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Hi All. I have a couple of random questions about LEDs that I know you clever people can help with! :) Firstly (and most importantly) - do you have to pre-heat LEDs (such as LED Pars etc)? Also, how do they manage to last for 11 years (approx)?! What are they made of that makes it possible, and is there any other reason that they are sooo expensive (other than that they last ages and so are better value for money)? Does an individual LED bulb have the ability to change colour, or is it purely by mixing it with other colours that par cans can colour change? Thankyou so much, you're all stars! :** laughs out loud **:
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No you don't need to preheat LED's as far as I'm aware. With regards to the cost I think it is purely because it is a fairly new technology. With regards to how they last so long, I'm not to sure, I will leave that to another poster to explain! Finally, no one LED does not create the millions of colours possible with LED fixtures. It is the mixing of Red, Green and Blue that does it, which is why if you're wanting a nice flat open white, you won't achieve it with LED's. Hope I have managed to answer some of your questions!
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Briefly to answer your questions:

 

1) Traditional filament bulbs fail because the filament is gradually evaporating away (and coating the inside of the glass with condensed filament). Eventually the filament becomes so weak that the thermal shock of switch on evaporates a section and it's no longer complete! Bulbs are filled with inert gas to slow this process. Filament bulbs work by passing a current through a resistor (filament) which heats up and glows.

 

LEDS use inorganic semiconductor technology and don't suffer from the same effect and operate at much lower temperatures. Light is generated by the transfer of electrons between the layers of the seminconductor enabled by the voltage across the device. The just last a long time. They are expensive because the facilities you need to make the LEDS are expensive to build and maintain.

 

Pre-heat - no. The reason for pre heating filament bulbs is to reduce the thermal shock on switch on by passing a small current all the time and keeping them warm to just below the point they visibly glow. This increases their life and also speeds up the reaction time when you turn them up.

 

LEDs as such operate at discrete frequencies and LED par cans and the like are made of different LEDS which operate at Red, green or blue frequencies usually so you can mix the intensity of the different colour clusters to get a range of colours. LEDs are relatively non-divergent compared to traditional bulbs which act like point source light sources. To get the best results you might use a lens to diffuse the light and mix the colours better.

 

With regards to power output, I'm sure others can help, but remember that if you want to colour a traditional filament bulb with a filter, the filter reduces the intensity of the light by removing parts of the colour spectrum (as heat and hence they burn out) whereas an LED only generates the colours needed.

 

You can get bi or even tri colour LEDS but they are really several in one package and are usually only found in low power applications (indicator lights for example).

 

There's a lot more to know, but I'm sure others will fill you in.

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No, they don't need to be pre-heated, although some manufacturers may keep a low voltage applied for other reasons.

 

I think the remainder of this question is better answered outside of this forum. I bet there's a good Wiki on Light Emitting Diodes for instance :** laughs out loud **:

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Hi,

 

"how do they manage to last for 11 years (approx)?! What are they made of that makes it possible"

 

They produce light as a result of an electro chemical reaction and not by heating up a filament until it glows.

They do fail, sometimes completely suddenly, but mostly they fade with age. They also only conduct electricity in one direction and are sensitive to over current/voltage.

 

"and is there any other reason that they are sooo expensive"

 

Can't remember prices lately, but they are something like 10p each (Prices vary on colour). But you don't get a lot of light from one LED, so there will typically be about 150 in a par can. (150 * 10 = £15 just for the equivalent of a 100W bulb. I imagine better quality LEDS cost a lot more though.)

 

(All figures are guesses BTW. Didn't have time to look up ;-)

 

HTH

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do you have to pre-heat LEDs (such as LED Pars etc)?

 

No preheat required, rise time of an LED is in micro seconds, millionths of a second, they strobe very well and are used for high speed scientific photography. Fast flashing dosen`t bother them at all. In fact its usually the way theyre dimmed by varying on/off ratio hundreds of times a second.

 

Also, how do they manage to last for 11 years (approx)?!

 

A lot of them don`t ;-)

In general they will fade out over extended periods, high quality white high power LED from Lumileds is quoted as putting out 70% of intial light output after 50,000 hours. This is in ideal cool conditions, LEDs DO generate heat, letting them get hot has short and long term negative effects on light output.

 

Cheap LEDs may fade to 10% of intial over a similar or shorter period.Or random failure which low cost LEDS are muich more prone to.

 

White LEDs can exhibit serious colour shifts towards green/yellow or blue as they age.

 

What are they made of that makes it possible, and is there any other reason that they are sooo expensive (other than that they last ages and so are better value for money)?

 

Grown on synthetic sapphire wafers, with exotic materials like Gallium Arsenide and Gallium Nitride in super clean enviroments.

 

Difference in cost , in quantity , between a cheap commodity LED and a high quality big name LED, just for the component, can be as much as 10 times more for a quality part.

 

Does an individual LED bulb have the ability to change colour, or is it purely by mixing it with other colours that par cans can colour change?

 

LEDs themselves are monochromatic.

