Question about regular LED production

[Nisei]

I've been wondering about something for some time now and can't seem to find an answer but maybe someone here can help me out.

Since white LEDs are very common these days, why do they still produce red, green, yellow, orange and blue? Just take a white LED and instead of using clear plastic, use colored plastic to get all the colors you want.
Perhaps it's because white LEDs are still more expensive to produce but it would also make it possible to get colors which don't exist nowadays. I would like to have purple LEDs for example (not UV) but can't find them anywhere.

 

[spencer]

Other colours are produced because a white LED just a blue die with phosphor that converts the blue to other colours somewhat efficiently. There is very little red and cyan light coming out of an LED so if that colour plastic was put in front it would block out nearly all of the light.

 

[Th232]

^ What spencer said.

It's also more efficient to put a different kind of phosphor on to the LED to get the desired colour, or at least a peak in the right area.

 

[Nisei]

OK thanks for the replies. I can understand that but what I said would make sense as well wouldn't it? If you take a white LED and cover it with a color filter it works great. Now perhaps it wouldn't work for some colors but others might work fine and they could produce a much wider range of colors.
Which brings me to another subject: today we see more and more clear LEDs but LEDs used to be colored. If the phosphor is making the color, why did they also use colored plastic on top of that?

 

[Th232]

OK thanks for the replies. I can understand that but what I said would make sense as well wouldn't it? If you take a white LED and cover it with a color filter it works great.

It would work, but the efficiency of such a combination would be terrible. Of course, if there aren't any LEDs in the colour you want, you don't have much choice.

Which brings me to another subject: today we see more and more clear LEDs but LEDs used to be colored. If the phosphor is making the color, why did they also use colored plastic on top of that?

Guessing here, their phosphors previously didn't get the right spectrum, so they had to filter the output? Given that their output would already be close to the right spectrum, the efficiency losses wouldn't be as bad compared to if they filtered a white LED.

 

[Dave_H]

I can think of several reasons. Generating a wide range of wavelengths
and filtering most of them out is going to be inefficient. The desired colour
may not be strongly present in the white light. Brightness and efficiency
do matter in most applications, and these would be compromised.

Surely the manufacturing steps to add prosphor to white LEDs which is
not necessary for other LEDs, adds cost and complexity. Whatever fade
effects can be related to phosphor (especially low-end product) can be
avoided with monochromatic LEDs.

I have solar driveway marker lights in blue, white, and amber and they
all use a white LED behind a coloured filter. The blue and amber look
anemic, pale and washed-out. compared to what a monchromatic LED
would provide. They did this to simplify product design.

Some drive circuits especially series strings can accommodate fewer
white LEDs than others due to higher Vf. For example, Christmas lights
with all colours except white seem to go up to 50 lights, whereas white
appears limited to ~35. This is lower overall efficiency. (Note: you can get
white strings greater than 35, but they are multiple parallel substrings).

I've played around with some old beehive lenses (from auto tail-lights)
over white LEDs, and the results are mediochre. Perhaps adding tint to
the LED moulding compound is simple, and I think it is done anyway
in some cases, but likely just to enhance a fairly narrow band of wavelengths.

Dave

 

[blasterman]

If you take a white LED and cover it with a color filter it works great.

Not exactly. Filters degrade over time, and they aren't as efficient as an LED emitting the desired color.

Next, cool-white LEDs, which are the most efficient type, have very little yellow and red spectrum. So, putting a red filter on a cool-white LED woulnd't be very bright. For instance, for plant growth RED light is extremely beneficial, and Red LEDs are excellent sources of this. Putting a red filter over a cool-white LED would deliver much less red light.

However, I just got done arguing the opposite in a DJ lighting forum, but for very different reasons. Because of the limitations of high powered RGB LEDs and drastically broader market of white LEDs, it's better (sometimes) to simply use a colored filter wheel that's used with standard white light sources.

 

[Ragiska]

i think you are confused what filters do. they do NOT "alter" the color of light, they only BLOCK all other colors of light which are not desired.

 

[HarryN]

LOL - perhaps you need a new search engine.

For most colors, you can get them from Philips Lumileds in the "rebel" line. They don't happen to make "purple", so I took the massivly difficult step of typing "purple led" into Bing and up came a whole bunch of choices, including this one from about 5 down. I didn't research it further, as you probably can do just as well yourself.

http://www.lc-led.com/View/itemNumber/165

 

[csshih]

It's also more efficient to put a different kind of phosphor on to the LED to get the desired colour, or at least a peak in the right area.

It is not a phosphor, the die itself has a different chemistry.

I believe only white emitters use a phosphor. <_ WRONG.

saabluster pointed out that lumileds (philips) also makes amber leds with a phosphor.

 

[John_Galt]

i think you are confused what filters do. they do NOT "alter" the color of light, they only BLOCK all other colors of light which are not desired.

This...

 

[IMSabbel]

Which brings me to another subject: today we see more and more clear LEDs but LEDs used to be colored. If the phosphor is making the color, why did they also use colored plastic on top of that?

LEDs were basically indicator lights. If the plastic is colored, you can still see what kind of light it should produce even when off.

 

[saabluster]

I believe only white emitters use a phosphor.

Actually Lumileds makes a very nice phosphor converted amber.

 

[Colorblinded]

Not exactly. Filters degrade over time, and they aren't as efficient as an LED emitting the desired color.

Well to that point, so do phosphors.

Ultimately it's just inefficient to make all that "white" light and then filter out so much just to emit one particular color, as others have said.

 

[csshih]

Actually Lumileds makes a very nice phosphor converted amber.

corrected! thank you!

 

[JohnR66]

Phosphor coated LEDs are in white of varying color temperatures. There are also hot pink, violet and amber.

Evidot LEDs use quantum dots to make several different hues across the spectrum. I guess this can be classified as a phosphor.

There has even been a non phosphor white LED.

 

[Nisei]

Thanks for all the replies. I do know that a color filter blocks all undesired wavelengths and yes, there are purple-like leds but afaik these are usually the UV type LED. Perhaps I should've made clear that I'm not looking for super bright LEDs but indicator LEDs. I don't care if they're not super bright and efficient at all. On the contrary, it can be very annoying when an indicator LED is super bright. I have an audio mixer with a blue power-on LED which is so bright I have to squint if I'm adjusting the settings of the buttons. Adding a resistor to dim the brightness turned out to be impossible because that made the other LEDs dim as well and none of the local electronics stores could sell me a low brightness blue LED.

 

[Colorblinded]

IIRC places like allied electronics, newark.com, digikey, mouser (and so on) all sell a great variety of color LEDs in varying intensities. If you're looking to tone down the overwhelming blue you could just look for a dimmer LED there.

I can't stand the blue LED indicators on a lot of things, very rarely do manufacturers do them right (in a non-blinding fashion).