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LED Evolution Could Spell The End For Bulbs
An anonymous reader writes "USA Today is running a story discussing how LED lamps were unthinkable until the technology cleared a major hurdle just a dozen years ago. Since then, LEDs have evolved quickly and are being adapted for many uses, including pool illumination and reading lights, as evidenced at the Lightfair trade show here this week. More widespread use could lead to big energy savings and a minor revolution in the way we think about lighting."
They haven't been used as sources of illumination because they, for a long time, could not produce white light -- only red, green and yellow. Nichia Chemical of Japan changed that in 1993 when it started producing blue LEDs, which combined with red and green produce white light, opening up a whole new field for the technology.
This is certainly one way to produce a white LED but it is not the common method today. Most white LEDs use a phosphor to convert a blue or ultraviolet LED into a white one. A quick google found the following page that talks about this in more detail:
http://www.marktechopto.com/engineering/white.cfm
I would speculate that for normal home lighting using a phosphor will give better results as:
Yes, I am. You might be too ...
Most people have another type of receptor, called a rod, which is not colour sensitive, unlike the three kinds of cones which are colour sensitive. However, my rods have a much wide spectral response than the normally accepted colour range of white light. I have known for a long time that light without significant ultraviolet content makes it hard for me to accurately resolve edges. I find technical drawing very difficult by incandescent light. Others may be the same too.
Remember 10% of men lack one kind of cone, and are partly colour blind. A lot more lack fashion sense, but you can't blame that on LEDs
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I'm not the one you replied to, but I did look up the spectrum -- it's shown here. It's definitely more spread out than I would have guessed, but it doesn't look like an incandescent,
Warm and cool are really terms used to describe white light. When you talk about white the question becomes what is it? A blend of all the colours is an elementary explination, but the fact is they aren't all present in equal levels, from any source.
The way that it is talked about, is called colour temperature, and it is spoken of in kelvins. The idea is if you heat a black body radiator to that temperature, that's the kind of white you get. The lower the temperature, the more red in it, the higher the temperature, the more blue.
On most monitors that aren't connected via DVI, you can see colour temperature changes for yourself. In its configuration there should be a colour temperature option, generally with three presets: 5000k, 6500k and 9300k. PLay with them and notice the change. You'll probably find that changing from the one you are used to looks "wrong", either too red or too blue depending on. That's an illusion, however. If you go away for awhile and come back, or just ignore it and keep working, you'll find your eyes adjust and consider that to be white.
With bulbs, it gets more complex because it's not just a function of the temperature of the white, but of it's spectral composition. Most incandesant bulbs have a spectrum that is low on the high frequencies (near violet) and high on the low frequencies (near red). Other lights, like many floursecants, have an uneven spectrum, with peaks all over.
Now ideally what you are shooting for usually is light as close to sunlight as you can get. That's what humans would generally think of as "normal" or "correct" lighting. Easier said than done, of course.
So I don't know what the spectrum for any of the varities of white LEDs looks like, but it is very possible, even likely, that they are different than an incandescant bulb. It may be that they have a generally higher temperature and thus really are cooler, colourwise.