacement of incandescents by CFLs is going to inevitably create dull looking spaces, distort colours of architectural finishes (stone, marble, timber, paint, stucco etc.), of furnishings – curtains, carpets, upholstery, furniture finishes, artwork etc.), warp the colour of skin (people are continuously going to look bad/sick in their mirrors as well as in the eyes of their partners).
Incandescent lamps are close to theoretical ‘point sources’ which allow for the design of precise optical systems around them to direct the light in an accurate manner. This permits the creation of accent lighting, a means to create visual interest and drama in spaces. CFLs are diffuse light sources and no engineering will truly make a diffuse light source into a ‘point-source’. Gone is accent lighting!
Example of how food can look when illuminated by a low colour rendering light source (top) such as CFL or LED, vs a high colour rendering light source (bottom) such as incandescent light:
When a whole room is lit by a fluorescent light it will look more like in the top picture, unless there is an incandescent lamp or halogen spotlight in the room, which will then immediately bring some warmth to the room and make colours come alive again.
See also this interview with a pro-CFL professor explaining the quality problems with CFLs: Why Efficient Light Bulbs Fail to Thrive
Edit: Some content moved to separate post about incandescent light quality.