Producing 2,700K to >10,000K Light Using Only Blue LEDs
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Old 01-07-2013, 09:07 PM   #1
DarkCobra
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Producing 2,700K to >10,000K Light Using Only Blue LEDs


My project queue is full. Overfull, really. It will be a while before I can try this. So I'm writing up my notes and ideas here, both for myself, and anyone else who might like to try before I do.


REMOTE PHOSPHOR INTRO:

Today I was reading about remote phosphor technology. If you haven't seen this yet, take a look at this video - the first demo starts just after the one minute mark:

http://www.youtube.com/watch?v=5iq97uwE93o

Cool! But what's going on here? Well, I'd best start with a description of how regular white LEDs work.

All white LEDs are really royal blue LEDs, with a built-in phosphor layer that converts part of the blue light to other colors, creating some shade of white. The phosphor also acts as a diffuser, which isn't really noticeable to us since it's so darn small and covered with a lens. But no diffuser is perfect. They send some light back to the source, which in this case is the LED die. Because the die isn't very reflective, this light is mostly absorbed and lost as heat. Some light also gets bounced around inside the diffuser, until it's absorbed and converted to heat. Both sources of heat add to the normal heating of the die, and as LED temperature goes up, efficiency goes down.

In remote phosphor tech, the phosphor is on a panel external to the blue LED. The panel is larger, and separated by some distance, allowing the blue light to spread out before conversion. A "mixing chamber" is also required, which is a fancy term for nothing more than a reflector:



Most of the light reflected by the phosphor now hits the reflector instead of the LED, and gets sent back in the right direction rather than absorbed (the diagram doesn't show this clearly). Between this and eliminating two sources of heat at the LED die, this can be 30% more efficient than a normal white LED! We also get a larger, more diffuse light source, which is often desirable.


DIY POTENTIAL:

I was surprised to find remote phosphor panels both readily available and inexpensive, in both a variety of flat and 3D shapes. For example, here is a 5,000K 21mm x 21mm panel for $0.62 in single quantity.

The "mixing chamber" (reflector) needs to be well-designed and highly reflective for the full 30% efficiency increase. However, even a poor one cobbled together from something like aluminized mylar should still provide efficiency better than a normal white LED; some recovered light is better than none.

And of course, royal blue LEDs are commonly available.

Just one small hitch. The phosphor panels are available in 2,700, 3,000, 3,500, 4,000, and 5,000K. We need higher color temperature. 6,500K is listed in some datasheets for a limited number of shapes, but doesn't appear to be sold anywhere. Intermatix, the manufacturer, will gladly make custom panels with any spectrum and size you want. But that will surely require a larger order than us hobbyists can manage, and who knows how long it will be until some aquarium lighting vendor steps up to do this.


MAKE YOUR OWN SPECTRUM:

I figure you can still make 6,500K. Or any color temperature you want, really. Take a look at the spectral distributions:



2,700K is heavy in red. 5,000K is heavy in green. All have some amount of blue, which is just some of the royal blue from the LED passing through unchanged.

So what if we make a panel from three materials?



These colors are not representative of the actual material color, but rather the effect it will have on the LED.

Blue is plain diffuser material, passing the blue light unchanged. Red and green are remote phosphors, with spectrum as labelled.

By changing the relative size of these materials, you can produce any spectral balance desired. Since all produce diffuse light from a larger area, the multicolored shadows typically seen when attempting to mix different colored LEDs will be greatly reduced. Done right, it may be as unnoticeable as when using different colored fluorescent tubes.

Or, make an oversized panel. Then, by simply repositioning the panel under the LED and reflector, you can adjust the spectrum on the fly.

Comments appreciated, experimentation encouraged.
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Old 01-07-2013, 11:04 PM   #2
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Very interesting! Thanks for posting the info. I'm going to have to order some to play with. I wonder what would happen if I slapped a bit of that material onto a 50 watt RGB led?
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Old 01-09-2013, 11:37 AM   #3
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Quote:
Originally Posted by O2surplus View Post
Very interesting! Thanks for posting the info. I'm going to have to order some to play with. I wonder what would happen if I slapped a bit of that material onto a 50 watt RGB led?
Cool, let us know if you do.

I did see one commercial LED bulb, I think it was a Philips, that had some red LEDs in addition to the royal blues. The reason wasn't clear, I'm guessing it allows the user to electronically tune the color. An RGB LED would be a neat experiment, but:

1) I'm not sure if the blue is the royal blue that best stimulates the remote phosphor. Something I never thought to check on before now.

2) RGB LEDs are awfully expensive for the amount of light they put out. Perhaps if you could use only a few, and get the light to blend well.

I do know that using RGB LEDs (without remote phosphor) to simulate white produces poor viewing results. For example, consider an object that reflects true yellow well, but red and green poorly. As the RGB LED produces strong, narrow peaks of red and green, but zero true yellow, this object will look washed out. Over a tank, some colors inevitably end up looking a bit off and artificial. Photographers usually hate RGB illumination. There are RGBW LEDs which help address this problem, with the majority of the illumination coming from white, and the RGB being used only to tune the exact shade.

Since the phosphor in a white LED or a remote phosphor system produces a more continuous spectrum, it looks better.
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Old 01-09-2013, 05:16 PM   #4
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Interesting idea, but how would you know what "color temperature" light you were getting? Researching something like this would seem to require a means of measuring what you were getting.
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Old 01-09-2013, 05:50 PM   #5
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Seems to me that prices are beginning to fall on some types of led since the release of the newer COB led. I recently purchased (3) 50 watt RGB led arrays for $14 each and a couple 7,000k 100 watt white led arrays for $22 each. I'm experimenting with them right now, as I'm attempting to create a lighting system with the broadest spectrum available using only leds.
For the sake of experimentation, I've also purchased a bunch of 10 watt leds in a myriad of colors. I plan to mount them as close together as physically possible and drive each color independent of the others for full "color control".
Below are the colors I'll be using to varying degrees, and I'll just have to wait and see how my plants respond. I know some of the wavelengths may be unnecessary for success but I'd rather include them now in a smaller build and eliminate them, if warranted, later on.
1. 10 watt "Violet 390 - 410nm"
2. 10 watt "Royal Blue 445nm"
3. 10 watt "Cyan 510 - 530nm"
4. 10 watt "Green 560nm"
5. 10 watt "Yellow 580nm"
6. 10 watt "Amber 590- 610nm"
7. 10 watt "Red 630 - 650nm"
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Old 01-09-2013, 09:50 PM   #6
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Quote:
Originally Posted by Hoppy View Post
Interesting idea, but how would you know what "color temperature" light you were getting? Researching something like this would seem to require a means of measuring what you were getting.
Easy answer. You wouldn't.

I only consider this a potential means of being able to almost perfectly tweak the appearance of a tank. How many times have you wished you had just a little more/less red/blue/whatever after building a LED light? Or added a new plant or fish, that a little differently colored light would really accentuate? You could add or swap out a few LEDs, but that may not get you exactly the desired aesthetic result. And if you're not careful, you may get multicolored shadows too.

If you're only interested in researching effects of spectra on plant growth and pigmentary response, then it's better to go with various colors of LEDs. Then you know exactly what you're getting, and probably don't care about stuff like multicolored shadows.

Quote:
Originally Posted by O2surplus View Post
I'm experimenting with them right now, as I'm attempting to create a lighting system with the broadest spectrum available using only leds.
Sounds like a great project. I'll be eagerly awaiting anything you discover, keep us informed.
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