I'm just reading this in the news today: http://www.canada.com/topics/news/na...34a064&k=95624
Ontario to ban incandescent bulbs, in favor of energy-efficient CFL ones.
Australia and at least one state (I think it was California or Washington) have similar legislation in the works or passed already.
This is a huge mistake for multiple reasons, such that it should be up to the market not forcing it down people's throats, but I'll concentrate on the technical reasons. Rod Elliott covers reliability and safety concerns, and some efficiency myths about CFLs, here: http://sound.westhost.com/articles/incandescent.htm
My biggest concern is with quality of the light, which I think fits here nicely.
The human eye has three types of color sensitive cone cells, covering three different frequency ranges, with response peaks near red, green, and blue parts of the spectrum. Here's what it looks like:
(the black line is the response of rod cells which are not associated with color perception, but only brightness)
Now, a display device which produces its own light, such as a computer monitor or a TV, obviously needs only three color types to produce any combination of visible colors (technically, it's a bit more restricted than that due to the specific emission frequencies of the phosphors used in display devices).
It would seem to follow that a light source with three or more narrow spectral emission regions, such as a fluorescent light (they use phosphor coatings on the glass tube to produce light from the UV the plasma inside generates), that was properly white-balanced, would be all that's needed. Unfortunately, that is completely false. I'll try to explain why that is, now, and why only an incandescent bulb can fully be made to mimic visually sunlight.
The problem is because the light you see is not what comes directly from the light source, but what has been reflected from the object. That is, the light you see is the product of the light source spectrum with the spectral reflectance of the object you're looking at. Finally, your eyes respond not to narrow spectral locations for RGB, but with the complex curves shown above. What this means is the following: take two lights that have been white balanced to appear white (excite your R, G, and B cones the same amount), but have different spectra.
Now, while some objects may appear to have the same color under one light (while having different reflectance spectra), they can have different colors under another light. This is known as illuminant metameric failure. That is, the fact that the light source is white balanced doesn't guarantee that object colors will appear the same as under a different white balanced light. Indeed, in general a CFL or other fluorescent light cannot be made to have illuminated objects appear the same color as when lit with sunlight, whereas an incandescent bulb can.
The remaining thing left to explain is why an incandescent bulb can be made to perfectly match sunlight. The reason is that, unlike a fluorescent one which has narrow emission peaks, an incandescent bulb is, like the sun, a blackbody radiator, producing a smooth and continuous spectrum. By running an incandescent filament at the right temperature, the spectrum can then be adjusted to match sunlight by using a filter on the reflector behind the bulb. There are a few commercial light bulbs that implement this. CFL and other fluorescent, even so called 'daylight' ones, simply cannot match the solar spectrum and thus a number of objects will necessarily appear the wrong color.
The following shows typical spectra between 'daylight' CFL and incandescent bulbs:
Ontario to ban incandescent bulbs, in favor of energy-efficient CFL ones.
Australia and at least one state (I think it was California or Washington) have similar legislation in the works or passed already.
This is a huge mistake for multiple reasons, such that it should be up to the market not forcing it down people's throats, but I'll concentrate on the technical reasons. Rod Elliott covers reliability and safety concerns, and some efficiency myths about CFLs, here: http://sound.westhost.com/articles/incandescent.htm
My biggest concern is with quality of the light, which I think fits here nicely.
The human eye has three types of color sensitive cone cells, covering three different frequency ranges, with response peaks near red, green, and blue parts of the spectrum. Here's what it looks like:
(the black line is the response of rod cells which are not associated with color perception, but only brightness)
Now, a display device which produces its own light, such as a computer monitor or a TV, obviously needs only three color types to produce any combination of visible colors (technically, it's a bit more restricted than that due to the specific emission frequencies of the phosphors used in display devices).
It would seem to follow that a light source with three or more narrow spectral emission regions, such as a fluorescent light (they use phosphor coatings on the glass tube to produce light from the UV the plasma inside generates), that was properly white-balanced, would be all that's needed. Unfortunately, that is completely false. I'll try to explain why that is, now, and why only an incandescent bulb can fully be made to mimic visually sunlight.
The problem is because the light you see is not what comes directly from the light source, but what has been reflected from the object. That is, the light you see is the product of the light source spectrum with the spectral reflectance of the object you're looking at. Finally, your eyes respond not to narrow spectral locations for RGB, but with the complex curves shown above. What this means is the following: take two lights that have been white balanced to appear white (excite your R, G, and B cones the same amount), but have different spectra.
Now, while some objects may appear to have the same color under one light (while having different reflectance spectra), they can have different colors under another light. This is known as illuminant metameric failure. That is, the fact that the light source is white balanced doesn't guarantee that object colors will appear the same as under a different white balanced light. Indeed, in general a CFL or other fluorescent light cannot be made to have illuminated objects appear the same color as when lit with sunlight, whereas an incandescent bulb can.
The remaining thing left to explain is why an incandescent bulb can be made to perfectly match sunlight. The reason is that, unlike a fluorescent one which has narrow emission peaks, an incandescent bulb is, like the sun, a blackbody radiator, producing a smooth and continuous spectrum. By running an incandescent filament at the right temperature, the spectrum can then be adjusted to match sunlight by using a filter on the reflector behind the bulb. There are a few commercial light bulbs that implement this. CFL and other fluorescent, even so called 'daylight' ones, simply cannot match the solar spectrum and thus a number of objects will necessarily appear the wrong color.
The following shows typical spectra between 'daylight' CFL and incandescent bulbs:
Also, incandescent light bulbs are not inherently white either (as they are not changing the lighting element, but are tinting the light that is being output). The daylight or white ones get closer, but they still don't quite get there. Some really expensive artificial light fixtures for artists have a combination of incandescent and florescent. CIE's definition of white daylight is 6,500 K: LEDs are actually the most readily available at this color range
