[APD] Idle Thoughts on LED Illumination

[Wright Huntley <whuntley at verizon_net>]

Everyone is doing, while I just sit and think. :-)

White LEDs are somewhat more expensive and probably not as efficient or 
versatile as RGB colored LEDs.

Quick e-bay survey shows bright whites at $0.23 ea in 1000 quantity plus 
shipping, while red, green and blues (high intensity) run at or below 
$0.20 ea *including* shipping at only 50 quantities from Hong Kong.

Only apparent advantage of whites is that you can pack more in a smaller 
space than three colors. Disadvantage is the lack of ability to control 
the exact "whiteness" (aka color temp.). [May be OK if you really like 
to look at 8600K (bluish) lamps.

The suppliers all expect you to waste power by converting 12Vdc to 20mA 
with a resistor. Since 9V will be dropped in the resistor and only 3 in 
the diode, that seems to be wasting about 3/4 of your input power. 
[Starting with 3X as much heat as you really need!] You possibly cannot 
operate with diodes in series to get around that, as their Voltage drop 
at a given current isn't predictable enough to keep current within safe 
bounds.

The safer solution may be to use a constant-current supply (aka battery 
charger) if the diodes are pretty well matched. You can then use 
series/parallel combinations without risking overdriving them.

Since the band gap, hence forward Voltage drop isn't the same for red, 
green and blue diodes, you may need three independent power sources for 
the three colors. At least that lets you tune for a pleasing color. :-)

Otherwise, I'd start by trying to put groups of three in series of RGB 
and parallel those up to see if the constant current through each set is 
close enough to not burn some out. A standard car battery charger might 
be perfect, as you can often set them at 3A or 6A as needed.

Wish they would spec LEDs with mW of output. Candelas are utterly 
useless for our purposes and impossible to easily convert to PAR. Note 
that some blue LEDs are around 470nm, which is bright in candelas and 
photosynthetically useful, while some are relatively dim, in candelas, 
at 400-440 nm and plant-useless. Be careful, here.

Red LEDs are readily available at a useful 670-630 nm. All LEDs put out 
a pretty wide spectrum (compared to a laser) so the photosynthesis isn't 
a problem if you just get close. That is, they aren't such a narrow line 
source that they won't activate the chlorophyl a and b spectra until you 
get below about 440 nm or well above 700 nm. [Those parts of the 
spectrum aren't visible enough to be useful, either.]

Grinding off and polishing or immersing have been suggested as ways to 
get the narrow beams spread out more. A third, fairly easy, fix would be 
to pot the lenses completely in a layer of clear GE silicone potting 
compound. You would lose about 5% of the light at the silicone air 
surface, due to reflection and again at the air-water boundary. Final 
efficiency would be about 90% (95% of 95%) of what immersion would give. 
Not too bad.

Enough idle thoughts for now. Asbestos britches going <ON> in case I got 
some of the above wrong. ;-)

Wright

--
Wright Huntley -- 760 872-3995 -- Rt. 001 Box K36, Bishop CA 93514

    "...there are only a limited number of things that government
 can do more effectively than individuals or other organizations
 can do."
     -- T. Sowell


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