White LED is a blue LED with a yellow phosphor, it dosen`t make sense to add gel in front though , efficiency would be awful.

Colour mixing LED is generally RGB , some with amber added, some with amber and white, some with amber,cyan and deep blue allows wider colour range and more convincing pastels.

 

good resource Craig Johnson`s Led Museum

 

http://www.ledmuseum.org/

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Different colour models, Cyan Magenta Yellow is usual in printing and moving lites which is subtractive mixing, start with white and subtract colours to get desired hue. Full CMY = Black

 

RGB is additive mixing, full RGB = White, Same as your monitor uses, and helps explain difficulty of matching monitor colours to printers.

 

Cyan LEDs are available with good efficiency, yellow are available but need towards to the red amber end to get any efficiency. Magenta as we perceive it is a mixed colour so you dont get them as an LED colour, though there are pink and purple phosphor type LEDs around, they certainly arent bright enough for colour mxing lights.

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The Stagebar 54 from Martin gives the option to create softer, subtler pastels which is a nice feature. It does it with the addition of Amber and White LED's and it looks fantastic, the colour options are incredible. Shame its so damn expensive though, however I do see why. :** laughs out loud **:
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Surely RGB is all you need for any form of full colour full spectrum light..like the phosphors on a TV.

 

Would be true, if not for the fact that LED's are monochromatic... Phosphors are not. If you look at the transmission spectrum in a gel swatch book for the primary colours you will see that even primarys have a big frequency range. LED's have a tiny frequency range hence do not replicate the colour effects you might otherwise expect..

 

Different colour models, Cyan Magenta Yellow is usual in printing and moving lites which is subtractive mixing, start with white and subtract colours to get desired hue. Full CMY = Black

 

RGB is additive mixing, full RGB = White, Same as your monitor uses, and helps explain difficulty of matching monitor colours to printers.

 

Hmm.. well CMY and RGB are neither additive nor subtractive by definition... Yes monitors are RGB, but mix subtractively. Printers (on white at least) also mix subractively.

 

RGB in LED's is additive yes, but only because its a multi source mixing system. Any form of single source system is subtractive, whether its RGB or CMY...

 

CMY is more flexible and efficient (generally, although both terms are a bit subjective) hence its dominance in lighting.

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`Full spectrum` a much abused term, technical term should be Spectral Power Distribution, measuring how the power accross the visible range is distributed.

 

Sunlight is `full spectrum` by definition, but then the deiniton would vary by how close to the equator you are.

 

Tungsten is `full spectrum` with an almost even power band through the visible spectrum ramping upwards towards the red end.

 

Fluoro and discharge sources are very spiky, big peaks of certain wavelenghts and troughs with almost nothing, by above definition it isnt `full spectrum`.

 

LED as said with RGB will give the appearnce of white but it wont be anywhere near `full spectrum` because of narrow emmision bands.

 

http://ioannis.virtualcomposer2000.com/spe...cope/index.html

 

</rant> a plague on the `full spectrum` snake oil salesman

 

Definately a bit subjective to understand, quite a good explanation of CMY and RGB here.

 

http://www.colorcube.com/articles/articles.htm

 

Can recommend the puzzle :-)

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The CIE diagrams (1931 or 1964) are useful guides, based on the observations of groups of real people deciding whether two colours are "the same"

 

It makes it very easy to determine the possible perceived colour gamut of multiple light sources

- Plot the points that show their colour (easy with LEDs as they are fundamentally monochromatic)

- The gamut is the area inside the polygon.

 

But, how that multiple source reflects off surfaces depends on the 'true' colour distribution (spectral power distribution) of the sources and not their perceived colour, although the two are related.

 

The concept of "Full Spectrum" is a strange one - it's usually considered the "equal energy" curve, where all frequencies (colours) of light are present at the same power (energy per second).

This means fewer blue photons to red photons of course, as blue photons carry more energy per each than red ones.

 

Colour Theory is one heck of a lot more complex than you'd think - your local (university) library probably has a lot of very interesting books on the subject!

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I knew you clever people would know! :-) cheers guys! I have a couple more questions; I have had no experience with fixtures that use LEDs at all, but would really like to understand them better! So for example, if I bought an LED par can, it would last ages, but eventually some of the LEDs would start to go off colour or just die; Could I then replace individual LEDs in a par? or would I just have to chuck it and buy a new one? how much is it to hire an LED par for a week? (the man from kave could answer that for me! :** laughs out loud **: ) I think that's all my questions for now, but I'm sure I'll remember some more! cheers for now.
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LEDs do have a lifetime problem, but it isn't the lifetime of the individual LEDs, it's the lifetime of availability of the product. When the things fail, there is no guarantee a replacement identical fixture will be available. I've got some 15 month old PAR56s - one has one red led permanently on. This model isn't made any more, and the new ones have different attribute implementation. So you need to batch buy them - always having a spare or two if poss. Then when they do cause problems, you skip 'em (or flog them on eBay) and start again. Don't worry about them at all - the LEDs are so much longer lasting, percentage wise, than incandescents
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