h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

great info
im 17 and have an intrest in chemistry especially organic chemistry.
the light emiting polymers you brifly mentioned intrested me. is it possible to make blue leds? with are red and green leds we should be able to make white leds. how bright can leds get? would ultra bright leds be the future of lighting the benifits would be obvious? you seem to be well learned in such matters could you tell me the chemicals that produce the green and red colours in leds?
thank you

Yes, blue LEDs have been available for several years, and torches based on white ones are fairly easy to buy now, with people attempting to produce decent designs of light bulbs based on LEDs as well, which would be far better than just about anything we currently have. I believe they produce far, far purer white light than conventional incandescent or flourescent bulbs.

I personally am looking forward to it!

It's funny you should mention that subject, because I went and wrote an article on it: A762662.

Inorganic Blue LED's are based on gallium nitride or indium gallium nitride. Nichia Corporation hold most of the fabrication patents

i read your item on leds and it answered my questions thankyou
but i would like to know at our current technolgical powers how bright is the best white light led is?, how expensive is it?, is it asthetical pleasing? and how long untill they are avalible in your average do it all?
thanks for answering a foolish persons insecnet pestering

Have a look at http://www.maplin.co.uk: that should answer your questions.

I have an LED keyring torch, which is really very very small indeed yet kicks out more light in one direction than any torch I've had before. Kind of comparable to the smallest size of Maglite, but with much lower energy consumption.

White LEDs seem to be exceptionally pure, so if we started lighting rooms with them it'd be a bit odd, because domestic incandescent (and to some extent flourescent) lamps are yellowish.

The way these white LED's work is that they couple a blue LED with a down-converter which emits two photons of yellow light for every single blue photon absorbed: blue LED's are very good at inducing fluorescence. The 'white' is a mixture of blue and yellow. Nichia Corporation now are producing a 'warm white' LED which also down converts the blue to red as well, giving it a warmer tinge. I'll give it about 10 years before the predominant form of lighting installed in new buildings is LED lighting. Good for our eyes, our pockets and the environment.

probally being foolish here but will the leds we use for houses be produced of a light bulb size or is the current size they only that can be produced?
also LEP that you mentioned in your artical thelinousn you mentioned that the organic polymers would be able to emit diffrent colours depending on the conditions i have an okish grasp of chemistry but i dont understand how that could happen would you be able to explain it to me pleas thanks

I suspect that house lighting would initially be made of arrays of small LEDs, in a package that fits in standard bulb mountings with appropriate transformer gear attached to get the voltage and current right. LEDs need DC, and I doubt they'd like 230V mains very much, but they should hopefully prove to be smaller than current energy-saving flourescent bulbs.

That's pure speculation, by the way.

I seem to remember reading somewhere recently that making *very* large white LEDs may soon be possible?

Regarding current devices, I use a 3-LED bulb in my caving headset as a backup light to my main carbide (gas) light. Compared to the standard incandescent lightbulb, I get a much longer lifetime (100+hrs compared to 24hrs), and have no worries about the LEDs ever burning out, but the colour of the light does vary somewhat across the beam - the spread of blue light from the LED chip itself isn't the same as the spread of light from the flourescent yellow reflective part, so some areas of light are bluer than others.
One definite advantage is that as the battery ages, the LED light just gets dimmer, whereas with a conventional bulb, the light rapidly yellows and becomes unusable.

Excellent!

Very large white LEDs are another possibility of course... it's going to be very interesting to see how this goes, but I'd love to be able to light at least part of my house with them, I can imagine that they'd be great in the kitchen! No heat output to speak of, so they're even better than flourescent tubes for mounting under cupboards to light worktops (my parents recently had under-cupboard lighting fitted, it's great but they have to use low-heat flourescents to stop the stuff in the cupboard from cooking). Some people might prefer a warmer light, but as has been mentioned, that could be arranged.

Especially if they can fix that colour irregularity.

Oh no! Lecture time!

What I think is quite exciting is the prospect of tunable colour lighting: any colour or hue to suit one's mood. I took my daughter up to Magna in Rotherham recently, and they use LED lighting quite extensively. They have some colour-tunable footlights in the room where they do the robot show.

This is what Nichia are producing right now:
http://www.nichia.co.jp/info/news/new20021112.html

read the newsletter thing you posted but as a layman there are some things i dont understand like what is the diffrences between the phosphurses you mentioned? what is YAG phosphur? the warm white light you mentioned would that be the same colour as your normal bulbs instead of pure wihte light.

I think YAG refers to Yttrium Aluminium Garnet (a material also used in lasers). I assume that YAG phosphors are good at converting blue to yellow light, and are used in standard white LEDs, but the 'warm white' LEDs use an extra red phosphor to lower the colour temperature of (ie to redden) the light given off.

YAG also is a good host for heavy metals such as neodymium which fluoresce at various wavelengths. But in coming back to your orginal point, organic LED's work by combining a polymer with hole and electron injecting electrodes. The holes and electrons combine to make 'excitons'; mobile excited states which spontaneously decay and give off light. There are two types of exciton: singlet and triplet: the former have electron spins opposed, whereas the latter have elcton spins aligned. Triplet excitons have a much longer lifetime which makes them prone to non-radiative (i.e. heat-emitting) decay.

Some studies recently have added transition metal complexes which convert the triplets to singlets and hence increase yield of light production.

so to put in simlpletons terms the triplet excitons have longer lifes but singlet have high light production so to convert triplet to singlet would get the best of both worlds.

personally i never thought leds were so fasinating

You wait until you've got a flexible OLED display in your living room instead of curtains... then it'll be exciting

Also, the radiative transition from triplet exciton (^^ spins) to singlet ground state (^v spins) is forbidden by quantum mechanical rules. This isn't to say it doesn't happen, it just happens very slowly, which means that the rate of light generation is very low and other processes can deactivate the exciton.