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:
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.
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.